Merge remote-tracking branch 'origin/master' into substitute-other-storedir

19 files changed, 318 insertions, 111 deletions

diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 8980bc09d..2e2a17b14 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -6,11 +6,11 @@
 namespace nix {
 
 
-static Strings parseAttrPath(const string & s)
+static Strings parseAttrPath(std::string_view s)
 {
     Strings res;
     string cur;
-    string::const_iterator i = s.begin();
+    auto i = s.begin();
     while (i != s.end()) {
         if (*i == '.') {
             res.push_back(cur);
@@ -32,6 +32,15 @@ static Strings parseAttrPath(const string & s)
 }
 
 
+std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s)
+{
+    std::vector<Symbol> res;
+    for (auto & a : parseAttrPath(s))
+        res.push_back(state.symbols.create(a));
+    return res;
+}
+
+
 std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attrPath,
     Bindings & autoArgs, Value & vIn)
 {
diff --git a/src/libexpr/attr-path.hh b/src/libexpr/attr-path.hh
index fce160da7..d9d74ab2d 100644
--- a/src/libexpr/attr-path.hh
+++ b/src/libexpr/attr-path.hh
@@ -16,4 +16,6 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 /* Heuristic to find the filename and lineno or a nix value. */
 Pos findDerivationFilename(EvalState & state, Value & v, std::string what);
 
+std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);
+
 }
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 8e71db2b8..b90a64357 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -161,12 +161,12 @@ const Value *getPrimOp(const Value &v) {
 }
 
 
-string showType(const Value & v)
+string showType(ValueType type)
 {
-    switch (v.type) {
+    switch (type) {
         case tInt: return "an integer";
-        case tBool: return "a boolean";
-        case tString: return v.string.context ? "a string with context" : "a string";
+        case tBool: return "a Boolean";
+        case tString: return "a string";
         case tPath: return "a path";
         case tNull: return "null";
         case tAttrs: return "a set";
@@ -175,14 +175,27 @@ string showType(const Value & v)
         case tApp: return "a function application";
         case tLambda: return "a function";
         case tBlackhole: return "a black hole";
+        case tPrimOp: return "a built-in function";
+        case tPrimOpApp: return "a partially applied built-in function";
+        case tExternal: return "an external value";
+        case tFloat: return "a float";
+    }
+    abort();
+}
+
+
+string showType(const Value & v)
+{
+    switch (v.type) {
+        case tString: return v.string.context ? "a string with context" : "a string";
         case tPrimOp:
             return fmt("the built-in function '%s'", string(v.primOp->name));
         case tPrimOpApp:
             return fmt("the partially applied built-in function '%s'", string(getPrimOp(v)->primOp->name));
         case tExternal: return v.external->showType();
-        case tFloat: return "a float";
+    default:
+        return showType(v.type);
     }
-    abort();
 }
 
 
@@ -323,6 +336,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sOutputHash(symbols.create("outputHash"))
     , sOutputHashAlgo(symbols.create("outputHashAlgo"))
     , sOutputHashMode(symbols.create("outputHashMode"))
+    , sRecurseForDerivations(symbols.create("recurseForDerivations"))
     , repair(NoRepair)
     , store(store)
     , baseEnv(allocEnv(128))
@@ -471,14 +485,21 @@ Value * EvalState::addConstant(const string & name, Value & v)
 Value * EvalState::addPrimOp(const string & name,
     size_t arity, PrimOpFun primOp)
 {
+    auto name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
+    Symbol sym = symbols.create(name2);
+
+    /* Hack to make constants lazy: turn them into a application of
+       the primop to a dummy value. */
     if (arity == 0) {
+        auto vPrimOp = allocValue();
+        vPrimOp->type = tPrimOp;
+        vPrimOp->primOp = new PrimOp(primOp, 1, sym);
         Value v;
-        primOp(*this, noPos, nullptr, v);
+        mkApp(v, *vPrimOp, *vPrimOp);
         return addConstant(name, v);
     }
+
     Value * v = allocValue();
-    string name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
-    Symbol sym = symbols.create(name2);
     v->type = tPrimOp;
     v->primOp = new PrimOp(primOp, arity, sym);
     staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 1485dc7fe..863365259 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -18,7 +18,7 @@ namespace nix {
 
 class Store;
 class EvalState;
-struct StorePath;
+class StorePath;
 enum RepairFlag : bool;
 
