Merge pull request #4330 from NixOS/ca/properly-store-outputs

Properly store the outputs of CA derivations − take 2

18 files changed, 379 insertions, 72 deletions

diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index a918b7208..5b081c1ae 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -443,6 +443,24 @@ StorePath BinaryCacheStore::addTextToStore(const string & name, const string & s
     })->path;
 }
 
+std::optional<const Realisation> BinaryCacheStore::queryRealisation(const DrvOutput & id)
+{
+    auto outputInfoFilePath = realisationsPrefix + "/" + id.to_string() + ".doi";
+    auto rawOutputInfo = getFile(outputInfoFilePath);
+
+    if (rawOutputInfo) {
+        return {Realisation::fromJSON(
+            nlohmann::json::parse(*rawOutputInfo), outputInfoFilePath)};
+    } else {
+        return std::nullopt;
+    }
+}
+
+void BinaryCacheStore::registerDrvOutput(const Realisation& info) {
+    auto filePath = realisationsPrefix + "/" + info.id.to_string() + ".doi";
+    upsertFile(filePath, info.toJSON(), "application/json");
+}
+
 ref<FSAccessor> BinaryCacheStore::getFSAccessor()
 {
     return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()), localNarCache);
diff --git a/src/libstore/binary-cache-store.hh b/src/libstore/binary-cache-store.hh
index 5224d7ec8..07a8b2beb 100644
--- a/src/libstore/binary-cache-store.hh
+++ b/src/libstore/binary-cache-store.hh
@@ -33,6 +33,9 @@ private:
 
 protected:
 
+    // The prefix under which realisation infos will be stored
+    const std::string realisationsPrefix = "/realisations";
+
     BinaryCacheStore(const Params & params);
 
 public:
@@ -99,6 +102,10 @@ public:
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
 
+    void registerDrvOutput(const Realisation & info) override;
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput &) override;
+
     void narFromPath(const StorePath & path, Sink & sink) override;
 
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index de58d9f06..b7bf866eb 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -493,8 +493,9 @@ void DerivationGoal::inputsRealised()
     if (useDerivation) {
         auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
 
-        if ((!fullDrv.inputDrvs.empty() && derivationIsCA(fullDrv.type()))
-            || fullDrv.type() == DerivationType::DeferredInputAddressed) {
+        if (settings.isExperimentalFeatureEnabled("ca-derivations") &&
+            ((!fullDrv.inputDrvs.empty() && derivationIsCA(fullDrv.type()))
+            || fullDrv.type() == DerivationType::DeferredInputAddressed)) {
             /* We are be able to resolve this derivation based on the
                now-known results of dependencies. If so, we become a stub goal
                aliasing that resolved derivation goal */
@@ -2094,6 +2095,20 @@ struct RestrictedStore : public LocalFSStore, public virtual RestrictedStoreConf
         /* Nothing to be done; 'path' must already be valid. */
     }
 
+    void registerDrvOutput(const Realisation & info) override
+    {
+        if (!goal.isAllowed(info.id.drvPath))
+            throw InvalidPath("cannot register unknown drv output '%s' in recursive Nix", printStorePath(info.id.drvPath));
+        next->registerDrvOutput(info);
+    }
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput & id) override
+    {
+        if (!goal.isAllowed(id.drvPath))
+            throw InvalidPath("cannot query the output info for unknown derivation '%s' in recursive Nix", printStorePath(id.drvPath));
+        return next->queryRealisation(id);
+    }
+
     void buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode) override
     {
         if (buildMode != bmNormal) throw Error("unsupported build mode");
@@ -3391,9 +3406,11 @@ void DerivationGoal::registerOutputs()
         drvPathResolved = writeDerivation(worker.store, drv2);
     }
 
