Merge branch 'master' of github.com:NixOS/nix into drv-outputs-map-allow-missing

29 files changed, 592 insertions, 277 deletions

diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index b791c125b..30150eeba 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -178,7 +178,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
     auto [fileHash, fileSize] = fileHashSink.finish();
     narInfo->fileHash = fileHash;
     narInfo->fileSize = fileSize;
-    narInfo->url = "nar/" + narInfo->fileHash.to_string(Base32, false) + ".nar"
+    narInfo->url = "nar/" + narInfo->fileHash->to_string(Base32, false) + ".nar"
         + (compression == "xz" ? ".xz" :
            compression == "bzip2" ? ".bz2" :
            compression == "br" ? ".br" :
@@ -372,7 +372,7 @@ StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath
        method for very large paths, but `copyPath' is mainly used for
        small files. */
     StringSink sink;
-    Hash h;
+    std::optional<Hash> h;
     if (method == FileIngestionMethod::Recursive) {
         dumpPath(srcPath, sink, filter);
         h = hashString(hashAlgo, *sink.s);
@@ -382,7 +382,7 @@ StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath
         h = hashString(hashAlgo, s);
     }
 
-    ValidPathInfo info(makeFixedOutputPath(method, h, name));
+    ValidPathInfo info(makeFixedOutputPath(method, *h, name));
 
     auto source = StringSource { *sink.s };
     addToStore(info, source, repair, CheckSigs);
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 3a55f24cb..1f40dc42a 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -297,7 +297,7 @@ public:
     GoalPtr makeDerivationGoal(const StorePath & drvPath, const StringSet & wantedOutputs, BuildMode buildMode = bmNormal);
     std::shared_ptr<DerivationGoal> makeBasicDerivationGoal(const StorePath & drvPath,
         const BasicDerivation & drv, BuildMode buildMode = bmNormal);
-    GoalPtr makeSubstitutionGoal(const StorePath & storePath, RepairFlag repair = NoRepair);
+    GoalPtr makeSubstitutionGoal(const StorePath & storePath, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
 
     /* Remove a dead goal. */
     void removeGoal(GoalPtr goal);
@@ -1047,7 +1047,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath, const BasicDerivation
 {
     this->drv = std::make_unique<BasicDerivation>(BasicDerivation(drv));
     state = &DerivationGoal::haveDerivation;
-    name = fmt("building of %s", worker.store.showPaths(drv.outputPaths()));
+    name = fmt("building of %s", worker.store.showPaths(drv.outputPaths(worker.store)));
     trace("created");
 
     mcExpectedBuilds = std::make_unique<MaintainCount<uint64_t>>(worker.expectedBuilds);
@@ -1182,7 +1182,7 @@ void DerivationGoal::haveDerivation()
     retrySubstitution = false;
 
     for (auto & i : drv->outputs)
-        worker.store.addTempRoot(i.second.path);
+        worker.store.addTempRoot(i.second.path(worker.store, drv->name));
 
     /* Check what outputs paths are not already valid. */
     auto invalidOutputs = checkPathValidity(false, buildMode == bmRepair);
@@ -1206,7 +1206,7 @@ void DerivationGoal::haveDerivation()
        them. */
     if (settings.useSubstitutes && parsedDrv->substitutesAllowed())
         for (auto & i : invalidOutputs)
-            addWaitee(worker.makeSubstitutionGoal(i, buildMode == bmRepair ? Repair : NoRepair));
+            addWaitee(worker.makeSubstitutionGoal(i, buildMode == bmRepair ? Repair : NoRepair, getDerivationCA(*drv)));
 
     if (waitees.empty()) /* to prevent hang (no wake-up event) */
         outputsSubstituted();
@@ -1290,12 +1290,12 @@ void DerivationGoal::repairClosure()
     StorePathSet outputClosure;
     for (auto & i : drv->outputs) {
         if (!wantOutput(i.first, wantedOutputs)) continue;
-        worker.store.computeFSClosure(i.second.path, outputClosure);
+        worker.store.computeFSClosure(i.second.path(worker.store, drv->name), outputClosure);
     }
 
     /* Filter out our own outputs (which we have already checked). */
     for (auto & i : drv->outputs)
-        outputClosure.erase(i.second.path);
+        outputClosure.erase(i.second.path(worker.store, drv->name));
 
     /* Get all dependencies of this derivation so that we know which
        derivation is responsible for which path in the output
@@ -1307,7 +1307,7 @@ void DerivationGoal::repairClosure()
         if (i.isDerivation()) {
             Derivation drv = worker.store.derivationFromPath(i);
             for (auto & j : drv.outputs)
-                outputsToDrv.insert_or_assign(j.second.path, i);
+                outputsToDrv.insert_or_assign(j.second.path(worker.store, drv.name), i);
         }
 
     /* Check each path (slow!). */
@@ -1379,7 +1379,7 @@ void DerivationGoal::inputsRealised()
             for (auto & j : i.second) {
                 auto k = inDrv.outputs.find(j);
                 if (k != inDrv.outputs.end())
-                    worker.store.computeFSClosure(k->second.path, inputPaths);
+                    worker.store.computeFSClosure(k->second.path(worker.store, inDrv.name), inputPaths);
                 else
                     throw Error(
                         "derivation '%s' requires non-existent output '%s' from input derivation '%s'",
@@ -1432,7 +1432,7 @@ void DerivationGoal::tryToBuild()
        goal can start a build, and if not, the main loop will sleep a
        few seconds and then retry this goal. */
     PathSet lockFiles;
-    for (auto & outPath : drv->outputPaths())
+    for (auto & outPath : drv->outputPaths(worker.store))
         lockFiles.insert(worker.store.Store::toRealPath(outPath));
 
     if (!outputLocks.lockPaths(lockFiles, "", false)) {
@@ -1460,16 +1460,16 @@ void DerivationGoal::tryToBuild()
         return;
     }
 
-    missingPaths = drv->outputPaths();
+    missingPaths = drv->outputPaths(worker.store);
     if (buildMode != bmCheck)
         for (auto & i : validPaths) missingPaths.erase(i);
 
     /* If any of the outputs already exist but are not valid, delete
        them. */
     for (auto & i : drv->outputs) {
-        if (worker.store.isValidPath(i.second.path)) continue;
-        debug("removing invalid path '%s'", worker.store.printStorePath(i.second.path));
-        deletePath(worker.store.Store::toRealPath(i.second.path));
+        if (worker.store.isValidPath(i.second.path(worker.store, drv->name))) continue;
+        debug("removing invalid path '%s'", worker.store.printStorePath(i.second.path(worker.store, drv->name)));
+        deletePath(worker.store.Store::toRealPath(i.second.path(worker.store, drv->name)));
     }
 
     /* Don't do a remote build if the derivation has the attribute
@@ -1646,13 +1646,13 @@ void DerivationGoal::buildDone()
                So instead, check if the disk is (nearly) full now.  If
                so, we don't mark this build as a permanent failure. */
 #if HAVE_STATVFS
-            unsigned long long required = 8ULL * 1024 * 1024; // FIXME: make configurable
+            uint64_t required = 8ULL * 1024 * 1024; // FIXME: make configurable
             struct statvfs st;
             if (statvfs(worker.store.realStoreDir.c_str(), &st) == 0 &&
-                (unsigned long long) st.f_bavail * st.f_bsize < required)
+                (uint64_t) st.f_bavail * st.f_bsize < required)
                 diskFull = true;
             if (statvfs(tmpDir.c_str(), &st) == 0 &&
-                (unsigned long long) st.f_bavail * st.f_bsize < required)
+                (uint64_t) st.f_bavail * st.f_bsize < required)
                 diskFull = true;
 #endif
 
@@ -1692,7 +1692,7 @@ void DerivationGoal::buildDone()
                 fmt("running post-build-hook '%s'", settings.postBuildHook),
                 Logger::Fields{worker.store.printStorePath(drvPath)});
             PushActivity pact(act.id);
-            auto outputPaths = drv->outputPaths();
+            auto outputPaths = drv->outputPaths(worker.store);
             std::map<std::string, std::string> hookEnvironment = getEnv();
 
             hookEnvironment.emplace("DRV_PATH", worker.store.printStorePath(drvPath));
@@ -1920,7 +1920,7 @@ StorePathSet DerivationGoal::exportReferences(const StorePathSet & storePaths)
         if (j.isDerivation()) {
             Derivation drv = worker.store.derivationFromPath(j);
             for (auto & k : drv.outputs)
-                worker.store.computeFSClosure(k.second.path, paths);
+                worker.store.computeFSClosure(k.second.path(worker.store, drv.name), paths);
         }
     }
 
@@ -2015,7 +2015,7 @@ void DerivationGoal::startBuilder()
 
     /* Substitute output placeholders with the actual output paths. */
     for (auto & output : drv->outputs)
-        inputRewrites[hashPlaceholder(output.first)] = worker.store.printStorePath(output.second.path);
+        inputRewrites[hashPlaceholder(output.first)] = worker.store.printStorePath(output.second.path(worker.store, drv->name));
 
     /* Construct the environment passed to the builder. */
     initEnv();
@@ -2200,7 +2200,7 @@ void DerivationGoal::startBuilder()
            (typically the dependencies of /bin/sh).  Throw them
            out. */
         for (auto & i : drv->outputs)
-            dirsInChroot.erase(worker.store.printStorePath(i.second.path));
+            dirsInChroot.erase(worker.store.printStorePath(i.second.path(worker.store, drv->name)));
 
 #elif __APPLE__
         /* We don't really have any parent prep work to do (yet?)
@@ -2613,7 +2613,7 @@ void DerivationGoal::writeStructuredAttrs()
     /* Add an "outputs" object containing the output paths. */
     nlohmann::json outputs;
     for (auto & i : drv->outputs)
-        outputs[i.first] = rewriteStrings(worker.store.printStorePath(i.second.path), inputRewrites);
+        outputs[i.first] = rewriteStrings(worker.store.printStorePath(i.second.path(worker.store, drv->name)), inputRewrites);
     json["outputs"] = outputs;
 
