Use BuildableReq for buildPaths and ensurePath

This avoids an ambiguity where the `StorePathWithOutputs { drvPath, {}
}` could mean "build `brvPath`" or "substitute `drvPath`" depending on
context.

It also brings the internals closer in line to the new CLI, by
generalizing the `Buildable` type is used there and makes that
distinction already.

In doing so, relegate `StorePathWithOutputs` to being a type just for
backwards compatibility (CLI and RPC).

src/libcmd/installables.cc

@@ -679,19 +679,20 @@ Buildables build(ref<Store> store, Realise mode,
 
     Buildables buildables;
 
-    std::vector<StorePathWithOutputs> pathsToBuild;
+    std::vector<BuildableReq> pathsToBuild;
 
     for (auto & i : installables) {
         for (auto & b : i->toBuildables()) {
             std::visit(overloaded {
                 [&](BuildableOpaque bo) {
-                    pathsToBuild.push_back({bo.path});
+                    pathsToBuild.push_back(bo);
                 },
                 [&](BuildableFromDrv bfd) {
                     StringSet outputNames;
                     for (auto & output : bfd.outputs)
                         outputNames.insert(output.first);
-                    pathsToBuild.push_back({bfd.drvPath, outputNames});
+                    pathsToBuild.push_back(
+                        BuildableReqFromDrv{bfd.drvPath, outputNames});
                 },
             }, b);
             buildables.push_back(std::move(b));

src/libexpr/get-drvs.cc

@@ -2,6 +2,7 @@
 #include "util.hh"
 #include "eval-inline.hh"
 #include "store-api.hh"
+#include "path-with-outputs.hh"
 
 #include <cstring>
 #include <regex>

src/libexpr/primops.cc

@@ -35,7 +35,7 @@ InvalidPathError::InvalidPathError(const Path & path) :
 
 void EvalState::realiseContext(const PathSet & context)
 {
-    std::vector<StorePathWithOutputs> drvs;
+    std::vector<BuildableReqFromDrv> drvs;
 
     for (auto & i : context) {
         auto [ctxS, outputName] = decodeContext(i);
@@ -43,7 +43,7 @@ void EvalState::realiseContext(const PathSet & context)
         if (!store->isValidPath(ctx))
             throw InvalidPathError(store->printStorePath(ctx));
         if (!outputName.empty() && ctx.isDerivation()) {
-            drvs.push_back(StorePathWithOutputs{ctx, {outputName}});
+            drvs.push_back({ctx, {outputName}});
         }
     }
 
@@ -51,14 +51,16 @@ void EvalState::realiseContext(const PathSet & context)
 
     if (!evalSettings.enableImportFromDerivation)
         throw EvalError("attempted to realize '%1%' during evaluation but 'allow-import-from-derivation' is false",
-            store->printStorePath(drvs.begin()->path));
+            store->printStorePath(drvs.begin()->drvPath));
 
     /* For performance, prefetch all substitute info. */
     StorePathSet willBuild, willSubstitute, unknown;
     uint64_t downloadSize, narSize;
-    store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
+    std::vector<BuildableReq> buildReqs;
+    for (auto & d : drvs) buildReqs.emplace_back(BuildableReq { d });
+    store->queryMissing(buildReqs, willBuild, willSubstitute, unknown, downloadSize, narSize);
 
-    store->buildPaths(drvs);
+    store->buildPaths(buildReqs);
 
     /* Add the output of this derivations to the allowed
        paths. */

src/libmain/shared.cc

@@ -36,7 +36,7 @@ void printGCWarning()
 }
 
 
-void printMissing(ref<Store> store, const std::vector<StorePathWithOutputs> & paths, Verbosity lvl)
+void printMissing(ref<Store> store, const std::vector<BuildableReq> & paths, Verbosity lvl)
 {
     uint64_t downloadSize, narSize;
     StorePathSet willBuild, willSubstitute, unknown;

src/libmain/shared.hh

@@ -4,6 +4,7 @@
 #include "args.hh"
 #include "common-args.hh"
 #include "path.hh"
+#include "buildable.hh"
 