-    if (useDerivation || isCaFloating)
-        for (auto & [outputName, newInfo] : infos)
-            worker.store.linkDeriverToPath(drvPathResolved, outputName, newInfo.path);
+    if (settings.isExperimentalFeatureEnabled("ca-derivations"))
+        for (auto& [outputName, newInfo] : infos)
+            worker.store.registerDrvOutput(Realisation{
+                .id = DrvOutput{drvPathResolved, outputName},
+                .outPath = newInfo.path});
 }
 
 
diff --git a/src/libstore/ca-specific-schema.sql b/src/libstore/ca-specific-schema.sql
new file mode 100644
index 000000000..93c442826
--- /dev/null
+++ b/src/libstore/ca-specific-schema.sql
@@ -0,0 +1,11 @@
+-- Extension of the sql schema for content-addressed derivations.
+-- Won't be loaded unless the experimental feature `ca-derivations`
+-- is enabled
+
+create table if not exists Realisations (
+    drvPath text not null,
+    outputName text not null, -- symbolic output id, usually "out"
+    outputPath integer not null,
+    primary key (drvPath, outputName),
+    foreign key (outputPath) references ValidPaths(id) on delete cascade
+);
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 2224d54d5..ba5788b64 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -868,6 +868,28 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         break;
     }
 
+    case wopRegisterDrvOutput: {
+        logger->startWork();
+        auto outputId = DrvOutput::parse(readString(from));
+        auto outputPath = StorePath(readString(from));
+        auto resolvedDrv = StorePath(readString(from));
+        store->registerDrvOutput(Realisation{
+            .id = outputId, .outPath = outputPath});
+        logger->stopWork();
+        break;
+    }
+
+    case wopQueryRealisation: {
+        logger->startWork();
+        auto outputId = DrvOutput::parse(readString(from));
+        auto info = store->queryRealisation(outputId);
+        logger->stopWork();
+        std::set<StorePath> outPaths;
+        if (info) outPaths.insert(info->outPath);
+        worker_proto::write(*store, to, outPaths);
+        break;
+    }
+
     default:
         throw Error("invalid operation %1%", op);
     }
diff --git a/src/libstore/dummy-store.cc b/src/libstore/dummy-store.cc
index 98b745c3a..91fc178db 100644
--- a/src/libstore/dummy-store.cc
+++ b/src/libstore/dummy-store.cc
@@ -60,6 +60,9 @@ struct DummyStore : public Store, public virtual DummyStoreConfig
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
         BuildMode buildMode) override
     { unsupported("buildDerivation"); }
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput&) override
+    { unsupported("queryRealisation"); }
 };
 
 static RegisterStoreImplementation<DummyStore, DummyStoreConfig> regDummyStore;
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 467169ce8..ad1779aea 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -333,6 +333,10 @@ public:
         auto conn(connections->get());
         return conn->remoteVersion;
     }
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput&) override
+    // TODO: Implement
+    { unsupported("queryRealisation"); }
 };
 
 static RegisterStoreImplementation<LegacySSHStore, LegacySSHStoreConfig> regLegacySSHStore;
diff --git a/src/libstore/local-binary-cache-store.cc b/src/libstore/local-binary-cache-store.cc
index 7d979c5c2..bb7464989 100644
--- a/src/libstore/local-binary-cache-store.cc
+++ b/src/libstore/local-binary-cache-store.cc
@@ -87,6 +87,7 @@ protected:
 void LocalBinaryCacheStore::init()
 {
     createDirs(binaryCacheDir + "/nar");
+    createDirs(binaryCacheDir + realisationsPrefix);
     if (writeDebugInfo)
         createDirs(binaryCacheDir + "/debuginfo");
     BinaryCacheStore::init();
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 2a47b3956..69ab821d9 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -52,12 +52,52 @@ struct LocalStore::State::Stmts {
     SQLiteStmt QueryReferrers;
     SQLiteStmt InvalidatePath;
     SQLiteStmt AddDerivationOutput;
+    SQLiteStmt RegisterRealisedOutput;
     SQLiteStmt QueryValidDerivers;
     SQLiteStmt QueryDerivationOutputs;
+    SQLiteStmt QueryRealisedOutput;
+    SQLiteStmt QueryAllRealisedOutputs;
     SQLiteStmt QueryPathFromHashPart;
     SQLiteStmt QueryValidPaths;
 };
 