 
@@ -74,7 +74,8 @@ public:
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
         sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
-        sOutputHash, sOutputHashAlgo, sOutputHashMode;
+        sOutputHash, sOutputHashAlgo, sOutputHashMode,
+        sRecurseForDerivations;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
@@ -324,6 +325,7 @@ private:
 
 
 /* Return a string representing the type of the value `v'. */
+string showType(ValueType type);
 string showType(const Value & v);
 
 /* Decode a context string ‘!<name>!<path>’ into a pair <path,
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index ca9c547fa..a4937e722 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -348,7 +348,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                    should we recurse into it?  => Only if it has a
                    `recurseForDerivations = true' attribute. */
                 if (i->value->type == tAttrs) {
-                    Bindings::iterator j = i->value->attrs->find(state.symbols.create("recurseForDerivations"));
+                    Bindings::iterator j = i->value->attrs->find(state.sRecurseForDerivations);
                     if (j != i->value->attrs->end() && state.forceBool(*j->value, *j->pos))
                         getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 907f15246..fe5358962 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -50,20 +50,20 @@ void EvalState::realiseContext(const PathSet & context)
     std::vector<StorePathWithOutputs> drvs;
 
     for (auto & i : context) {
-        std::pair<string, string> decoded = decodeContext(i);
-        auto ctx = store->parseStorePath(decoded.first);
+        auto [ctxS, outputName] = decodeContext(i);
+        auto ctx = store->parseStorePath(ctxS);
         if (!store->isValidPath(ctx))
             throw InvalidPathError(store->printStorePath(ctx));
-        if (!decoded.second.empty() && ctx.isDerivation()) {
-            drvs.push_back(StorePathWithOutputs{ctx, {decoded.second}});
+        if (!outputName.empty() && ctx.isDerivation()) {
+            drvs.push_back(StorePathWithOutputs{ctx, {outputName}});
 
             /* Add the output of this derivation to the allowed
                paths. */
             if (allowedPaths) {
-                auto drv = store->derivationFromPath(store->parseStorePath(decoded.first));
-                DerivationOutputs::iterator i = drv.outputs.find(decoded.second);
+                auto drv = store->derivationFromPath(ctx);
+                DerivationOutputs::iterator i = drv.outputs.find(outputName);
                 if (i == drv.outputs.end())
-                    throw Error("derivation '%s' does not have an output named '%s'", decoded.first, decoded.second);
+                    throw Error("derivation '%s' does not have an output named '%s'", ctxS, outputName);
                 allowedPaths->insert(store->printStorePath(i->second.path));
             }
         }
@@ -79,6 +79,7 @@ void EvalState::realiseContext(const PathSet & context)
     StorePathSet willBuild, willSubstitute, unknown;
     unsigned long long downloadSize, narSize;
     store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
+
     store->buildPaths(drvs);
 }
 
@@ -775,10 +776,11 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         auto outPath = state.store->makeFixedOutputPath(ingestionMethod, h, drvName);
         if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);
         drv.outputs.insert_or_assign("out", DerivationOutput {
-            std::move(outPath),
-            (ingestionMethod == FileIngestionMethod::Recursive ? "r:" : "")
-                + printHashType(h.type),
-            h.to_string(Base16, false),
+            .path = std::move(outPath),
+            .hash = DerivationOutputHash {
+                .method = ingestionMethod,
+                .hash = std::move(h),
+            },
         });
     }
 
@@ -792,7 +794,10 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         for (auto & i : outputs) {
             if (!jsonObject) drv.env[i] = "";
             drv.outputs.insert_or_assign(i,
-                DerivationOutput { StorePath::dummy, "", "" });
+                DerivationOutput {
+                    .path = StorePath::dummy,
+                    .hash = std::optional<DerivationOutputHash> {},
+                });
         }
 