     /* Handle exportReferencesGraph. */
@@ -2821,7 +2821,7 @@ struct RestrictedStore : public LocalFSStore
                 auto drv = derivationFromPath(path.path);
                 for (auto & output : drv.outputs)
                     if (wantOutput(output.first, path.outputs))
-                        newPaths.insert(output.second.path);
+                        newPaths.insert(output.second.path(*this, drv.name));
             } else if (!goal.isAllowed(path.path))
                 throw InvalidPath("cannot build unknown path '%s' in recursive Nix", printStorePath(path.path));
         }
@@ -2855,7 +2855,7 @@ struct RestrictedStore : public LocalFSStore
 
     void queryMissing(const std::vector<StorePathWithOutputs> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-        unsigned long long & downloadSize, unsigned long long & narSize) override
+        uint64_t & downloadSize, uint64_t & narSize) override
     {
         /* This is slightly impure since it leaks information to the
            client about what paths will be built/substituted or are
@@ -3583,7 +3583,7 @@ StorePathSet parseReferenceSpecifiers(Store & store, const BasicDerivation & drv
         if (store.isStorePath(i))
             result.insert(store.parseStorePath(i));
         else if (drv.outputs.count(i))
-            result.insert(drv.outputs.find(i)->second.path);
+            result.insert(drv.outputs.find(i)->second.path(store, drv.name));
         else throw BuildError("derivation contains an illegal reference specifier '%s'", i);
     }
     return result;
@@ -3621,7 +3621,7 @@ void DerivationGoal::registerOutputs()
     if (hook) {
         bool allValid = true;
         for (auto & i : drv->outputs)
-            if (!worker.store.isValidPath(i.second.path)) allValid = false;
+            if (!worker.store.isValidPath(i.second.path(worker.store, drv->name))) allValid = false;
         if (allValid) return;
     }
 
@@ -3642,23 +3642,23 @@ void DerivationGoal::registerOutputs()
        Nix calls. */
     StorePathSet referenceablePaths;
     for (auto & p : inputPaths) referenceablePaths.insert(p);
-    for (auto & i : drv->outputs) referenceablePaths.insert(i.second.path);
+    for (auto & i : drv->outputs) referenceablePaths.insert(i.second.path(worker.store, drv->name));
     for (auto & p : addedPaths) referenceablePaths.insert(p);
 
     /* Check whether the output paths were created, and grep each
        output path to determine what other paths it references.  Also make all
        output paths read-only. */
     for (auto & i : drv->outputs) {
-        auto path = worker.store.printStorePath(i.second.path);
-        if (!missingPaths.count(i.second.path)) continue;
+        auto path = worker.store.printStorePath(i.second.path(worker.store, drv->name));
+        if (!missingPaths.count(i.second.path(worker.store, drv->name))) continue;
 
         Path actualPath = path;
         if (needsHashRewrite()) {
-            auto r = redirectedOutputs.find(i.second.path);
+            auto r = redirectedOutputs.find(i.second.path(worker.store, drv->name));
             if (r != redirectedOutputs.end()) {
                 auto redirected = worker.store.Store::toRealPath(r->second);
                 if (buildMode == bmRepair
-                    && redirectedBadOutputs.count(i.second.path)
+                    && redirectedBadOutputs.count(i.second.path(worker.store, drv->name))
                     && pathExists(redirected))
                     replaceValidPath(path, redirected);
                 if (buildMode == bmCheck)
@@ -3727,7 +3727,9 @@ void DerivationGoal::registerOutputs()
 
         if (fixedOutput) {
 
-            if (i.second.hash->method == FileIngestionMethod::Flat) {
+            FixedOutputHash outputHash = std::get<DerivationOutputFixed>(i.second.output).hash;
+
+            if (outputHash.method == FileIngestionMethod::Flat) {
                 /* The output path should be a regular file without execute permission. */
                 if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0)
                     throw BuildError(
@@ -3738,13 +3740,13 @@ void DerivationGoal::registerOutputs()
 
             /* Check the hash. In hash mode, move the path produced by
                the derivation to its content-addressed location. */
-            Hash h2 = i.second.hash->method == FileIngestionMethod::Recursive
-                ? hashPath(*i.second.hash->hash.type, actualPath).first
-                : hashFile(*i.second.hash->hash.type, actualPath);
+            Hash h2 = outputHash.method == FileIngestionMethod::Recursive
+                ? hashPath(outputHash.hash.type, actualPath).first
+                : hashFile(outputHash.hash.type, actualPath);
 
-            auto dest = worker.store.makeFixedOutputPath(i.second.hash->method, h2, i.second.path.name());
+            auto dest = worker.store.makeFixedOutputPath(outputHash.method, h2, i.second.path(worker.store, drv->name).name());
 
-            if (i.second.hash->hash != h2) {
+            if (outputHash.hash != h2) {
 
                 /* Throw an error after registering the path as
                    valid. */
@@ -3752,7 +3754,7 @@ void DerivationGoal::registerOutputs()
                 delayedException = std::make_exception_ptr(
                     BuildError("hash mismatch in fixed-output derivation '%s':\n  wanted: %s\n  got:    %s",
                         worker.store.printStorePath(dest),
-                        i.second.hash->hash.to_string(SRI, true),
+                        outputHash.hash.to_string(SRI, true),
                         h2.to_string(SRI, true)));
 
                 Path actualDest = worker.store.Store::toRealPath(dest);
@@ -3774,7 +3776,7 @@ void DerivationGoal::registerOutputs()
                 assert(worker.store.parseStorePath(path) == dest);
 
             ca = FixedOutputHash {
-                .method = i.second.hash->method,
+                .method = outputHash.method,
                 .hash = h2,
             };
         }
@@ -3789,8 +3791,10 @@ void DerivationGoal::registerOutputs()
            time.  The hash is stored in the database so that we can
            verify later on whether nobody has messed with the store. */
         debug("scanning for references inside '%1%'", path);
-        HashResult hash;
-        auto references = worker.store.parseStorePathSet(scanForReferences(actualPath, worker.store.printStorePathSet(referenceablePaths), hash));
+        // HashResult hash;
+        auto pathSetAndHash = scanForReferences(actualPath, worker.store.printStorePathSet(referenceablePaths));
+        auto references = worker.store.parseStorePathSet(pathSetAndHash.first);
+        HashResult hash = pathSetAndHash.second;
 
         if (buildMode == bmCheck) {
             if (!worker.store.isValidPath(worker.store.parseStorePath(path))) continue;
@@ -3898,7 +3902,7 @@ void DerivationGoal::registerOutputs()
     /* If this is the first round of several, then move the output out of the way. */
     if (nrRounds > 1 && curRound == 1 && curRound < nrRounds && keepPreviousRound) {
         for (auto & i : drv->outputs) {
-            auto path = worker.store.printStorePath(i.second.path);
+            auto path = worker.store.printStorePath(i.second.path(worker.store, drv->name));
             Path prev = path + checkSuffix;
             deletePath(prev);
             Path dst = path + checkSuffix;
@@ -3916,7 +3920,7 @@ void DerivationGoal::registerOutputs()
        if the result was not determistic? */
     if (curRound == nrRounds) {
         for (auto & i : drv->outputs) {
-            Path prev = worker.store.printStorePath(i.second.path) + checkSuffix;
+            Path prev = worker.store.printStorePath(i.second.path(worker.store, drv->name)) + checkSuffix;
             deletePath(prev);
         }
     }
@@ -4217,9 +4221,9 @@ StorePathSet DerivationGoal::checkPathValidity(bool returnValid, bool checkHash)
     for (auto & i : drv->outputs) {
         if (!wantOutput(i.first, wantedOutputs)) continue;
         bool good =
-            worker.store.isValidPath(i.second.path) &&
-            (!checkHash || worker.pathContentsGood(i.second.path));
-        if (good == returnValid) result.insert(i.second.path);
+            worker.store.isValidPath(i.second.path(worker.store, drv->name)) &&
+            (!checkHash || worker.pathContentsGood(i.second.path(worker.store, drv->name)));
+        if (good == returnValid) result.insert(i.second.path(worker.store, drv->name));
     }
     return result;
 }
@@ -4276,6 +4280,10 @@ private:
     /* The store path that should be realised through a substitute. */
     StorePath storePath;
 
+    /* The path the substituter refers to the path as. This will be
+     * different when the stores have different names. */
+    std::optional<StorePath> subPath;
+
     /* The remaining substituters. */
     std::list<ref<Store>> subs;
 
@@ -4309,8 +4317,11 @@ private:
     typedef void (SubstitutionGoal::*GoalState)();
     GoalState state;
 
+    /* Content address for recomputing store path */
+    std::optional<ContentAddress> ca;
+
 public:
-    SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair = NoRepair);
+    SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
     ~SubstitutionGoal();
 
     void timedOut(Error && ex) override { abort(); };
@@ -4340,10 +4351,11 @@ public:
 };
 
 
-SubstitutionGoal::SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair)
+SubstitutionGoal::SubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair, std::optional<ContentAddress> ca)
     : Goal(worker)
     , storePath(storePath)
     , repair(repair)
+    , ca(ca)
 {
     state = &SubstitutionGoal::init;
     name = fmt("substitution of '%s'", worker.store.printStorePath(this->storePath));
@@ -4418,14 +4430,18 @@ void SubstitutionGoal::tryNext()
     sub = subs.front();
     subs.pop_front();
 
-    if (sub->storeDir != worker.store.storeDir) {
+    if (ca) {
+        subPath = sub->makeFixedOutputPathFromCA(storePath.name(), *ca);
+        if (sub->storeDir == worker.store.storeDir)
+            assert(subPath == storePath);
+    } else if (sub->storeDir != worker.store.storeDir) {
         tryNext();
         return;
     }
 
     try {
         // FIXME: make async
-        info = sub->queryPathInfo(storePath);
+        info = sub->queryPathInfo(subPath ? *subPath : storePath);
     } catch (InvalidPath &) {
         tryNext();
         return;
@@ -4444,6 +4460,19 @@ void SubstitutionGoal::tryNext()
         throw;
     }
 