+int getSchema(Path schemaPath)
+{
+    int curSchema = 0;
+    if (pathExists(schemaPath)) {
+        string s = readFile(schemaPath);
+        if (!string2Int(s, curSchema))
+            throw Error("'%1%' is corrupt", schemaPath);
+    }
+    return curSchema;
+}
+
+void migrateCASchema(SQLite& db, Path schemaPath, AutoCloseFD& lockFd)
+{
+    const int nixCASchemaVersion = 1;
+    int curCASchema = getSchema(schemaPath);
+    if (curCASchema != nixCASchemaVersion) {
+        if (curCASchema > nixCASchemaVersion) {
+            throw Error("current Nix store ca-schema is version %1%, but I only support %2%",
+                 curCASchema, nixCASchemaVersion);
+        }
+
+        if (!lockFile(lockFd.get(), ltWrite, false)) {
+            printInfo("waiting for exclusive access to the Nix store for ca drvs...");
+            lockFile(lockFd.get(), ltWrite, true);
+        }
+
+        if (curCASchema == 0) {
+            static const char schema[] =
+              #include "ca-specific-schema.sql.gen.hh"
+                ;
+            db.exec(schema);
+        }
+        writeFile(schemaPath, fmt("%d", nixCASchemaVersion));
+        lockFile(lockFd.get(), ltRead, true);
+    }
+}
+
 LocalStore::LocalStore(const Params & params)
     : StoreConfig(params)
     , Store(params)
@@ -238,6 +278,10 @@ LocalStore::LocalStore(const Params & params)
 
     else openDB(*state, false);
 
+    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+        migrateCASchema(state->db, dbDir + "/ca-schema", globalLock);
+    }
+
     /* Prepare SQL statements. */
     state->stmts->RegisterValidPath.create(state->db,
         "insert into ValidPaths (path, hash, registrationTime, deriver, narSize, ultimate, sigs, ca) values (?, ?, ?, ?, ?, ?, ?, ?);");
@@ -264,6 +308,28 @@ LocalStore::LocalStore(const Params & params)
     state->stmts->QueryPathFromHashPart.create(state->db,
         "select path from ValidPaths where path >= ? limit 1;");
     state->stmts->QueryValidPaths.create(state->db, "select path from ValidPaths");
+    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+        state->stmts->RegisterRealisedOutput.create(state->db,
+            R"(
+                insert or replace into Realisations (drvPath, outputName, outputPath)
+                values (?, ?, (select id from ValidPaths where path = ?))
+                ;
+            )");
+        state->stmts->QueryRealisedOutput.create(state->db,
+            R"(
+                select Output.path from Realisations
+                    inner join ValidPaths as Output on Output.id = Realisations.outputPath
+                    where drvPath = ? and outputName = ?
+                    ;
+            )");
+        state->stmts->QueryAllRealisedOutputs.create(state->db,
+            R"(
+                select outputName, Output.path from Realisations
+                    inner join ValidPaths as Output on Output.id = Realisations.outputPath
+                    where drvPath = ?
+                    ;
+            )");
+    }
 }
 
 
@@ -301,16 +367,7 @@ std::string LocalStore::getUri()
 
 
 int LocalStore::getSchema()
-{
-    int curSchema = 0;
-    if (pathExists(schemaPath)) {
-        string s = readFile(schemaPath);
-        if (!string2Int(s, curSchema))
-            throw Error("'%1%' is corrupt", schemaPath);
-    }
-    return curSchema;
-}
-
+{ return nix::getSchema(schemaPath); }
 
 void LocalStore::openDB(State & state, bool create)
 {
@@ -597,13 +654,19 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
 }
 