         Hash h = hashDerivationModulo(*state.store, Derivation(drv), true);
@@ -801,7 +806,10 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             auto outPath = state.store->makeOutputPath(i, h, drvName);
             if (!jsonObject) drv.env[i] = state.store->printStorePath(outPath);
             drv.outputs.insert_or_assign(i,
-                DerivationOutput { std::move(outPath), "", "" });
+                DerivationOutput {
+                    .path = std::move(outPath),
+                    .hash = std::optional<DerivationOutputHash>(),
+                });
         }
     }
 
@@ -2201,10 +2209,11 @@ static void prim_splitVersion(EvalState & state, const Pos & pos, Value * * args
 RegisterPrimOp::PrimOps * RegisterPrimOp::primOps;
 
 
-RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun)
+RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun,
+    std::optional<std::string> requiredFeature)
 {
     if (!primOps) primOps = new PrimOps;
-    primOps->emplace_back(name, arity, fun);
+    primOps->push_back({name, arity, fun, requiredFeature});
 }
 
 
@@ -2396,7 +2405,8 @@ void EvalState::createBaseEnv()
 
     if (RegisterPrimOp::primOps)
         for (auto & primOp : *RegisterPrimOp::primOps)
-            addPrimOp(std::get<0>(primOp), std::get<1>(primOp), std::get<2>(primOp));
+            if (!primOp.requiredFeature || settings.isExperimentalFeatureEnabled(*primOp.requiredFeature))
+                addPrimOp(primOp.name, primOp.arity, primOp.primOp);
 
     /* Now that we've added all primops, sort the `builtins' set,
        because attribute lookups expect it to be sorted. */
diff --git a/src/libexpr/primops.hh b/src/libexpr/primops.hh
index 05d0792ef..75c460ecf 100644
--- a/src/libexpr/primops.hh
+++ b/src/libexpr/primops.hh
@@ -7,12 +7,25 @@ namespace nix {
 
 struct RegisterPrimOp
 {
-    typedef std::vector<std::tuple<std::string, size_t, PrimOpFun>> PrimOps;
+    struct Info
+    {
+        std::string name;
+        size_t arity;
+        PrimOpFun primOp;
+        std::optional<std::string> requiredFeature;
+    };
+
+    typedef std::vector<Info> PrimOps;
     static PrimOps * primOps;
+
     /* You can register a constant by passing an arity of 0. fun
        will get called during EvalState initialization, so there
        may be primops not yet added and builtins is not yet sorted. */
-    RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun);
+    RegisterPrimOp(
+        std::string name,
+        size_t arity,
+        PrimOpFun fun,
+        std::optional<std::string> requiredFeature = {});
 };
 
 /* These primops are disabled without enableNativeCode, but plugins
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index dc6d5e413..1cb422967 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -48,7 +48,10 @@ void printMissing(ref<Store> store, const StorePathSet & willBuild,
     unsigned long long downloadSize, unsigned long long narSize, Verbosity lvl)
 {
     if (!willBuild.empty()) {
-        printMsg(lvl, "these derivations will be built:");
+        if (willBuild.size() == 1)
+            printMsg(lvl, fmt("this derivation will be built:"));
+        else
+            printMsg(lvl, fmt("these %d derivations will be built:", willBuild.size()));
         auto sorted = store->topoSortPaths(willBuild);
         reverse(sorted.begin(), sorted.end());
         for (auto & i : sorted)
@@ -56,9 +59,18 @@ void printMissing(ref<Store> store, const StorePathSet & willBuild,
     }
 