+    if (info->path != storePath) {
+        if (info->isContentAddressed(*sub) && info->references.empty()) {
+            auto info2 = std::make_shared<ValidPathInfo>(*info);
+            info2->path = storePath;
+            info = info2;
+        } else {
+            printError("asked '%s' for '%s' but got '%s'",
+                sub->getUri(), worker.store.printStorePath(storePath), sub->printStorePath(info->path));
+            tryNext();
+            return;
+        }
+    }
+
     /* Update the total expected download size. */
     auto narInfo = std::dynamic_pointer_cast<const NarInfo>(info);
 
@@ -4533,7 +4562,7 @@ void SubstitutionGoal::tryToRun()
             PushActivity pact(act.id);
 
             copyStorePath(ref<Store>(sub), ref<Store>(worker.store.shared_from_this()),
-                storePath, repair, sub->isTrusted ? NoCheckSigs : CheckSigs);
+                subPath ? *subPath : storePath, repair, sub->isTrusted ? NoCheckSigs : CheckSigs);
 
             promise.set_value();
         } catch (...) {
@@ -4666,11 +4695,11 @@ std::shared_ptr<DerivationGoal> Worker::makeBasicDerivationGoal(const StorePath
 }
 
 
-GoalPtr Worker::makeSubstitutionGoal(const StorePath & path, RepairFlag repair)
+GoalPtr Worker::makeSubstitutionGoal(const StorePath & path, RepairFlag repair, std::optional<ContentAddress> ca)
 {
     GoalPtr goal = substitutionGoals[path].lock(); // FIXME
     if (!goal) {
-        goal = std::make_shared<SubstitutionGoal>(path, *this, repair);
+        goal = std::make_shared<SubstitutionGoal>(path, *this, repair, ca);
         substitutionGoals.insert_or_assign(path, goal);
         wakeUp(goal);
     }
@@ -5012,7 +5041,7 @@ bool Worker::pathContentsGood(const StorePath & path)
     if (!pathExists(store.printStorePath(path)))
         res = false;
     else {
-        HashResult current = hashPath(*info->narHash.type, store.printStorePath(path));
+        HashResult current = hashPath(info->narHash->type, store.printStorePath(path));
         Hash nullHash(htSHA256);
         res = info->narHash == nullHash || info->narHash == current.first;
     }
@@ -5038,7 +5067,7 @@ void Worker::markContentsGood(const StorePath & path)
 static void primeCache(Store & store, const std::vector<StorePathWithOutputs> & paths)
 {
     StorePathSet willBuild, willSubstitute, unknown;
-    unsigned long long downloadSize, narSize;
+    uint64_t downloadSize, narSize;
     store.queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
 
     if (!willBuild.empty() && 0 == settings.maxBuildJobs && getMachines().empty())
diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index e630cf6f1..6585a480d 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -58,23 +58,6 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         }
     };
 
-    /* We always have one output, and if it's a fixed-output derivation (as
-       checked below) it must be the only output */
-    auto & output = drv.outputs.begin()->second;
-
-    /* Try the hashed mirrors first. */
-    if (output.hash && output.hash->method == FileIngestionMethod::Flat)
-        for (auto hashedMirror : settings.hashedMirrors.get())
-            try {
-                if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
-                auto & h = output.hash->hash;
-                fetch(hashedMirror + printHashType(*h.type) + "/" + h.to_string(Base16, false));
-                return;
-            } catch (Error & e) {
-                debug(e.what());
-            }
-
-    /* Otherwise try the specified URL. */
     fetch(mainUrl);
 }
 
diff --git a/src/libstore/content-address.cc b/src/libstore/content-address.cc
index 6cb69d0a9..f83b98a98 100644
--- a/src/libstore/content-address.cc
+++ b/src/libstore/content-address.cc
@@ -3,7 +3,7 @@
 namespace nix {
 
 std::string FixedOutputHash::printMethodAlgo() const {
-    return makeFileIngestionPrefix(method) + printHashType(*hash.type);
+    return makeFileIngestionPrefix(method) + printHashType(hash.type);
 }
 
 std::string makeFileIngestionPrefix(const FileIngestionMethod m) {
@@ -46,7 +46,7 @@ ContentAddress parseContentAddress(std::string_view rawCa) {
         if (prefix == "text") {
             auto hashTypeAndHash = rawCa.substr(prefixSeparator+1, string::npos);
             Hash hash = Hash(string(hashTypeAndHash));
-            if (*hash.type != htSHA256) {
+            if (hash.type != htSHA256) {
                 throw Error("parseContentAddress: the text hash should have type SHA256");
             }
             return TextHash { hash };
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 82833f5c3..9f7be6e1a 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -86,7 +86,7 @@ struct TunnelLogger : public Logger
     }
 
     /* startWork() means that we're starting an operation for which we
-      want to send out stderr to the client. */
+       want to send out stderr to the client. */
     void startWork()
     {
         auto state(state_.lock());
@@ -289,7 +289,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         auto hash = store->queryPathInfo(path)->narHash;
         logger->stopWork();
-        to << hash.to_string(Base16, false);
+        to << hash->to_string(Base16, false);
         break;
     }
 
@@ -451,7 +451,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     case wopBuildDerivation: {
         auto drvPath = store->parseStorePath(readString(from));
         BasicDerivation drv;
-        readDerivation(from, *store, drv);
+        readDerivation(from, *store, drv, Derivation::nameFromPath(drvPath));
         BuildMode buildMode = (BuildMode) readInt(from);
         logger->startWork();
         if (!trusted)
@@ -579,7 +579,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         auto path = store->parseStorePath(readString(from));
         logger->startWork();
         SubstitutablePathInfos infos;
-        store->querySubstitutablePathInfos({path}, infos);
+        store->querySubstitutablePathInfos({{path, std::nullopt}}, infos);
         logger->stopWork();
         auto i = infos.find(path);
         if (i == infos.end())
@@ -595,10 +595,16 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopQuerySubstitutablePathInfos: {
-        auto paths = readStorePaths<StorePathSet>(*store, from);
-        logger->startWork();
         SubstitutablePathInfos infos;
-        store->querySubstitutablePathInfos(paths, infos);
+        StorePathCAMap pathsMap = {};
+        if (GET_PROTOCOL_MINOR(clientVersion) < 22) {
+            auto paths = readStorePaths<StorePathSet>(*store, from);
+            for (auto & path : paths)
+                pathsMap.emplace(path, std::nullopt);
+        } else
+            pathsMap = readStorePathCAMap(*store, from);
+        logger->startWork();
+        store->querySubstitutablePathInfos(pathsMap, infos);
         logger->stopWork();
         to << infos.size();
         for (auto & i : infos) {
@@ -632,7 +638,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             if (GET_PROTOCOL_MINOR(clientVersion) >= 17)
                 to << 1;
             to << (info->deriver ? store->printStorePath(*info->deriver) : "")
-               << info->narHash.to_string(Base16, false);
+               << info->narHash->to_string(Base16, false);
             writeStorePaths(*store, to, info->references);
             to << info->registrationTime << info->narSize;
             if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
@@ -703,24 +709,84 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (!trusted)
             info.ultimate = false;
 
-        std::unique_ptr<Source> source;
-        if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
-            source = std::make_unique<TunnelSource>(from, to);
-        else {
-            StringSink saved;
-            TeeSource tee { from, saved };
-            ParseSink ether;
-            parseDump(ether, tee);
-            source = std::make_unique<StringSource>(std::move(*saved.s));
+        if (GET_PROTOCOL_MINOR(clientVersion) >= 23) {
+
+            struct FramedSource : Source
+            {
+                Source & from;
+                bool eof = false;
+                std::vector<unsigned char> pending;
+                size_t pos = 0;
+
+                FramedSource(Source & from) : from(from)
+                { }
+
+                ~FramedSource()
+                {
+                    if (!eof) {
+                        while (true) {
+                            auto n = readInt(from);
+                            if (!n) break;
+                            std::vector<unsigned char> data(n);
+                            from(data.data(), n);
+                        }
+                    }
+                }
+
+                size_t read(unsigned char * data, size_t len) override
+                {
+                    if (eof) throw EndOfFile("reached end of FramedSource");
+
+                    if (pos >= pending.size()) {
+                        size_t len = readInt(from);
+                        if (!len) {
+                            eof = true;
+                            return 0;
+                        }
+                        pending = std::vector<unsigned char>(len);
+                        pos = 0;
+                        from(pending.data(), len);
+                    }
+
+                    auto n = std::min(len, pending.size() - pos);
+                    memcpy(data, pending.data() + pos, n);
+                    pos += n;
+                    return n;
+                }
+            };
+
+            logger->startWork();
+
+            {
+                FramedSource source(from);
+                store->addToStore(info, source, (RepairFlag) repair,
+                    dontCheckSigs ? NoCheckSigs : CheckSigs);
+            }
+
+            logger->stopWork();
         }
 
-        logger->startWork();
+        else {
+            std::unique_ptr<Source> source;
+            if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
+                source = std::make_unique<TunnelSource>(from, to);
+            else {
+                StringSink saved;
+                TeeSource tee { from, saved };
+                ParseSink ether;
+                parseDump(ether, tee);
+                source = std::make_unique<StringSource>(std::move(*saved.s));
+            }
 
-        // FIXME: race if addToStore doesn't read source?
-        store->addToStore(info, *source, (RepairFlag) repair,
-            dontCheckSigs ? NoCheckSigs : CheckSigs);
+            logger->startWork();
+
+            // FIXME: race if addToStore doesn't read source?
+            store->addToStore(info, *source, (RepairFlag) repair,
+                dontCheckSigs ? NoCheckSigs : CheckSigs);
+
+            logger->stopWork();
+        }
 
-        logger->stopWork();
         break;
     }
 
@@ -730,7 +796,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             targets.push_back(store->parsePathWithOutputs(s));
         logger->startWork();
         StorePathSet willBuild, willSubstitute, unknown;
-        unsigned long long downloadSize, narSize;
+        uint64_t downloadSize, narSize;
         store->queryMissing(targets, willBuild, willSubstitute, unknown, downloadSize, narSize);
         logger->stopWork();
         writeStorePaths(*store, to, willBuild);
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 870901421..7d0a5abeb 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -7,6 +7,22 @@
 
 namespace nix {
 
+// FIXME Put this somewhere?
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
+
+StorePath DerivationOutput::path(const Store & store, std::string_view drvName) const
+{
+    return std::visit(overloaded {
+        [](DerivationOutputInputAddressed doi) {
+            return doi.path;
+        },
+        [&](DerivationOutputFixed dof) {
+            return store.makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
+        }
+    }, output);
+}
+
 
 bool BasicDerivation::isBuiltin() const
 {
@@ -99,7 +115,6 @@ static DerivationOutput parseDerivationOutput(const Store & store, std::istrings
     expect(str, ","); const auto hash = parseString(str);
     expect(str, ")");
 