 #include <signal.h>
 
@@ -42,7 +43,7 @@ struct StorePathWithOutputs;
 
 void printMissing(
     ref<Store> store,
-    const std::vector<StorePathWithOutputs> & paths,
+    const std::vector<BuildableReq> & paths,
     Verbosity lvl = lvlInfo);
 
 void printMissing(ref<Store> store, const StorePathSet & willBuild,

src/libstore/build/derivation-goal.cc

@@ -73,7 +73,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath,
     state = &DerivationGoal::getDerivation;
     name = fmt(
         "building of '%s' from .drv file",
-        StorePathWithOutputs { drvPath, wantedOutputs }.to_string(worker.store));
+        to_string(worker.store, BuildableReqFromDrv { drvPath, wantedOutputs }));
     trace("created");
 
     mcExpectedBuilds = std::make_unique<MaintainCount<uint64_t>>(worker.expectedBuilds);
@@ -94,7 +94,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath, const BasicDerivation
     state = &DerivationGoal::haveDerivation;
     name = fmt(
         "building of '%s' from in-memory derivation",
-        StorePathWithOutputs { drvPath, drv.outputNames() }.to_string(worker.store));
+        to_string(worker.store, BuildableReqFromDrv { drvPath, drv.outputNames() }));
     trace("created");
 
     mcExpectedBuilds = std::make_unique<MaintainCount<uint64_t>>(worker.expectedBuilds);

src/libstore/build/entry-points.cc

@@ -6,16 +6,20 @@
 
 namespace nix {
 
-void Store::buildPaths(const std::vector<StorePathWithOutputs> & drvPaths, BuildMode buildMode)
+void Store::buildPaths(const std::vector<BuildableReq> & reqs, BuildMode buildMode)
 {
     Worker worker(*this);
 
     Goals goals;
-    for (auto & path : drvPaths) {
+    for (auto & br : reqs) {
-        if (path.path.isDerivation())
+        std::visit(overloaded {
-            goals.insert(worker.makeDerivationGoal(path.path, path.outputs, buildMode));
+            [&](BuildableReqFromDrv bfd) {
-        else
+                goals.insert(worker.makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode));
-            goals.insert(worker.makePathSubstitutionGoal(path.path, buildMode == bmRepair ? Repair : NoRepair));
+            },
+            [&](BuildableOpaque bo) {
+                goals.insert(worker.makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair));
+            },
+        }, br);
     }
 
     worker.run(goals);

src/libstore/build/local-derivation-goal.cc

@@ -1190,6 +1190,26 @@ void LocalDerivationGoal::writeStructuredAttrs()
     chownToBuilder(tmpDir + "/.attrs.sh");
 }
 
+
+static StorePath pathPartOfReq(const BuildableReq & req)
+{
+    return std::visit(overloaded {
+        [&](BuildableOpaque bo) {
+            return bo.path;
+        },
+        [&](BuildableReqFromDrv bfd)  {
+            return bfd.drvPath;
+        },
+    }, req);
+}
+
+
+bool LocalDerivationGoal::isAllowed(const BuildableReq & req)
+{
+    return this->isAllowed(pathPartOfReq(req));
+}
+
+
 struct RestrictedStoreConfig : virtual LocalFSStoreConfig
 {
     using LocalFSStoreConfig::LocalFSStoreConfig;
@@ -1312,25 +1332,27 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
     // an allowed derivation
     { throw Error("queryRealisation"); }
 
-    void buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode) override
+    void buildPaths(const std::vector<BuildableReq> & paths, BuildMode buildMode) override
     {
         if (buildMode != bmNormal) throw Error("unsupported build mode");
 
         StorePathSet newPaths;
 
-        for (auto & path : paths) {
+        for (auto & req : paths) {
-            if (!goal.isAllowed(path.path))
+            if (!goal.isAllowed(req))
-                throw InvalidPath("cannot build unknown path '%s' in recursive Nix", printStorePath(path.path));
+                throw InvalidPath("cannot build '%s' in recursive Nix because path is unknown", to_string(*next, req));
         }
 
         next->buildPaths(paths, buildMode);
 
         for (auto & path : paths) {
-            if (!path.path.isDerivation()) continue;
+            auto p =  std::get_if<BuildableReqFromDrv>(&path);
-            auto outputs = next->queryDerivationOutputMap(path.path);
+            if (!p) continue;
-            for (auto & output : outputs)
+            auto & bfd = *p;
-                if (wantOutput(output.first, path.outputs))
+            auto outputs = next->queryDerivationOutputMap(bfd.drvPath);
-                    newPaths.insert(output.second);
+            for (auto & [outputName, outputPath] : outputs)
+                if (wantOutput(outputName, bfd.outputs))
+                    newPaths.insert(outputPath);
         }
 
         StorePathSet closure;
@@ -1358,7 +1380,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
     void addSignatures(const StorePath & storePath, const StringSet & sigs) override
     { unsupported("addSignatures"); }
 
-    void queryMissing(const std::vector<StorePathWithOutputs> & targets,
+    void queryMissing(const std::vector<BuildableReq> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
         uint64_t & downloadSize, uint64_t & narSize) override
     {
@@ -1366,12 +1388,12 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
            client about what paths will be built/substituted or are
            already present. Probably not a big deal. */
 