     if (!willSubstitute.empty()) {
-        printMsg(lvl, fmt("these paths will be fetched (%.2f MiB download, %.2f MiB unpacked):",
-                downloadSize / (1024.0 * 1024.0),
-                narSize / (1024.0 * 1024.0)));
+        const float downloadSizeMiB = downloadSize / (1024.f * 1024.f);
+        const float narSizeMiB = narSize / (1024.f * 1024.f);
+        if (willSubstitute.size() == 1) {
+            printMsg(lvl, fmt("this path will be fetched (%.2f MiB download, %.2f MiB unpacked):",
+                downloadSizeMiB,
+                narSizeMiB));
+        } else {
+            printMsg(lvl, fmt("these %d paths will be fetched (%.2f MiB download, %.2f MiB unpacked):",
+                willSubstitute.size(),
+                downloadSizeMiB,
+                narSizeMiB));
+        }
         for (auto & i : willSubstitute)
             printMsg(lvl, fmt("  %s", store->printStorePath(i)));
     }
diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index f8eff508c..9f52ddafa 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -388,8 +388,6 @@ void BinaryCacheStore::addSignatures(const StorePath & storePath, const StringSe
 
     narInfo->sigs.insert(sigs.begin(), sigs.end());
 
-    auto narInfoFile = narInfoFileFor(narInfo->path);
-
     writeNarInfo(narInfo);
 }
 
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 63cb70ef2..bd2b38833 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -86,7 +86,7 @@ struct HookInstance;
 
 
 /* A pointer to a goal. */
-class Goal;
+struct Goal;
 class DerivationGoal;
 typedef std::shared_ptr<Goal> GoalPtr;
 typedef std::weak_ptr<Goal> WeakGoalPtr;
@@ -1196,8 +1196,8 @@ void DerivationGoal::haveDerivation()
     parsedDrv = std::make_unique<ParsedDerivation>(drvPath, *drv);
 
     if (parsedDrv->contentAddressed()) {
-        settings.requireExperimentalFeature("content-addressed-paths");
-        throw Error("content-addressed-paths isn't implemented yet");
+        settings.requireExperimentalFeature("ca-derivations");
+        throw Error("ca-derivations isn't implemented yet");
     }
 
 
@@ -3718,10 +3718,7 @@ void DerivationGoal::registerOutputs()
 
         if (fixedOutput) {
 
-            FileIngestionMethod outputHashMode; Hash h;
-            i.second.parseHashInfo(outputHashMode, h);
-
-            if (outputHashMode == FileIngestionMethod::Flat) {
+            if (i.second.hash->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(
@@ -3732,20 +3729,22 @@ void DerivationGoal::registerOutputs()
 
             /* Check the hash. In hash mode, move the path produced by
                the derivation to its content-addressed location. */
-            Hash h2 = outputHashMode == FileIngestionMethod::Recursive
-                ? hashPath(h.type, actualPath).first
-                : hashFile(h.type, actualPath);
+            Hash h2 = i.second.hash->method == FileIngestionMethod::Recursive
+                ? hashPath(i.second.hash->hash.type, actualPath).first
+                : hashFile(i.second.hash->hash.type, actualPath);
 
-            auto dest = worker.store.makeFixedOutputPath(outputHashMode, h2, i.second.path.name());
+            auto dest = worker.store.makeFixedOutputPath(i.second.hash->method, h2, i.second.path.name());
 
-            if (h != h2) {
+            if (i.second.hash->hash != h2) {
 
                 /* Throw an error after registering the path as
                    valid. */
                 worker.hashMismatch = true;
                 delayedException = std::make_exception_ptr(
                     BuildError("hash mismatch in fixed-output derivation '%s':\n  wanted: %s\n  got:    %s",
-                        worker.store.printStorePath(dest), h.to_string(SRI, true), h2.to_string(SRI, true)));
+                        worker.store.printStorePath(dest),
+                        i.second.hash->hash.to_string(SRI, true),
+                        h2.to_string(SRI, true)));
 
                 Path actualDest = worker.store.Store::toRealPath(dest);
 
@@ -3765,7 +3764,7 @@ void DerivationGoal::registerOutputs()
             else
                 assert(worker.store.parseStorePath(path) == dest);
 
-            ca = makeFixedOutputCA(outputHashMode, h2);
+            ca = makeFixedOutputCA(i.second.hash->method, h2);
         }
 