-    std::optional<FixedOutputHash> fsh;
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
@@ -107,22 +122,29 @@ static DerivationOutput parseDerivationOutput(const Store & store, std::istrings
             hashAlgo = string(hashAlgo, 2);
         }
         const HashType hashType = parseHashType(hashAlgo);
-        fsh = FixedOutputHash {
-            .method = std::move(method),
-            .hash = Hash(hash, hashType),
-        };
-    }
 
-    return DerivationOutput {
-        .path = std::move(path),
-        .hash = std::move(fsh),
-    };
+        return DerivationOutput {
+            .output = DerivationOutputFixed {
+                .hash = FixedOutputHash {
+                    .method = std::move(method),
+                    .hash = Hash(hash, hashType),
+                },
+            }
+        };
+    } else
+        return DerivationOutput {
+            .output = DerivationOutputInputAddressed {
+                .path = std::move(path),
+            }
+        };
 }
 
 
-static Derivation parseDerivation(const Store & store, std::string && s)
+static Derivation parseDerivation(const Store & store, std::string && s, std::string_view name)
 {
     Derivation drv;
+    drv.name = name;
+
     std::istringstream str(std::move(s));
     expect(str, "Derive([");
 
@@ -166,10 +188,10 @@ static Derivation parseDerivation(const Store & store, std::string && s)
 }
 
 
-Derivation readDerivation(const Store & store, const Path & drvPath)
+Derivation readDerivation(const Store & store, const Path & drvPath, std::string_view name)
 {
     try {
-        return parseDerivation(store, readFile(drvPath));
+        return parseDerivation(store, readFile(drvPath), name);
     } catch (FormatError & e) {
         throw Error("error parsing derivation '%1%': %2%", drvPath, e.msg());
     }
@@ -187,7 +209,7 @@ Derivation Store::readDerivation(const StorePath & drvPath)
 {
     auto accessor = getFSAccessor();
     try {
-        return parseDerivation(*this, accessor->readFile(printStorePath(drvPath)));
+        return parseDerivation(*this, accessor->readFile(printStorePath(drvPath)), Derivation::nameFromPath(drvPath));
     } catch (FormatError & e) {
         throw Error("error parsing derivation '%s': %s", printStorePath(drvPath), e.msg());
     }
@@ -255,10 +277,14 @@ string Derivation::unparse(const Store & store, bool maskOutputs,
     for (auto & i : outputs) {
         if (first) first = false; else s += ',';
         s += '('; printUnquotedString(s, i.first);
-        s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(i.second.path));
-        s += ','; printUnquotedString(s, i.second.hash ? i.second.hash->printMethodAlgo() : "");
-        s += ','; printUnquotedString(s,
-            i.second.hash ? i.second.hash->hash.to_string(Base16, false) : "");
+        s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(i.second.path(store, name)));
+        if (auto hash = std::get_if<DerivationOutputFixed>(&i.second.output)) {
+            s += ','; printUnquotedString(s, hash->hash.printMethodAlgo());
+            s += ','; printUnquotedString(s, hash->hash.hash.to_string(Base16, false));
+        } else {
+            s += ','; printUnquotedString(s, "");
+            s += ','; printUnquotedString(s, "");
+        }
         s += ')';
     }
 
@@ -314,7 +340,7 @@ bool BasicDerivation::isFixedOutput() const
 {
     return outputs.size() == 1 &&
         outputs.begin()->first == "out" &&
-        outputs.begin()->second.hash;
+        std::holds_alternative<DerivationOutputFixed>(outputs.begin()->second.output);
 }
 
 
@@ -346,10 +372,11 @@ Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutput
     /* Return a fixed hash for fixed-output derivations. */
     if (drv.isFixedOutput()) {
         DerivationOutputs::const_iterator i = drv.outputs.begin();
+        auto hash = std::get<DerivationOutputFixed>(i->second.output);
         return hashString(htSHA256, "fixed:out:"
-            + i->second.hash->printMethodAlgo() + ":"
-            + i->second.hash->hash.to_string(Base16, false) + ":"
-            + store.printStorePath(i->second.path));
+            + hash.hash.printMethodAlgo() + ":"
+            + hash.hash.hash.to_string(Base16, false) + ":"
+            + store.printStorePath(i->second.path(store, drv.name)));
     }
 
     /* For other derivations, replace the inputs paths with recursive
@@ -383,11 +410,11 @@ bool wantOutput(const string & output, const std::set<string> & wanted)
 }
 
 
-StorePathSet BasicDerivation::outputPaths() const
+StorePathSet BasicDerivation::outputPaths(const Store & store) const
 {
     StorePathSet paths;
     for (auto & i : outputs)
-        paths.insert(i.second.path);
+        paths.insert(i.second.path(store, name));
     return paths;
 }
 
@@ -397,7 +424,6 @@ static DerivationOutput readDerivationOutput(Source & in, const Store & store)
     auto hashAlgo = readString(in);
     auto hash = readString(in);
 
-    std::optional<FixedOutputHash> fsh;
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
@@ -405,16 +431,20 @@ static DerivationOutput readDerivationOutput(Source & in, const Store & store)
             hashAlgo = string(hashAlgo, 2);
         }
         auto hashType = parseHashType(hashAlgo);
-        fsh = FixedOutputHash {
-            .method = std::move(method),
-            .hash = Hash(hash, hashType),
+        return DerivationOutput {
+            .output = DerivationOutputFixed {
+                .hash = FixedOutputHash {
+                    .method = std::move(method),
+                    .hash = Hash(hash, hashType),
+                },
+            }
+        };
+    } else
+        return DerivationOutput {
+            .output = DerivationOutputInputAddressed {
+                .path = std::move(path),
+            }
         };
-    }
-
-    return DerivationOutput {
-        .path = std::move(path),
-        .hash = std::move(fsh),
-    };
 }
 
 StringSet BasicDerivation::outputNames() const
@@ -426,8 +456,19 @@ StringSet BasicDerivation::outputNames() const
 }
 
 
-Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv)
+std::string_view BasicDerivation::nameFromPath(const StorePath & drvPath) {
+    auto nameWithSuffix = drvPath.name();
+    constexpr std::string_view extension = ".drv";
+    assert(hasSuffix(nameWithSuffix, extension));
+    nameWithSuffix.remove_suffix(extension.size());
+    return nameWithSuffix;
+}
+
+
+Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv, std::string_view name)
 {
+    drv.name = name;
+
     drv.outputs.clear();
     auto nr = readNum<size_t>(in);
     for (size_t n = 0; n < nr; n++) {
@@ -456,10 +497,10 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
     out << drv.outputs.size();
     for (auto & i : drv.outputs) {
         out << i.first
-            << store.printStorePath(i.second.path);
-        if (i.second.hash) {
-            out << i.second.hash->printMethodAlgo()
-                << i.second.hash->hash.to_string(Base16, false);
+            << store.printStorePath(i.second.path(store, drv.name));
+        if (auto hash = std::get_if<DerivationOutputFixed>(&i.second.output)) {
+            out << hash->hash.printMethodAlgo()
+                << hash->hash.hash.to_string(Base16, false);
         } else {
             out << "" << "";
         }
diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index 00cf1bb08..133ffe50e 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -13,10 +13,20 @@ namespace nix {
 
 /* Abstract syntax of derivations. */
 
-struct DerivationOutput
+struct DerivationOutputInputAddressed
 {
     StorePath path;
-    std::optional<FixedOutputHash> hash; /* hash used for expected hash computation */
+};
+
+struct DerivationOutputFixed
+{
+    FixedOutputHash hash; /* hash used for expected hash computation */
+};
+
+struct DerivationOutput
+{
+    std::variant<DerivationOutputInputAddressed, DerivationOutputFixed> output;
+    StorePath path(const Store & store, std::string_view drvName) const;
 };
 
 typedef std::map<string, DerivationOutput> DerivationOutputs;
@@ -35,6 +45,7 @@ struct BasicDerivation
     Path builder;
     Strings args;
     StringPairs env;
+    std::string name;
 
     BasicDerivation() { }
     virtual ~BasicDerivation() { };
@@ -45,10 +56,12 @@ struct BasicDerivation
     bool isFixedOutput() const;
 
     /* Return the output paths of a derivation. */
-    StorePathSet outputPaths() const;
+    StorePathSet outputPaths(const Store & store) const;
 
     /* Return the output names of a derivation. */
     StringSet outputNames() const;
+
+    static std::string_view nameFromPath(const StorePath & storePath);
 };
 
 struct Derivation : BasicDerivation
@@ -72,7 +85,7 @@ StorePath writeDerivation(ref<Store> store,
     const Derivation & drv, std::string_view name, RepairFlag repair = NoRepair);
 
 /* Read a derivation from a file. */
-Derivation readDerivation(const Store & store, const Path & drvPath);
+Derivation readDerivation(const Store & store, const Path & drvPath, std::string_view name);
 
 // FIXME: remove
 bool isDerivation(const string & fileName);
@@ -89,7 +102,7 @@ bool wantOutput(const string & output, const std::set<string> & wanted);
 struct Source;
 struct Sink;
 
-Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv);
+Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv, std::string_view name);
 void writeDerivation(Sink & out, const Store & store, const BasicDerivation & drv);
 
 std::string hashPlaceholder(const std::string & outputName);
diff --git a/src/libstore/export-import.cc b/src/libstore/export-import.cc
index b963d64d7..a0fc22264 100644
--- a/src/libstore/export-import.cc
+++ b/src/libstore/export-import.cc
@@ -38,9 +38,9 @@ void Store::exportPath(const StorePath & path, Sink & sink)
        filesystem corruption from spreading to other machines.
        Don't complain if the stored hash is zero (unknown). */
     Hash hash = hashSink.currentHash().first;
-    if (hash != info->narHash && info->narHash != Hash(*info->narHash.type))
+    if (hash != info->narHash && info->narHash != Hash(info->narHash->type))
         throw Error("hash of path '%s' has changed from '%s' to '%s'!",
-            printStorePath(path), info->narHash.to_string(Base32, true), hash.to_string(Base32, true));
+            printStorePath(path), info->narHash->to_string(Base32, true), hash.to_string(Base32, true));
 
     teeSink
         << exportMagic
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index aaed5c218..e74382ed2 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -500,7 +500,7 @@ struct LocalStore::GCState
     StorePathSet alive;
     bool gcKeepOutputs;
     bool gcKeepDerivations;
-    unsigned long long bytesInvalidated;
+    uint64_t bytesInvalidated;
     bool moveToTrash = true;
     bool shouldDelete;
     GCState(const GCOptions & options, GCResults & results)
@@ -518,7 +518,7 @@ bool LocalStore::isActiveTempFile(const GCState & state,
 
 void LocalStore::deleteGarbage(GCState & state, const Path & path)
 {
-    unsigned long long bytesFreed;
+    uint64_t bytesFreed;
     deletePath(path, bytesFreed);
     state.results.bytesFreed += bytesFreed;
 }
@@ -528,7 +528,7 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
 {
     checkInterrupt();
 
-    unsigned long long size = 0;
+    uint64_t size = 0;
 
     auto storePath = maybeParseStorePath(path);
     if (storePath && isValidPath(*storePath)) {
@@ -687,7 +687,7 @@ void LocalStore::removeUnusedLinks(const GCState & state)
     AutoCloseDir dir(opendir(linksDir.c_str()));
     if (!dir) throw SysError("opening directory '%1%'", linksDir);
 
-    long long actualSize = 0, unsharedSize = 0;
+    int64_t actualSize = 0, unsharedSize = 0;
 
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir.get())) {
@@ -717,10 +717,10 @@ void LocalStore::removeUnusedLinks(const GCState & state)
     struct stat st;
     if (stat(linksDir.c_str(), &st) == -1)
         throw SysError("statting '%1%'", linksDir);
-    long long overhead = st.st_blocks * 512ULL;
+    auto overhead = st.st_blocks * 512ULL;
 
-    printInfo(format("note: currently hard linking saves %.2f MiB")
-        % ((unsharedSize - actualSize - overhead) / (1024.0 * 1024.0)));
+    printInfo("note: currently hard linking saves %.2f MiB",
+        ((unsharedSize - actualSize - overhead) / (1024.0 * 1024.0)));
 }
 
 
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index d47e0b6b5..3406a9331 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -335,9 +335,6 @@ public:
         "setuid/setgid bits or with file capabilities."};
 #endif
 
-    Setting<Strings> hashedMirrors{this, {"http://tarballs.nixos.org/"}, "hashed-mirrors",
-        "A list of servers used by builtins.fetchurl to fetch files by hash."};
-
     Setting<uint64_t> minFree{this, 0, "min-free",
         "Automatically run the garbage collector when free disk space drops below the specified amount."};
 
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index a8bd8a972..5d7566121 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -113,7 +113,7 @@ struct LegacySSHStore : public Store
 
             if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4) {
                 auto s = readString(conn->from);
-                info->narHash = s.empty() ? Hash() : Hash(s);
+                info->narHash = s.empty() ? std::optional<Hash>{} : Hash{s};
                 info->ca = parseContentAddressOpt(readString(conn->from));
                 info->sigs = readStrings<StringSet>(conn->from);
             }
@@ -138,7 +138,7 @@ struct LegacySSHStore : public Store
                 << cmdAddToStoreNar
                 << printStorePath(info.path)
                 << (info.deriver ? printStorePath(*info.deriver) : "")
-                << info.narHash.to_string(Base16, false);
+                << info.narHash->to_string(Base16, false);
             writeStorePaths(*this, conn->to, info.references);
             conn->to
                 << info.registrationTime
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 4db4f7d6a..e2cd3c0a5 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -560,19 +560,12 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
         DerivationOutputs::const_iterator out = drv.outputs.find("out");
         if (out == drv.outputs.end())
             throw Error("derivation '%s' does not have an output named 'out'", printStorePath(drvPath));
-
-        check(
-            makeFixedOutputPath(
-                out->second.hash->method,
-                out->second.hash->hash,
-                drvName),
-            out->second.path, "out");
     }
 
     else {
         Hash h = hashDerivationModulo(*this, drv, true);
         for (auto & i : drv.outputs)
-            check(makeOutputPath(i.first, h, drvName), i.second.path, i.first);
+            check(makeOutputPath(i.first, h, drvName), i.second.path(*this, drv.name), i.first);
     }
 }
 
@@ -586,7 +579,7 @@ uint64_t LocalStore::addValidPath(State & state,
 
     state.stmtRegisterValidPath.use()
         (printStorePath(info.path))
-        (info.narHash.to_string(Base16, true))
+        (info.narHash->to_string(Base16, true))
         (info.registrationTime == 0 ? time(0) : info.registrationTime)
         (info.deriver ? printStorePath(*info.deriver) : "", (bool) info.deriver)
         (info.narSize, info.narSize != 0)
@@ -614,7 +607,7 @@ uint64_t LocalStore::addValidPath(State & state,
             state.stmtAddDerivationOutput.use()
                 (id)
                 (i.first)
-                (printStorePath(i.second.path))
+                (printStorePath(i.second.path(*this, drv.name)))
                 .exec();
         }
     }
@@ -686,7 +679,7 @@ void LocalStore::updatePathInfo(State & state, const ValidPathInfo & info)
 {
     state.stmtUpdatePathInfo.use()
         (info.narSize, info.narSize != 0)
-        (info.narHash.to_string(Base16, true))
+        (info.narHash->to_string(Base16, true))
         (info.ultimate ? 1 : 0, info.ultimate)
         (concatStringsSep(" ", info.sigs), !info.sigs.empty())
         (renderContentAddress(info.ca), (bool) info.ca)
@@ -850,20 +843,32 @@ StorePathSet LocalStore::querySubstitutablePaths(const StorePathSet & paths)
 }
 
 
-void LocalStore::querySubstitutablePathInfos(const StorePathSet & paths,
-    SubstitutablePathInfos & infos)
+void LocalStore::querySubstitutablePathInfos(const StorePathCAMap & paths, SubstitutablePathInfos & infos)
 {
     if (!settings.useSubstitutes) return;
     for (auto & sub : getDefaultSubstituters()) {
-        if (sub->storeDir != storeDir) continue;
         for (auto & path : paths) {
-            if (infos.count(path)) continue;
-            debug("checking substituter '%s' for path '%s'", sub->getUri(), printStorePath(path));
+            auto subPath(path.first);
+
+            // recompute store path so that we can use a different store root
+            if (path.second) {
+                subPath = makeFixedOutputPathFromCA(path.first.name(), *path.second);
+                if (sub->storeDir == storeDir)
+                    assert(subPath == path.first);
+                if (subPath != path.first)
+                    debug("replaced path '%s' with '%s' for substituter '%s'", printStorePath(path.first), sub->printStorePath(subPath), sub->getUri());
+            } else if (sub->storeDir != storeDir) continue;
+
+            debug("checking substituter '%s' for path '%s'", sub->getUri(), sub->printStorePath(subPath));
             try {
-                auto info = sub->queryPathInfo(path);
+                auto info = sub->queryPathInfo(subPath);
+
+                if (sub->storeDir != storeDir && !(info->isContentAddressed(*sub) && info->references.empty()))
+                    continue;
+
                 auto narInfo = std::dynamic_pointer_cast<const NarInfo>(
                     std::shared_ptr<const ValidPathInfo>(info));
-                infos.insert_or_assign(path, SubstitutablePathInfo{
+                infos.insert_or_assign(path.first, SubstitutablePathInfo{
                     info->deriver,
                     info->references,
                     narInfo ? narInfo->fileSize : 0,
@@ -904,7 +909,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
         StorePathSet paths;
 
         for (auto & i : infos) {
-            assert(i.narHash.type == htSHA256);
+            assert(i.narHash && i.narHash->type == htSHA256);
             if (isValidPath_(*state, i.path))
                 updatePathInfo(*state, i);
             else
@@ -1017,7 +1022,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 
             if (hashResult.first != info.narHash)
                 throw Error("hash mismatch importing path '%s';\n  wanted: %s\n  got:    %s",
-                    printStorePath(info.path), info.narHash.to_string(Base32, true), hashResult.first.to_string(Base32, true));
+                    printStorePath(info.path), info.narHash->to_string(Base32, true), hashResult.first.to_string(Base32, true));
 
             if (hashResult.second != info.narSize)
                 throw Error("size mismatch importing path '%s';\n  wanted: %s\n  got:   %s",
@@ -1313,9 +1318,9 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 
                 std::unique_ptr<AbstractHashSink> hashSink;
                 if (!info->ca || !info->references.count(info->path))
-                    hashSink = std::make_unique<HashSink>(*info->narHash.type);
+                    hashSink = std::make_unique<HashSink>(info->narHash->type);
                 else
-                    hashSink = std::make_unique<HashModuloSink>(*info->narHash.type, std::string(info->path.hashPart()));
+                    hashSink = std::make_unique<HashModuloSink>(info->narHash->type, std::string(info->path.hashPart()));
 
                 dumpPath(Store::toRealPath(i), *hashSink);
                 auto current = hashSink->finish();
@@ -1324,7 +1329,7 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                     logError({
                         .name = "Invalid hash - path modified",
                         .hint = hintfmt("path '%s' was modified! expected hash '%s', got '%s'",
-                        printStorePath(i), info->narHash.to_string(Base32, true), current.first.to_string(Base32, true))
+                        printStorePath(i), info->narHash->to_string(Base32, true), current.first.to_string(Base32, true))
                     });
                     if (repair) repairPath(i); else errors = true;
                 } else {
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 56ef1d251..285f46658 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -29,8 +29,8 @@ struct Derivation;
 struct OptimiseStats
 {
     unsigned long filesLinked = 0;
-    unsigned long long bytesFreed = 0;
-    unsigned long long blocksFreed = 0;
+    uint64_t bytesFreed = 0;
+    uint64_t blocksFreed = 0;
 };
 