 
-void LocalStore::linkDeriverToPath(const StorePath & deriver, const string & outputName, const StorePath & output)
+void LocalStore::registerDrvOutput(const Realisation & info)
 {
     auto state(_state.lock());
-    return linkDeriverToPath(*state, queryValidPathId(*state, deriver), outputName, output);
+    retrySQLite<void>([&]() {
+        state->stmts->RegisterRealisedOutput.use()
+            (info.id.drvPath.to_string())
+            (info.id.outputName)
+            (printStorePath(info.outPath))
+            .exec();
+    });
 }
 
-void LocalStore::linkDeriverToPath(State & state, uint64_t deriver, const string & outputName, const StorePath & output)
+void LocalStore::cacheDrvOutputMapping(State & state, const uint64_t deriver, const string & outputName, const StorePath & output)
 {
     retrySQLite<void>([&]() {
         state.stmts->AddDerivationOutput.use()
@@ -653,7 +716,7 @@ uint64_t LocalStore::addValidPath(State & state,
             /* Floating CA derivations have indeterminate output paths until
                they are built, so don't register anything in that case */
             if (i.second.second)
-                linkDeriverToPath(state, id, i.first, *i.second.second);
+                cacheDrvOutputMapping(state, id, i.first, *i.second.second);
         }
     }
 
@@ -814,70 +877,85 @@ StorePathSet LocalStore::queryValidDerivers(const StorePath & path)
     });
 }
 
-
-std::map<std::string, std::optional<StorePath>> LocalStore::queryPartialDerivationOutputMap(const StorePath & path_)
+// Try to resolve the derivation at path `original`, with a caching layer
+// to make it more efficient
+std::optional<StorePath> cachedResolve(
+    LocalStore & store,
+    const StorePath & original)
 {
-    auto path = path_;
-    std::map<std::string, std::optional<StorePath>> outputs;
-    Derivation drv = readDerivation(path);
-    for (auto & [outName, _] : drv.outputs) {
-        outputs.insert_or_assign(outName, std::nullopt);
-    }
-    bool haveCached = false;
     {
         auto resolutions = drvPathResolutions.lock();
-        auto resolvedPathOptIter = resolutions->find(path);
+        auto resolvedPathOptIter = resolutions->find(original);
         if (resolvedPathOptIter != resolutions->end()) {
             auto & [_, resolvedPathOpt] = *resolvedPathOptIter;
             if (resolvedPathOpt)
-                path = *resolvedPathOpt;
-            haveCached = true;
+                return resolvedPathOpt;
         }
     }
-    /* can't just use else-if instead of `!haveCached` because we need to unlock
-       `drvPathResolutions` before it is locked in `Derivation::resolve`. */
-    if (!haveCached && (drv.type() == DerivationType::CAFloating || drv.type() == DerivationType::DeferredInputAddressed)) {
-        /* Try resolve drv and use that path instead. */
-        auto attempt = drv.tryResolve(*this);
-        if (!attempt)
-            /* If we cannot resolve the derivation, we cannot have any path
-               assigned so we return the map of all std::nullopts. */
-            return outputs;
-        /* Just compute store path */
-        auto pathResolved = writeDerivation(*this, *std::move(attempt), NoRepair, true);
-        /* Store in memo table. */
-        /* FIXME: memo logic should not be local-store specific, should have
-           wrapper-method instead. */
-        drvPathResolutions.lock()->insert_or_assign(path, pathResolved);
-        path = std::move(pathResolved);
-    }
-    return retrySQLite<std::map<std::string, std::optional<StorePath>>>([&]() {
-        auto state(_state.lock());
 