         /* Get rid of all weird permissions.  This also checks that
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index b95f7bfdc..528b7ccea 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -8,22 +8,8 @@
 
 namespace nix {
 
-
-void DerivationOutput::parseHashInfo(FileIngestionMethod & recursive, Hash & hash) const
-{
-    recursive = FileIngestionMethod::Flat;
-    string algo = hashAlgo;
-
-    if (string(algo, 0, 2) == "r:") {
-        recursive = FileIngestionMethod::Recursive;
-        algo = string(algo, 2);
-    }
-
-    HashType hashType = parseHashType(algo);
-    if (hashType == htUnknown)
-        throw Error("unknown hash algorithm '%s'", algo);
-
-    hash = Hash(this->hash, hashType);
+std::string DerivationOutputHash::printMethodAlgo() const {
+    return makeFileIngestionPrefix(method) + printHashType(hash.type);
 }
 
 
@@ -120,6 +106,36 @@ static StringSet parseStrings(std::istream & str, bool arePaths)
 }
 
 
+static DerivationOutput parseDerivationOutput(const Store & store, istringstream_nocopy & str)
+{
+    expect(str, ","); auto path = store.parseStorePath(parsePath(str));
+    expect(str, ","); auto hashAlgo = parseString(str);
+    expect(str, ","); const auto hash = parseString(str);
+    expect(str, ")");
+
+    std::optional<DerivationOutputHash> fsh;
+    if (hashAlgo != "") {
+        auto method = FileIngestionMethod::Flat;
+        if (string(hashAlgo, 0, 2) == "r:") {
+            method = FileIngestionMethod::Recursive;
+            hashAlgo = string(hashAlgo, 2);
+        }
+        const HashType hashType = parseHashType(hashAlgo);
+        if (hashType == htUnknown)
+            throw Error("unknown hash hashAlgorithm '%s'", hashAlgo);
+        fsh = DerivationOutputHash {
+            .method = std::move(method),
+            .hash = Hash(hash, hashType),
+        };
+    }
+
+    return DerivationOutput {
+        .path = std::move(path),
+        .hash = std::move(fsh),
+    };
+}
+
+
 static Derivation parseDerivation(const Store & store, const string & s)
 {
     Derivation drv;
@@ -129,15 +145,8 @@ static Derivation parseDerivation(const Store & store, const string & s)
     /* Parse the list of outputs. */
     while (!endOfList(str)) {
         expect(str, "("); std::string id = parseString(str);
-        expect(str, ","); auto path = store.parseStorePath(parsePath(str));
-        expect(str, ","); auto hashAlgo = parseString(str);
-        expect(str, ","); auto hash = parseString(str);
-        expect(str, ")");
-        drv.outputs.emplace(id, DerivationOutput {
-            .path = std::move(path),
-            .hashAlgo = std::move(hashAlgo),
-            .hash = std::move(hash)
-        });
+        auto output = parseDerivationOutput(store, str);
+        drv.outputs.emplace(std::move(id), std::move(output));
     }
 
     /* Parse the list of input derivations. */
@@ -263,8 +272,9 @@ string Derivation::unparse(const Store & store, bool maskOutputs,
         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.hashAlgo);
-        s += ','; printUnquotedString(s, i.second.hash);
+        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 += ')';
     }
 
@@ -320,7 +330,7 @@ bool BasicDerivation::isFixedOutput() const
 {
     return outputs.size() == 1 &&
         outputs.begin()->first == "out" &&
-        outputs.begin()->second.hash != "";
+        outputs.begin()->second.hash;
 }
 
 
@@ -353,8 +363,8 @@ Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutput
     if (drv.isFixedOutput()) {
         DerivationOutputs::const_iterator i = drv.outputs.begin();
         return hashString(htSHA256, "fixed:out:"
-            + i->second.hashAlgo + ":"
-            + i->second.hash + ":"
+            + i->second.hash->printMethodAlgo() + ":"
+            + i->second.hash->hash.to_string(Base16, false) + ":"
             + store.printStorePath(i->second.path));
     }
 