 
@@ -139,7 +139,7 @@ public:
 
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
 
-    void querySubstitutablePathInfos(const StorePathSet & paths,
+    void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
     void addToStore(const ValidPathInfo & info, Source & source,
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index e68edb38c..7f1b62f26 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -108,9 +108,19 @@ void Store::computeFSClosure(const StorePath & startPath,
 }
 
 
+std::optional<ContentAddress> getDerivationCA(const BasicDerivation & drv)
+{
+    auto out = drv.outputs.find("out");
+    if (out != drv.outputs.end()) {
+        if (auto v = std::get_if<DerivationOutputFixed>(&out->second.output))
+            return v->hash;
+    }
+    return std::nullopt;
+}
+
 void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
     StorePathSet & willBuild_, StorePathSet & willSubstitute_, StorePathSet & unknown_,
-    unsigned long long & downloadSize_, unsigned long long & narSize_)
+    uint64_t & downloadSize_, uint64_t & narSize_)
 {
     Activity act(*logger, lvlDebug, actUnknown, "querying info about missing paths");
 
@@ -122,8 +132,8 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
     {
         std::unordered_set<std::string> done;
         StorePathSet & unknown, & willSubstitute, & willBuild;
-        unsigned long long & downloadSize;
-        unsigned long long & narSize;
+        uint64_t & downloadSize;
+        uint64_t & narSize;
     };
 
     struct DrvState
@@ -157,7 +167,7 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
         auto outPath = parseStorePath(outPathS);
 
         SubstitutablePathInfos infos;
-        querySubstitutablePathInfos({outPath}, infos);
+        querySubstitutablePathInfos({{outPath, getDerivationCA(*drv)}}, infos);
 
         if (infos.empty()) {
             drvState_->lock()->done = true;
@@ -198,8 +208,8 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
             PathSet invalid;
             for (auto & j : drv->outputs)
                 if (wantOutput(j.first, path.outputs)
-                    && !isValidPath(j.second.path))
-                    invalid.insert(printStorePath(j.second.path));
+                    && !isValidPath(j.second.path(*this, drv->name)))
+                    invalid.insert(printStorePath(j.second.path(*this, drv->name)));
             if (invalid.empty()) return;
 
             if (settings.useSubstitutes && parsedDrv.substitutesAllowed()) {
@@ -214,7 +224,7 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
             if (isValidPath(path.path)) return;
 
             SubstitutablePathInfos infos;
-            querySubstitutablePathInfos({path.path}, infos);
+            querySubstitutablePathInfos({{path.path, std::nullopt}}, infos);
 
             if (infos.empty()) {
                 auto state(state_.lock());
diff --git a/src/libstore/nar-accessor.cc b/src/libstore/nar-accessor.cc
index d884a131e..59ec164b6 100644
--- a/src/libstore/nar-accessor.cc
+++ b/src/libstore/nar-accessor.cc
@@ -79,14 +79,14 @@ struct NarAccessor : public FSAccessor
             parents.top()->isExecutable = true;
         }
 
-        void preallocateContents(unsigned long long size) override
+        void preallocateContents(uint64_t size) override
         {
             assert(size <= std::numeric_limits<uint64_t>::max());
             parents.top()->size = (uint64_t) size;
             parents.top()->start = pos;
         }
 
-        void receiveContents(unsigned char * data, unsigned int len) override
+        void receiveContents(unsigned char * data, size_t len) override
         { }
 
         void createSymlink(const Path & path, const string & target) override
diff --git a/src/libstore/nar-info-disk-cache.cc b/src/libstore/nar-info-disk-cache.cc
index 012dea6ea..9ddb9957f 100644
--- a/src/libstore/nar-info-disk-cache.cc
+++ b/src/libstore/nar-info-disk-cache.cc
@@ -230,9 +230,9 @@ public:
                     (std::string(info->path.name()))
                     (narInfo ? narInfo->url : "", narInfo != 0)
                     (narInfo ? narInfo->compression : "", narInfo != 0)
-                    (narInfo && narInfo->fileHash ? narInfo->fileHash.to_string(Base32, true) : "", narInfo && narInfo->fileHash)
+                    (narInfo && narInfo->fileHash ? narInfo->fileHash->to_string(Base32, true) : "", narInfo && narInfo->fileHash)
                     (narInfo ? narInfo->fileSize : 0, narInfo != 0 && narInfo->fileSize)
-                    (info->narHash.to_string(Base32, true))
+                    (info->narHash->to_string(Base32, true))
                     (info->narSize)
                     (concatStringsSep(" ", info->shortRefs()))
                     (info->deriver ? std::string(info->deriver->to_string()) : "", (bool) info->deriver)
diff --git a/src/libstore/nar-info.cc b/src/libstore/nar-info.cc
index 04550ed97..ca471463c 100644
--- a/src/libstore/nar-info.cc
+++ b/src/libstore/nar-info.cc
@@ -7,15 +7,14 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
     : ValidPathInfo(StorePath(StorePath::dummy)) // FIXME: hack
 {
     auto corrupt = [&]() {
-        throw Error("NAR info file '%1%' is corrupt", whence);
+        return Error("NAR info file '%1%' is corrupt", whence);
     };
 
     auto parseHashField = [&](const string & s) {
         try {
             return Hash(s);
         } catch (BadHash &) {
-            corrupt();
-            return Hash(); // never reached
+            throw corrupt();
         }
     };
 
@@ -25,12 +24,12 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
     while (pos < s.size()) {
 
         size_t colon = s.find(':', pos);
-        if (colon == std::string::npos) corrupt();
+        if (colon == std::string::npos) throw corrupt();
 
         std::string name(s, pos, colon - pos);
 
         size_t eol = s.find('\n', colon + 2);
-        if (eol == std::string::npos) corrupt();
+        if (eol == std::string::npos) throw corrupt();
 
         std::string value(s, colon + 2, eol - colon - 2);
 
@@ -45,16 +44,16 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "FileHash")
             fileHash = parseHashField(value);
         else if (name == "FileSize") {
-            if (!string2Int(value, fileSize)) corrupt();
+            if (!string2Int(value, fileSize)) throw corrupt();
         }
         else if (name == "NarHash")
             narHash = parseHashField(value);
         else if (name == "NarSize") {
-            if (!string2Int(value, narSize)) corrupt();
+            if (!string2Int(value, narSize)) throw corrupt();
         }
         else if (name == "References") {
             auto refs = tokenizeString<Strings>(value, " ");
-            if (!references.empty()) corrupt();
+            if (!references.empty()) throw corrupt();
             for (auto & r : refs)
                 references.insert(StorePath(r));
         }
@@ -67,7 +66,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "Sig")
             sigs.insert(value);
         else if (name == "CA") {
-            if (ca) corrupt();
+            if (ca) throw corrupt();
             // FIXME: allow blank ca or require skipping field?
             ca = parseContentAddressOpt(value);
         }
@@ -77,7 +76,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
 
     if (compression == "") compression = "bzip2";
 
-    if (!havePath || url.empty() || narSize == 0 || !narHash) corrupt();
+    if (!havePath || url.empty() || narSize == 0 || !narHash) throw corrupt();
 }
 
 std::string NarInfo::to_string(const Store & store) const
@@ -87,11 +86,11 @@ std::string NarInfo::to_string(const Store & store) const
     res += "URL: " + url + "\n";
     assert(compression != "");
     res += "Compression: " + compression + "\n";
-    assert(fileHash.type == htSHA256);
-    res += "FileHash: " + fileHash.to_string(Base32, true) + "\n";
+    assert(fileHash && fileHash->type == htSHA256);
+    res += "FileHash: " + fileHash->to_string(Base32, true) + "\n";
     res += "FileSize: " + std::to_string(fileSize) + "\n";
-    assert(narHash.type == htSHA256);
-    res += "NarHash: " + narHash.to_string(Base32, true) + "\n";
+    assert(narHash && narHash->type == htSHA256);
+    res += "NarHash: " + narHash->to_string(Base32, true) + "\n";
     res += "NarSize: " + std::to_string(narSize) + "\n";
 
     res += "References: " + concatStringsSep(" ", shortRefs()) + "\n";
diff --git a/src/libstore/nar-info.hh b/src/libstore/nar-info.hh
index 373c33427..eff19f0ef 100644
--- a/src/libstore/nar-info.hh
+++ b/src/libstore/nar-info.hh
@@ -10,7 +10,7 @@ struct NarInfo : ValidPathInfo
 {
     std::string url;
     std::string compression;
-    Hash fileHash;
+    std::optional<Hash> fileHash;
     uint64_t fileSize = 0;
     std::string system;
 
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index b2b2412a3..e4b4b6213 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -282,7 +282,7 @@ void LocalStore::optimiseStore(OptimiseStats & stats)
     }
 }
 
-static string showBytes(unsigned long long bytes)
+static string showBytes(uint64_t bytes)
 {
     return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
 }
diff --git a/src/libstore/path.hh b/src/libstore/path.hh
index e43a8b50c..b03a0f69d 100644
--- a/src/libstore/path.hh
+++ b/src/libstore/path.hh
@@ -64,6 +64,8 @@ typedef std::set<StorePath> StorePathSet;
 typedef std::vector<StorePath> StorePaths;
 typedef std::map<string, StorePath> OutputPathMap;
 
+typedef std::map<StorePath, std::optional<ContentAddress>> StorePathCAMap;
+
 /* Extension of derivations in the Nix store. */
 const std::string drvExtension = ".drv";
 
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index 968e2e425..62a3cda61 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -76,8 +76,8 @@ void RefScanSink::operator () (const unsigned char * data, size_t len)
 }
 
 
-PathSet scanForReferences(const string & path,
-    const PathSet & refs, HashResult & hash)
+std::pair<PathSet, HashResult> scanForReferences(const string & path,
+    const PathSet & refs)
 {
     RefScanSink refsSink;
     HashSink hashSink { htSHA256 };
@@ -111,9 +111,9 @@ PathSet scanForReferences(const string & path,
         found.insert(j->second);
     }
 