+    /* Try resolve drv and use that path instead. */
+    auto drv = store.readDerivation(original);
+    auto attempt = drv.tryResolve(store);
+    if (!attempt)
+        return std::nullopt;
+    /* Just compute store path */
+    auto pathResolved =
+        writeDerivation(store, *std::move(attempt), NoRepair, true);
+    /* Store in memo table. */
+    drvPathResolutions.lock()->insert_or_assign(original, pathResolved);
+    return pathResolved;
+}
+
+std::map<std::string, std::optional<StorePath>>
+LocalStore::queryPartialDerivationOutputMap(const StorePath& path_)
+{
+    auto path = path_;
+    auto outputs = retrySQLite<std::map<std::string, std::optional<StorePath>>>([&]() {
+        auto state(_state.lock());
+        std::map<std::string, std::optional<StorePath>> outputs;
         uint64_t drvId;
         try {
             drvId = queryValidPathId(*state, path);
-        } catch (InvalidPath &) {
-            /* FIXME? if the derivation doesn't exist, we cannot have a mapping
-               for it. */
-            return outputs;
+        } catch (InvalidPath&) {
+            // Ignore non-existing drvs as they might still have an output map
+            // defined if ca-derivations is enabled
         }
+        auto use(state->stmts->QueryDerivationOutputs.use()(drvId));
+        while (use.next())
+            outputs.insert_or_assign(
+                use.getStr(0), parseStorePath(use.getStr(1)));
 
-        auto useQueryDerivationOutputs {
-            state->stmts->QueryDerivationOutputs.use()
-            (drvId)
-        };
+        return outputs;
+    });
+
+    if (!settings.isExperimentalFeatureEnabled("ca-derivations"))
+        return outputs;
+
+    auto drv = readDerivation(path);
+
+    for (auto & output : drv.outputsAndOptPaths(*this)) {
+        outputs.emplace(output.first, std::nullopt);
+    }
+
+    auto resolvedDrv = cachedResolve(*this, path);
+
+    if (!resolvedDrv)
+        return outputs;
+
+    retrySQLite<void>([&]() {
+        auto state(_state.lock());
+        path = *resolvedDrv;
+        auto useQueryDerivationOutputs{
+            state->stmts->QueryAllRealisedOutputs.use()(path.to_string())};
 
         while (useQueryDerivationOutputs.next())
             outputs.insert_or_assign(
                 useQueryDerivationOutputs.getStr(0),
-                parseStorePath(useQueryDerivationOutputs.getStr(1))
-            );
-
-        return outputs;
+                parseStorePath(useQueryDerivationOutputs.getStr(1)));
     });
-}
 
+    return outputs;
+}
 
 std::optional<StorePath> LocalStore::queryPathFromHashPart(const std::string & hashPart)
 {
@@ -1612,5 +1690,19 @@ void LocalStore::createUser(const std::string & userName, uid_t userId)
     }
 }
 
-
+std::optional<const Realisation> LocalStore::queryRealisation(
+    const DrvOutput& id) {
+    typedef std::optional<const Realisation> Ret;
+    return retrySQLite<Ret>([&]() -> Ret {
+        auto state(_state.lock());
+        auto use(state->stmts->QueryRealisedOutput.use()(id.drvPath.to_string())(
+            id.outputName));
+        if (!use.next())
+            return std::nullopt;
+        auto outputPath = parseStorePath(use.getStr(0));
+        auto resolvedDrv = StorePath(use.getStr(1));
+        return Ret{
+            Realisation{.id = id, .outPath = outputPath}};
+    });
 }
+}  // namespace nix
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 332718af4..69559e346 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -208,6 +208,13 @@ public:
        garbage until it exceeds maxFree. */
     void autoGC(bool sync = true);
 
+    /* Register the store path 'output' as the output named 'outputName' of
+       derivation 'deriver'. */
+    void registerDrvOutput(const Realisation & info) override;
+    void cacheDrvOutputMapping(State & state, const uint64_t deriver, const string & outputName, const StorePath & output);
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput&) override;
+
 private:
 
     int getSchema();
@@ -276,11 +283,6 @@ private:
        specified by the ‘secret-key-files’ option. */
     void signPathInfo(ValidPathInfo & info);
 