@@ -397,6 +407,33 @@ StorePathSet BasicDerivation::outputPaths() const
     return paths;
 }
 
+static DerivationOutput readDerivationOutput(Source & in, const Store & store)
+{
+    auto path = store.parseStorePath(readString(in));
+    auto hashAlgo = readString(in);
+    const auto hash = readString(in);
+
+    std::optional<DerivationOutputHash> fsh;
+    if (hashAlgo != "") {
+        auto method = FileIngestionMethod::Flat;
+        if (string(hashAlgo, 0, 2) == "r:") {
+            method = FileIngestionMethod::Recursive;
+            hashAlgo = string(hashAlgo, 2);
+        }
+        const HashType hashType = parseHashType(hashAlgo);
+        if (hashType == htUnknown)
+            throw Error("unknown hash hashAlgorithm '%s'", hashAlgo);
+        fsh = DerivationOutputHash {
+            .method = std::move(method),
+            .hash = Hash(hash, hashType),
+        };
+    }
+
+    return DerivationOutput {
+        .path = std::move(path),
+        .hash = std::move(fsh),
+    };
+}
 
 StringSet BasicDerivation::outputNames() const
 {
@@ -413,14 +450,8 @@ Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv)
     auto nr = readNum<size_t>(in);
     for (size_t n = 0; n < nr; n++) {
         auto name = readString(in);
-        auto path = store.parseStorePath(readString(in));
-        auto hashAlgo = readString(in);
-        auto hash = readString(in);
-        drv.outputs.emplace(name, DerivationOutput {
-            .path = std::move(path),
-            .hashAlgo = std::move(hashAlgo),
-            .hash = std::move(hash)
-        });
+        auto output = readDerivationOutput(in, store);
+        drv.outputs.emplace(std::move(name), std::move(output));
     }
 
     drv.inputSrcs = readStorePaths<StorePathSet>(store, in);
@@ -442,7 +473,10 @@ 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) << i.second.hashAlgo << i.second.hash;
+        out << i.first
+            << store.printStorePath(i.second.path)
+            << i.second.hash->printMethodAlgo()
+            << i.second.hash->hash.to_string(Base16, false);
     writeStorePaths(store, out, drv.inputSrcs);
     out << drv.platform << drv.builder << drv.args;
     out << drv.env.size();
diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index d349c6d4d..7b677ca49 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -12,11 +12,17 @@ namespace nix {
 
 /* Abstract syntax of derivations. */
 
+/// Pair of a hash, and how the file system was ingested
+struct DerivationOutputHash {
+    FileIngestionMethod method;
+    Hash hash;
+    std::string printMethodAlgo() const;
+};
+
 struct DerivationOutput
 {
     StorePath path;
-    std::string hashAlgo; /* hash used for expected hash computation */
-    std::string hash; /* expected hash, may be null */
+    std::optional<DerivationOutputHash> hash; /* hash used for expected hash computation */
     void parseHashInfo(FileIngestionMethod & recursive, Hash & hash) const;
 };
 
diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index c954ace7f..531b85af8 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -72,6 +72,7 @@ struct curlFileTransfer : public FileTransfer
 
         curl_off_t writtenToSink = 0;
 
+        inline static const std::set<long> successfulStatuses {200, 201, 204, 206, 304, 0 /* other protocol */};
         /* Get the HTTP status code, or 0 for other protocols. */
         long getHTTPStatus()
         {
@@ -98,7 +99,7 @@ struct curlFileTransfer : public FileTransfer
 
                     /* Only write data to the sink if this is a
                        successful response. */
-                    if (httpStatus == 0 || httpStatus == 200 || httpStatus == 201 || httpStatus == 206) {
+                    if (successfulStatuses.count(httpStatus)) {
                         writtenToSink += len;
                         this->request.dataCallback((char *) data, len);
                     }
@@ -352,8 +353,7 @@ struct curlFileTransfer : public FileTransfer
             if (writeException)
                 failEx(writeException);
 