-    hash = hashSink.finish();
+    auto hash = hashSink.finish();
 
-    return found;
+    return std::pair<PathSet, HashResult>(found, hash);
 }
 
 
diff --git a/src/libstore/references.hh b/src/libstore/references.hh
index c38bdd720..598a3203a 100644
--- a/src/libstore/references.hh
+++ b/src/libstore/references.hh
@@ -5,8 +5,7 @@
 
 namespace nix {
 
-PathSet scanForReferences(const Path & path, const PathSet & refs,
-    HashResult & hash);
+std::pair<PathSet, HashResult> scanForReferences(const Path & path, const PathSet & refs);
 
 struct RewritingSink : Sink
 {
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 3b3cdbc02..1200ab200 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -39,13 +39,45 @@ void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths
         out << store.printStorePath(i);
 }
 
-
 StorePath read(const Store & store, Source & from, Proxy<StorePath> _)
 {
 	auto path = readString(from);
 	return store.parseStorePath(path);
 }
 
+StorePathCAMap readStorePathCAMap(const Store & store, Source & from)
+{
+    StorePathCAMap paths;
+    auto count = readNum<size_t>(from);
+    while (count--)
+        paths.insert_or_assign(store.parseStorePath(readString(from)), parseContentAddressOpt(readString(from)));
+    return paths;
+}
+
+void writeStorePathCAMap(const Store & store, Sink & out, const StorePathCAMap & paths)
+{
+    out << paths.size();
+    for (auto & i : paths) {
+        out << store.printStorePath(i.first);
+        out << renderContentAddress(i.second);
+    }
+}
+
+std::map<string, StorePath> readOutputPathMap(const Store & store, Source & from)
+{
+    std::map<string, StorePath> pathMap;
+    auto rawInput = readStrings<Strings>(from);
+    if (rawInput.size() % 2)
+        throw Error("got an odd number of elements from the daemon when trying to read a output path map");
+    auto curInput = rawInput.begin();
+    while (curInput != rawInput.end()) {
+        auto thisKey = *curInput++;
+        auto thisValue = *curInput++;
+        pathMap.emplace(thisKey, store.parseStorePath(thisValue));
+    }
+    return pathMap;
+}
+
 
 void write(const Store & store, Sink & out, const StorePath & storePath)
 {
@@ -323,18 +355,17 @@ StorePathSet RemoteStore::querySubstitutablePaths(const StorePathSet & paths)
 }
 
 
-void RemoteStore::querySubstitutablePathInfos(const StorePathSet & paths,
-    SubstitutablePathInfos & infos)
+void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, SubstitutablePathInfos & infos)
 {
-    if (paths.empty()) return;
+    if (pathsMap.empty()) return;
 
     auto conn(getConnection());
 
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
 
-        for (auto & i : paths) {
+        for (auto & i : pathsMap) {
             SubstitutablePathInfo info;
-            conn->to << wopQuerySubstitutablePathInfo << printStorePath(i);
+            conn->to << wopQuerySubstitutablePathInfo << printStorePath(i.first);
             conn.processStderr();
             unsigned int reply = readInt(conn->from);
             if (reply == 0) continue;
@@ -344,13 +375,19 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathSet & paths,
             info.references = readStorePaths<StorePathSet>(*this, conn->from);
             info.downloadSize = readLongLong(conn->from);
             info.narSize = readLongLong(conn->from);
-            infos.insert_or_assign(i, std::move(info));
+            infos.insert_or_assign(i.first, std::move(info));
         }
 
     } else {
 
         conn->to << wopQuerySubstitutablePathInfos;
-        writeStorePaths(*this, conn->to, paths);
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 22) {
+            StorePathSet paths;
+            for (auto & path : pathsMap)
+                paths.insert(path.first);
+            writeStorePaths(*this, conn->to, paths);
+        } else
+            writeStorePathCAMap(*this, conn->to, pathsMap);
         conn.processStderr();
         size_t count = readNum<size_t>(conn->from);
         for (size_t n = 0; n < count; n++) {
@@ -489,14 +526,89 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
         conn->to << wopAddToStoreNar
                  << printStorePath(info.path)
                  << (info.deriver ? printStorePath(*info.deriver) : "")
-                 << info.narHash.to_string(Base16, false);
+                 << info.narHash->to_string(Base16, false);
         writeStorePaths(*this, conn->to, info.references);
         conn->to << info.registrationTime << info.narSize
                  << info.ultimate << info.sigs << renderContentAddress(info.ca)
                  << repair << !checkSigs;
-        bool tunnel = GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21;
-        if (!tunnel) copyNAR(source, conn->to);
-        conn.processStderr(0, tunnel ? &source : nullptr);
+
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 23) {
+
+            std::exception_ptr ex;
+
+            struct FramedSink : BufferedSink
+            {
+                ConnectionHandle & conn;
+                std::exception_ptr & ex;
+
+                FramedSink(ConnectionHandle & conn, std::exception_ptr & ex) : conn(conn), ex(ex)
+                { }
+
+                ~FramedSink()
+                {
+                    try {
+                        conn->to << 0;
+                        conn->to.flush();
+                    } catch (...) {
+                        ignoreException();
+                    }
+                }
+
+                void write(const unsigned char * data, size_t len) override
+                {
+                    /* Don't send more data if the remote has
+                       encountered an error. */
+                    if (ex) {
+                        auto ex2 = ex;
+                        ex = nullptr;
+                        std::rethrow_exception(ex2);
+                    }
+                    conn->to << len;
+                    conn->to(data, len);
+                };
+            };
+
+            /* Handle log messages / exceptions from the remote on a
+               separate thread. */
+            std::thread stderrThread([&]()
+            {
+                try {
+                    conn.processStderr();
+                } catch (...) {
+                    ex = std::current_exception();
+                }
+            });
+
+            Finally joinStderrThread([&]()
+            {
+                if (stderrThread.joinable()) {
+                    stderrThread.join();
+                    if (ex) {
+                        try {
+                            std::rethrow_exception(ex);
+                        } catch (...) {
+                            ignoreException();
+                        }
+                    }
+                }
+            });
+
+            {
+                FramedSink sink(conn, ex);
+                copyNAR(source, sink);
+                sink.flush();
+            }
+
+            stderrThread.join();
+            if (ex)
+                std::rethrow_exception(ex);
+
+        } else if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21) {
+            conn.processStderr(0, &source);
+        } else {
+            copyNAR(source, conn->to);
+            conn.processStderr(0, nullptr);
+        }
     }
 }
 
@@ -698,7 +810,7 @@ void RemoteStore::addSignatures(const StorePath & storePath, const StringSet & s
 
 void RemoteStore::queryMissing(const std::vector<StorePathWithOutputs> & targets,
     StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-    unsigned long long & downloadSize, unsigned long long & narSize)
+    uint64_t & downloadSize, uint64_t & narSize)
 {
     {
         auto conn(getConnection());
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index b39a8de48..4b093ad3b 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -56,7 +56,7 @@ public:
 
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
 
-    void querySubstitutablePathInfos(const StorePathSet & paths,
+    void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) override;
 
     void addToStore(const ValidPathInfo & info, Source & nar,
@@ -94,7 +94,7 @@ public:
 
     void queryMissing(const std::vector<StorePathWithOutputs> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-        unsigned long long & downloadSize, unsigned long long & narSize) override;
+        uint64_t & downloadSize, uint64_t & narSize) override;
 
     void connect() override;
 
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 1b7dff085..67935f3ba 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -343,13 +343,10 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
             std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1)
                 .count();
 
-        auto size = istream->tellg();
-
-        printInfo("uploaded 's3://%s/%s' (%d bytes) in %d ms",
-            bucketName, path, size, duration);
+        printInfo("uploaded 's3://%s/%s' in %d ms",
+            bucketName, path, duration);
 
         stats.putTimeMs += duration;
-        stats.putBytes += size;
         stats.put++;
     }
 
diff --git a/src/libstore/s3-binary-cache-store.hh b/src/libstore/s3-binary-cache-store.hh
index 4d43fe4d2..b2b75d498 100644
--- a/src/libstore/s3-binary-cache-store.hh
+++ b/src/libstore/s3-binary-cache-store.hh
@@ -19,7 +19,6 @@ public:
     struct Stats
     {
         std::atomic<uint64_t> put{0};
-        std::atomic<uint64_t> putBytes{0};
         std::atomic<uint64_t> putTimeMs{0};
         std::atomic<uint64_t> get{0};
         std::atomic<uint64_t> getBytes{0};
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index a624fafdd..f46e22874 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -193,6 +193,23 @@ StorePath Store::makeFixedOutputPath(
     }
 }
 
+// FIXME Put this somewhere?
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
+
+StorePath Store::makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
+    const StorePathSet & references, bool hasSelfReference) const
+{
+    // New template
+    return std::visit(overloaded {
+        [&](TextHash th) {
+            return makeTextPath(name, th.hash, references);
+        },
+        [&](FixedOutputHash fsh) {
+            return makeFixedOutputPath(fsh.method, fsh.hash, name, references, hasSelfReference);
+        }
+    }, ca);
+}
 
 StorePath Store::makeTextPath(std::string_view name, const Hash & hash,
     const StorePathSet & references) const
@@ -549,7 +566,7 @@ string Store::makeValidityRegistration(const StorePathSet & paths,
         auto info = queryPathInfo(i);
 
         if (showHash) {
-            s += info->narHash.to_string(Base16, false) + "\n";
+            s += info->narHash->to_string(Base16, false) + "\n";
             s += (format("%1%\n") % info->narSize).str();
         }
 
@@ -581,7 +598,7 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
             auto info = queryPathInfo(storePath);
 
             jsonPath
-                .attr("narHash", info->narHash.to_string(hashBase, true))
+                .attr("narHash", info->narHash->to_string(hashBase, true))
                 .attr("narSize", info->narSize);
 
             {
@@ -624,7 +641,7 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
                     if (!narInfo->url.empty())
                         jsonPath.attr("url", narInfo->url);
                     if (narInfo->fileHash)
-                        jsonPath.attr("downloadHash", narInfo->fileHash.to_string(hashBase, true));
+                        jsonPath.attr("downloadHash", narInfo->fileHash->to_string(hashBase, true));
                     if (narInfo->fileSize)
                         jsonPath.attr("downloadSize", narInfo->fileSize);
                     if (showClosureSize)
@@ -700,6 +717,15 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 
     uint64_t total = 0;
 