-    /* Register the store path 'output' as the output named 'outputName' of
-       derivation 'deriver'. */
-    void linkDeriverToPath(const StorePath & deriver, const string & outputName, const StorePath & output);
-    void linkDeriverToPath(State & state, uint64_t deriver, const string & outputName, const StorePath & output);
-
     Path getRealStoreDir() override { return realStoreDir; }
 
     void createUser(const std::string & userName, uid_t userId) override;
diff --git a/src/libstore/local.mk b/src/libstore/local.mk
index dfe1e2cc4..03c4351ac 100644
--- a/src/libstore/local.mk
+++ b/src/libstore/local.mk
@@ -48,7 +48,7 @@ ifneq ($(sandbox_shell),)
 libstore_CXXFLAGS += -DSANDBOX_SHELL="\"$(sandbox_shell)\""
 endif
 
-$(d)/local-store.cc: $(d)/schema.sql.gen.hh
+$(d)/local-store.cc: $(d)/schema.sql.gen.hh $(d)/ca-specific-schema.sql.gen.hh
 
 $(d)/build.cc:
 
@@ -58,7 +58,7 @@ $(d)/build.cc:
 	@echo ')foo"' >> $@.tmp
 	@mv $@.tmp $@
 
-clean-files += $(d)/schema.sql.gen.hh
+clean-files += $(d)/schema.sql.gen.hh $(d)/ca-specific-schema.sql.gen.hh
 
 $(eval $(call install-file-in, $(d)/nix-store.pc, $(prefix)/lib/pkgconfig, 0644))
 
diff --git a/src/libstore/realisation.cc b/src/libstore/realisation.cc
new file mode 100644
index 000000000..47db1ec9f
--- /dev/null
+++ b/src/libstore/realisation.cc
@@ -0,0 +1,49 @@
+#include "realisation.hh"
+#include "store-api.hh"
+#include <nlohmann/json.hpp>
+
+namespace nix {
+
+MakeError(InvalidDerivationOutputId, Error);
+
+DrvOutput DrvOutput::parse(const std::string &strRep) {
+    const auto &[rawPath, outputs] = parsePathWithOutputs(strRep);
+    if (outputs.size() != 1)
+        throw InvalidDerivationOutputId("Invalid derivation output id %s", strRep);
+
+    return DrvOutput{
+        .drvPath = StorePath(rawPath),
+        .outputName = *outputs.begin(),
+    };
+}
+
+std::string DrvOutput::to_string() const {
+    return std::string(drvPath.to_string()) + "!" + outputName;
+}
+
+nlohmann::json Realisation::toJSON() const {
+    return nlohmann::json{
+        {"id", id.to_string()},
+        {"outPath", outPath.to_string()},
+    };
+}
+
+Realisation Realisation::fromJSON(
+    const nlohmann::json& json,
+    const std::string& whence) {
+    auto getField = [&](std::string fieldName) -> std::string {
+        auto fieldIterator = json.find(fieldName);
+        if (fieldIterator == json.end())
+            throw Error(
+                "Drv output info file '%1%' is corrupt, missing field %2%",
+                whence, fieldName);
+        return *fieldIterator;
+    };
+
+    return Realisation{
+        .id = DrvOutput::parse(getField("id")),
+        .outPath = StorePath(getField("outPath")),
+    };
+}
+
+} // namespace nix
diff --git a/src/libstore/realisation.hh b/src/libstore/realisation.hh
new file mode 100644
index 000000000..08579b739
--- /dev/null
+++ b/src/libstore/realisation.hh
@@ -0,0 +1,35 @@
+#pragma once
+
+#include "path.hh"
+#include <nlohmann/json_fwd.hpp>
+
+namespace nix {
+
+struct DrvOutput {
+    StorePath drvPath;
+    std::string outputName;
+
+    std::string to_string() const;
+
+    static DrvOutput parse(const std::string &);
+
+    bool operator<(const DrvOutput& other) const { return to_pair() < other.to_pair(); }
+    bool operator==(const DrvOutput& other) const { return to_pair() == other.to_pair(); }
+
+private:
+    // Just to make comparison operators easier to write
+    std::pair<StorePath, std::string> to_pair() const
+    { return std::make_pair(drvPath, outputName); }
+};
+
+struct Realisation {
+    DrvOutput id;
+    StorePath outPath;
+
+    nlohmann::json toJSON() const;
+    static Realisation fromJSON(const nlohmann::json& json, const std::string& whence);
+};
+
+typedef std::map<DrvOutput, Realisation> DrvOutputs;
+
+}
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index be29f8e6f..f1f4d0516 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -609,6 +609,27 @@ StorePath RemoteStore::addTextToStore(const string & name, const string & s,
     return addCAToStore(source, name, TextHashMethod{}, references, repair)->path;
 }
 