-            else if (code == CURLE_OK &&
-                (httpStatus == 200 || httpStatus == 201 || httpStatus == 204 || httpStatus == 206 || httpStatus == 304 || httpStatus == 0 /* other protocol */))
+            else if (code == CURLE_OK && successfulStatuses.count(httpStatus))
             {
                 result.cached = httpStatus == 304;
                 act.progress(result.bodySize, result.bodySize);
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 3901d532a..ca6f3562a 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -561,10 +561,12 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
         if (out == drv.outputs.end())
             throw Error("derivation '%s' does not have an output named 'out'", printStorePath(drvPath));
 
-        FileIngestionMethod method; Hash h;
-        out->second.parseHashInfo(method, h);
-
-        check(makeFixedOutputPath(method, h, drvName), out->second.path, "out");
+        check(
+            makeFixedOutputPath(
+                out->second.hash->method,
+                out->second.hash->hash,
+                drvName),
+            out->second.path, "out");
     }
 
     else {
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index c8097da52..661dea141 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -173,20 +173,20 @@ static std::string makeType(
 
 
 StorePath Store::makeFixedOutputPath(
-    FileIngestionMethod recursive,
+    FileIngestionMethod method,
     const Hash & hash,
     std::string_view name,
     const StorePathSet & references,
     bool hasSelfReference) const
 {
-    if (hash.type == htSHA256 && recursive == FileIngestionMethod::Recursive) {
+    if (hash.type == htSHA256 && method == FileIngestionMethod::Recursive) {
         return makeStorePath(makeType(*this, "source", references, hasSelfReference), hash, name);
     } else {
         assert(references.empty());
         return makeStorePath("output:out",
             hashString(htSHA256,
                 "fixed:out:"
-                + (recursive == FileIngestionMethod::Recursive ? (string) "r:" : "")
+                + makeFileIngestionPrefix(method)
                 + hash.to_string(Base16, true) + ":"),
             name);
     }
@@ -864,10 +864,21 @@ Strings ValidPathInfo::shortRefs() const
 }
 
 
-std::string makeFixedOutputCA(FileIngestionMethod recursive, const Hash & hash)
+std::string makeFileIngestionPrefix(const FileIngestionMethod m) {
+    switch (m) {
+    case FileIngestionMethod::Flat:
+        return "";
+    case FileIngestionMethod::Recursive:
+        return "r:";
+    default:
+        throw Error("impossible, caught both cases");
+    }
+}
+
+std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)
 {
     return "fixed:"
-        + (recursive == FileIngestionMethod::Recursive ? (std::string) "r:" : "")
+        + makeFileIngestionPrefix(method)
         + hash.to_string(Base32, true);
 }
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 12a6b6939..fd3652588 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -189,9 +189,10 @@ struct ValidPathInfo
 
     Strings shortRefs() const;
 
-    ValidPathInfo(const StorePath & path) : path(path) { }
+    ValidPathInfo(const ValidPathInfo & other) = default;
 
-    ValidPathInfo(StorePath && path) : path(std::move(path)) { }
+    ValidPathInfo(StorePath && path) : path(std::move(path)) { };
+    ValidPathInfo(const StorePath & path) : path(path) { };
 
     virtual ~ValidPathInfo() { }
 };
@@ -842,6 +843,9 @@ std::optional<ValidPathInfo> decodeValidPathInfo(
     std::istream & str,
     bool hashGiven = false);
 
+/* Compute the prefix to the hash algorithm which indicates how the files were
+   ingested. */
+std::string makeFileIngestionPrefix(const FileIngestionMethod m);
 