+    // recompute store path on the chance dstStore does it differently
+    if (info->ca && info->references.empty()) {
+        auto info2 = make_ref<ValidPathInfo>(*info);
+        info2->path = dstStore->makeFixedOutputPathFromCA(info->path.name(), *info->ca);
+        if (dstStore->storeDir == srcStore->storeDir)
+            assert(info->path == info2->path);
+        info = info2;
+    }
+
     if (!info->narHash) {
         StringSink sink;
         srcStore->narFromPath({storePath}, sink);
@@ -735,16 +761,20 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 }
 
 
-void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
+std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
     RepairFlag repair, CheckSigsFlag checkSigs, SubstituteFlag substitute)
 {
     auto valid = dstStore->queryValidPaths(storePaths, substitute);
 
-    PathSet missing;
+    StorePathSet missing;
+    for (auto & path : storePaths)
+        if (!valid.count(path)) missing.insert(path);
+
+    std::map<StorePath, StorePath> pathsMap;
     for (auto & path : storePaths)
-        if (!valid.count(path)) missing.insert(srcStore->printStorePath(path));
+        pathsMap.insert_or_assign(path, path);
 
-    if (missing.empty()) return;
+    if (missing.empty()) return pathsMap;
 
     Activity act(*logger, lvlInfo, actCopyPaths, fmt("copying %d paths", missing.size()));
 
@@ -759,30 +789,49 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
 
     ThreadPool pool;
 
-    processGraph<Path>(pool,
-        PathSet(missing.begin(), missing.end()),
+    processGraph<StorePath>(pool,
+        StorePathSet(missing.begin(), missing.end()),
+
+        [&](const StorePath & storePath) {
+            auto info = srcStore->queryPathInfo(storePath);
+            auto storePathForDst = storePath;
+            if (info->ca && info->references.empty()) {
+                storePathForDst = dstStore->makeFixedOutputPathFromCA(storePath.name(), *info->ca);
+                if (dstStore->storeDir == srcStore->storeDir)
+                    assert(storePathForDst == storePath);
+                if (storePathForDst != storePath)
+                    debug("replaced path '%s' to '%s' for substituter '%s'", srcStore->printStorePath(storePath), dstStore->printStorePath(storePathForDst), dstStore->getUri());
+            }
+            pathsMap.insert_or_assign(storePath, storePathForDst);
 
-        [&](const Path & storePath) {
-            if (dstStore->isValidPath(dstStore->parseStorePath(storePath))) {
+            if (dstStore->isValidPath(storePath)) {
                 nrDone++;
                 showProgress();
-                return PathSet();
+                return StorePathSet();
             }
 
-            auto info = srcStore->queryPathInfo(srcStore->parseStorePath(storePath));
-
             bytesExpected += info->narSize;
             act.setExpected(actCopyPath, bytesExpected);
 
-            return srcStore->printStorePathSet(info->references);
+            return info->references;
         },
 
-        [&](const Path & storePathS) {
+        [&](const StorePath & storePath) {
             checkInterrupt();
 
-            auto storePath = dstStore->parseStorePath(storePathS);
+            auto info = srcStore->queryPathInfo(storePath);
+
+            auto storePathForDst = storePath;
+            if (info->ca && info->references.empty()) {
+                storePathForDst = dstStore->makeFixedOutputPathFromCA(storePath.name(), *info->ca);
+                if (dstStore->storeDir == srcStore->storeDir)
+                    assert(storePathForDst == storePath);
+                if (storePathForDst != storePath)
+                    debug("replaced path '%s' to '%s' for substituter '%s'", srcStore->printStorePath(storePath), dstStore->printStorePath(storePathForDst), dstStore->getUri());
+            }
+            pathsMap.insert_or_assign(storePath, storePathForDst);
 
-            if (!dstStore->isValidPath(storePath)) {
+            if (!dstStore->isValidPath(storePathForDst)) {
                 MaintainCount<decltype(nrRunning)> mc(nrRunning);
                 showProgress();
                 try {
@@ -791,7 +840,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
                     nrFailed++;
                     if (!settings.keepGoing)
                         throw e;
-                    logger->log(lvlError, fmt("could not copy %s: %s", storePathS, e.what()));
+                    logger->log(lvlError, fmt("could not copy %s: %s", dstStore->printStorePath(storePath), e.what()));
                     showProgress();
                     return;
                 }
@@ -800,6 +849,8 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
             nrDone++;
             showProgress();
         });
+
+    return pathsMap;
 }
 
 
@@ -865,7 +916,7 @@ std::string ValidPathInfo::fingerprint(const Store & store) const
             store.printStorePath(path));
     return
         "1;" + store.printStorePath(path) + ";"
-        + narHash.to_string(Base32, true) + ";"
+        + narHash->to_string(Base32, true) + ";"
         + std::to_string(narSize) + ";"
         + concatStringsSep(",", store.printStorePathSet(references));
 }
@@ -876,10 +927,6 @@ void ValidPathInfo::sign(const Store & store, const SecretKey & secretKey)
     sigs.insert(secretKey.signDetached(fingerprint(store)));
 }
 
-// FIXME Put this somewhere?
-template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
-template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
-
 bool ValidPathInfo::isContentAddressed(const Store & store) const
 {
     if (! ca) return false;
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index a83a2ff10..3f8866a2c 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -85,7 +85,7 @@ struct GCOptions
     StorePathSet pathsToDelete;
 
     /* Stop after at least `maxFreed' bytes have been freed. */
-    unsigned long long maxFreed{std::numeric_limits<unsigned long long>::max()};
+    uint64_t maxFreed{std::numeric_limits<uint64_t>::max()};
 };
 
 
@@ -97,7 +97,7 @@ struct GCResults
 
     /* For `gcReturnDead', `gcDeleteDead' and `gcDeleteSpecific', the
        number of bytes that would be or was freed. */
-    unsigned long long bytesFreed = 0;
+    uint64_t bytesFreed = 0;
 };
 
 
@@ -105,8 +105,8 @@ struct SubstitutablePathInfo
 {
     std::optional<StorePath> deriver;
     StorePathSet references;
-    unsigned long long downloadSize; /* 0 = unknown or inapplicable */
-    unsigned long long narSize; /* 0 = unknown */
+    uint64_t downloadSize; /* 0 = unknown or inapplicable */
+    uint64_t narSize; /* 0 = unknown */
 };
 
 typedef std::map<StorePath, SubstitutablePathInfo> SubstitutablePathInfos;
@@ -115,7 +115,8 @@ struct ValidPathInfo
 {
     StorePath path;
     std::optional<StorePath> deriver;
-    Hash narHash;
+    // TODO document this
+    std::optional<Hash> narHash;
     StorePathSet references;
     time_t registrationTime = 0;
     uint64_t narSize = 0; // 0 = unknown
@@ -343,7 +344,11 @@ public:
         bool hasSelfReference = false) const;
 
     StorePath makeTextPath(std::string_view name, const Hash & hash,
-        const StorePathSet & references) const;
+        const StorePathSet & references = {}) const;
+
+    StorePath makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
+        const StorePathSet & references = {},
+        bool hasSelfReference = false) const;
 
     /* This is the preparatory part of addToStore(); it computes the
        store path to which srcPath is to be copied.  Returns the store
@@ -441,9 +446,10 @@ public:
     virtual StorePathSet querySubstitutablePaths(const StorePathSet & paths) { return {}; };
 
     /* Query substitute info (i.e. references, derivers and download
-       sizes) of a set of paths.  If a path does not have substitute
-       info, it's omitted from the resulting ‘infos’ map. */
-    virtual void querySubstitutablePathInfos(const StorePathSet & paths,
+       sizes) of a map of paths to their optional ca values. If a path
+       does not have substitute info, it's omitted from the resulting
+       ‘infos’ map. */
+    virtual void querySubstitutablePathInfos(const StorePathCAMap & paths,
         SubstitutablePathInfos & infos) { return; };
 
     /* Import a path into the store. */
@@ -615,7 +621,7 @@ public:
        that will be substituted. */
     virtual void queryMissing(const std::vector<StorePathWithOutputs> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
-        unsigned long long & downloadSize, unsigned long long & narSize);
+        uint64_t & downloadSize, uint64_t & narSize);
 
     /* Sort a set of paths topologically under the references
        relation.  If p refers to q, then p precedes q in this list. */
@@ -745,11 +751,13 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 
 
 /* Copy store paths from one store to another. The paths may be copied
-   in parallel. They are copied in a topologically sorted order
-   (i.e. if A is a reference of B, then A is copied before B), but
-   the set of store paths is not automatically closed; use
-   copyClosure() for that. */
-void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
+   in parallel. They are copied in a topologically sorted order (i.e.
+   if A is a reference of B, then A is copied before B), but the set
+   of store paths is not automatically closed; use copyClosure() for
+   that. Returns a map of what each path was copied to the dstStore
+   as. */
+std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStore,
+    const StorePathSet & storePaths,
     RepairFlag repair = NoRepair,
     CheckSigsFlag checkSigs = CheckSigs,
     SubstituteFlag substitute = NoSubstitute);
@@ -848,4 +856,6 @@ std::optional<ValidPathInfo> decodeValidPathInfo(
 /* Split URI into protocol+hierarchy part and its parameter set. */
 std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri);
 
+std::optional<ContentAddress> getDerivationCA(const BasicDerivation & drv);
+
 }
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index 6a6e29640..ad5854c85 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x116
+#define PROTOCOL_VERSION 0x117
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -122,4 +122,10 @@ StorePath read(const Store & store, Source & from, Proxy<StorePath> _);
 
 void write(const Store & store, Sink & out, const StorePath & storePath);
 
+StorePathCAMap readStorePathCAMap(const Store & store, Source & from);
+
+void writeStorePathCAMap(const Store & store, Sink & out, const StorePathCAMap & paths);
+
+void writeOutputPathMap(const Store & store, Sink & out, const OutputPathMap & paths);
+
 }