+void RemoteStore::registerDrvOutput(const Realisation & info)
+{
+    auto conn(getConnection());
+    conn->to << wopRegisterDrvOutput;
+    conn->to << info.id.to_string();
+    conn->to << std::string(info.outPath.to_string());
+    conn.processStderr();
+}
+
+std::optional<const Realisation> RemoteStore::queryRealisation(const DrvOutput & id)
+{
+    auto conn(getConnection());
+    conn->to << wopQueryRealisation;
+    conn->to << id.to_string();
+    conn.processStderr();
+    auto outPaths = worker_proto::read(*this, conn->from, Phantom<std::set<StorePath>>{});
+    if (outPaths.empty())
+        return std::nullopt;
+    return {Realisation{.id = id, .outPath = *outPaths.begin()}};
+}
+
 
 void RemoteStore::buildPaths(const std::vector<StorePathWithOutputs> & drvPaths, BuildMode buildMode)
 {
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 9f78fcb02..fdd53e6ed 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -81,6 +81,10 @@ public:
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
 
+    void registerDrvOutput(const Realisation & info) override;
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput &) override;
+
     void buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode) override;
 
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 6b9331495..7cdadc1f3 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -1,5 +1,6 @@
 #pragma once
 
+#include "realisation.hh"
 #include "path.hh"
 #include "hash.hh"
 #include "content-address.hh"
@@ -396,6 +397,8 @@ protected:
 
 public:
 
+    virtual std::optional<const Realisation> queryRealisation(const DrvOutput &) = 0;
+
     /* Queries the set of incoming FS references for a store path.
        The result is not cleared. */
     virtual void queryReferrers(const StorePath & path, StorePathSet & referrers)
@@ -468,6 +471,18 @@ public:
     virtual StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair = NoRepair) = 0;
 
+    /**
+     * Add a mapping indicating that `deriver!outputName` maps to the output path
+     * `output`.
+     *
+     * This is redundant for known-input-addressed and fixed-output derivations
+     * as this information is already present in the drv file, but necessary for
+     * floating-ca derivations and their dependencies as there's no way to
+     * retrieve this information otherwise.
+     */
+    virtual void registerDrvOutput(const Realisation & output)
+    { unsupported("registerDrvOutput"); }
+
     /* Write a NAR dump of a store path. */
     virtual void narFromPath(const StorePath & path, Sink & sink) = 0;
 
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index 63bd6ea49..f2cdc7ca3 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -1,5 +1,8 @@
 #pragma once
 
+#include "store-api.hh"
+#include "serialise.hh"
+
 namespace nix {
 
 
@@ -50,6 +53,8 @@ typedef enum {
     wopAddToStoreNar = 39,
     wopQueryMissing = 40,
     wopQueryDerivationOutputMap = 41,
+    wopRegisterDrvOutput = 42,
+    wopQueryRealisation = 43,
 } WorkerOp;
 
 
diff --git a/tests/content-addressed.sh b/tests/content-addressed.sh
index bc37a99c1..e8ac88609 100644
--- a/tests/content-addressed.sh
+++ b/tests/content-addressed.sh
@@ -55,7 +55,8 @@ testNixCommand () {
     nix build --experimental-features 'nix-command ca-derivations' --file ./content-addressed.nix --no-link
 }
 
-testRemoteCache
+# Disabled until we have it properly working
+# testRemoteCache
 testDeterministicCA
 testCutoff
 testGC