 /* Compute the content-addressability assertion (ValidPathInfo::ca)
    for paths created by makeFixedOutputPath() / addToStore(). */
diff --git a/src/libutil/tests/compression.cc b/src/libutil/tests/compression.cc
new file mode 100644
index 000000000..5b7a2c5b9
--- /dev/null
+++ b/src/libutil/tests/compression.cc
@@ -0,0 +1,78 @@
+#include "compression.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    /* ----------------------------------------------------------------------------
+     * compress / decompress
+     * --------------------------------------------------------------------------*/
+
+    TEST(compress, compressWithUnknownMethod) {
+        ASSERT_THROW(compress("invalid-method", "something-to-compress"), UnknownCompressionMethod);
+    }
+
+    TEST(compress, noneMethodDoesNothingToTheInput) {
+        ref<std::string> o = compress("none", "this-is-a-test");
+
+        ASSERT_EQ(*o, "this-is-a-test");
+    }
+
+    TEST(decompress, decompressXzCompressed) {
+        auto method = "xz";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        ref<std::string> o = decompress(method, *compress(method, str));
+
+        ASSERT_EQ(*o, str);
+    }
+
+    TEST(decompress, decompressBzip2Compressed) {
+        auto method = "bzip2";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        ref<std::string> o = decompress(method, *compress(method, str));
+
+        ASSERT_EQ(*o, str);
+    }
+
+    TEST(decompress, decompressBrCompressed) {
+        auto method = "br";
+        auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        ref<std::string> o = decompress(method, *compress(method, str));
+
+        ASSERT_EQ(*o, str);
+    }
+
+    TEST(decompress, decompressInvalidInputThrowsCompressionError) {
+        auto method = "bzip2";
+        auto str = "this is a string that does not qualify as valid bzip2 data";
+
+        ASSERT_THROW(decompress(method, str), CompressionError);
+    }
+
+    /* ----------------------------------------------------------------------------
+     * compression sinks
+     * --------------------------------------------------------------------------*/
+
+    TEST(makeCompressionSink, noneSinkDoesNothingToInput) {
+        StringSink strSink;
+        auto inputString = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto sink = makeCompressionSink("none", strSink);
+        (*sink)(inputString);
+        sink->finish();
+
+        ASSERT_STREQ((*strSink.s).c_str(), inputString);
+    }
+
+    TEST(makeCompressionSink, compressAndDecompress) {
+        StringSink strSink;
+        auto inputString = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+        auto decompressionSink = makeDecompressionSink("bzip2", strSink);
+        auto sink = makeCompressionSink("bzip2", *decompressionSink);
+
+        (*sink)(inputString);
+        sink->finish();
+        decompressionSink->finish();
+
+        ASSERT_STREQ((*strSink.s).c_str(), inputString);
+    }
+
+}
diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index 05a9b9cd9..8b85caf82 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -135,7 +135,13 @@ StorePath getDerivationEnvironment(ref<Store> store, const StorePath & drvPath)
     drv.inputSrcs.insert(std::move(getEnvShPath));
     Hash h = hashDerivationModulo(*store, drv, true);
     auto shellOutPath = store->makeOutputPath("out", h, drvName);
-    drv.outputs.insert_or_assign("out", DerivationOutput { shellOutPath, "", "" });
+    drv.outputs.insert_or_assign("out", DerivationOutput {
+        .path = shellOutPath,
+        .hash = DerivationOutputHash {
+            .method = FileIngestionMethod::Flat,
+            .hash = Hash { },
+        },
+    });
     drv.env["out"] = store->printStorePath(shellOutPath);
     auto shellDrvPath2 = writeDerivation(store, drv, drvName);
 
diff --git a/src/nix/show-derivation.cc b/src/nix/show-derivation.cc
index 2d31894c2..5d77cfdca 100644
--- a/src/nix/show-derivation.cc
+++ b/src/nix/show-derivation.cc
@@ -70,9 +70,9 @@ struct CmdShowDerivation : InstallablesCommand
                 for (auto & output : drv.outputs) {
                     auto outputObj(outputsObj.object(output.first));
                     outputObj.attr("path", store->printStorePath(output.second.path));
-                    if (output.second.hash != "") {
-                        outputObj.attr("hashAlgo", output.second.hashAlgo);
-                        outputObj.attr("hash", output.second.hash);
+                    if (output.second.hash) {
+                        outputObj.attr("hashAlgo", output.second.hash->printMethodAlgo());
+                        outputObj.attr("hash", output.second.hash->hash.to_string(Base16, false));
                     }
                 }
             }