-        std::vector<StorePathWithOutputs> allowed;
+        std::vector<BuildableReq> allowed;
-        for (auto & path : targets) {
+        for (auto & req : targets) {
-            if (goal.isAllowed(path.path))
+            if (goal.isAllowed(req))
-                allowed.emplace_back(path);
+                allowed.emplace_back(req);
             else
-                unknown.insert(path.path);
+                unknown.insert(pathPartOfReq(req));
         }
 
         next->queryMissing(allowed, willBuild, willSubstitute,

src/libstore/build/local-derivation-goal.hh

@@ -116,6 +116,7 @@ struct LocalDerivationGoal : public DerivationGoal
     {
         return inputPaths.count(path) || addedPaths.count(path);
     }
+    bool isAllowed(const BuildableReq & req);
 
     friend struct RestrictedStore;
 

src/libstore/build/worker.cc

@@ -226,14 +226,14 @@ void Worker::waitForAWhile(GoalPtr goal)
 
 void Worker::run(const Goals & _topGoals)
 {
-    std::vector<nix::StorePathWithOutputs> topPaths;
+    std::vector<nix::BuildableReq> topPaths;
 
     for (auto & i : _topGoals) {
         topGoals.insert(i);
         if (auto goal = dynamic_cast<DerivationGoal *>(i.get())) {
-            topPaths.push_back({goal->drvPath, goal->wantedOutputs});
+            topPaths.push_back(BuildableReqFromDrv{goal->drvPath, goal->wantedOutputs});
         } else if (auto goal = dynamic_cast<PathSubstitutionGoal *>(i.get())) {
-            topPaths.push_back({goal->storePath});
+            topPaths.push_back(BuildableOpaque{goal->storePath});
         }
     }
 

src/libstore/buildable.cc

@@ -11,6 +11,7 @@ nlohmann::json BuildableOpaque::toJSON(ref<Store> store) const {
     return res;
 }
 
+template<>
 nlohmann::json BuildableFromDrv::toJSON(ref<Store> store) const {
     nlohmann::json res;
     res["drvPath"] = store->printStorePath(drvPath);
@@ -30,4 +31,50 @@ nlohmann::json buildablesToJSON(const Buildables & buildables, ref<Store> store)
     return res;
 }
 
+
+std::string BuildableOpaque::to_string(const Store & store) const {
+    return store.printStorePath(path);
+}
+
+template<>
+std::string BuildableReqFromDrv::to_string(const Store & store) const {
+    return store.printStorePath(drvPath)
+        + "!"
+        + (outputs.empty() ? std::string { "*" } : concatStringsSep(",", outputs));
+}
+
+std::string to_string(const Store & store, const BuildableReq & req)
+{
+    return std::visit(
+        [&](const auto & req) { return req.to_string(store); },
+        req);
+}
+
+
+BuildableOpaque BuildableOpaque::parse(const Store & store, std::string_view s)
+{
+    return {store.parseStorePath(s)};
+}
+
+template<>
+BuildableReqFromDrv BuildableReqFromDrv::parse(const Store & store, std::string_view s)
+{
+    size_t n = s.find("!");
+    assert(n != s.npos);
+    auto drvPath = store.parseStorePath(s.substr(0, n));
+    auto outputsS = s.substr(n + 1);
+    std::set<string> outputs;
+    if (outputsS != "*")
+        outputs = tokenizeString<std::set<string>>(outputsS);
+    return {drvPath, outputs};
+}
+
+BuildableReq parseBuildableReq(const Store & store, std::string_view s)
+{
+    size_t n = s.find("!");
+    return n == s.npos
+        ? (BuildableReq) BuildableOpaque::parse(store, s)
+        : (BuildableReq) BuildableReqFromDrv::parse(store, s);
+}
+
 }

src/libstore/buildable.hh

@@ -2,6 +2,7 @@
 
 #include "util.hh"
 #include "path.hh"
+#include "path.hh"
 
 #include <optional>
 
@@ -13,19 +14,37 @@ class Store;
 
 struct BuildableOpaque {
     StorePath path;
+
     nlohmann::json toJSON(ref<Store> store) const;
+    std::string to_string(const Store & store) const;
+    static BuildableOpaque parse(const Store & store, std::string_view);
 };
 
-struct BuildableFromDrv {
+template<typename Outputs>
+struct BuildableForFromDrv {
     StorePath drvPath;
-    std::map<std::string, std::optional<StorePath>> outputs;
+    Outputs outputs;
+
     nlohmann::json toJSON(ref<Store> store) const;
+    std::string to_string(const Store & store) const;
+    static BuildableForFromDrv<Outputs> parse(const Store & store, std::string_view);
 };
 
-typedef std::variant<
+template <typename Outputs>
+using BuildableFor = std::variant<
     BuildableOpaque,
-    BuildableFromDrv
+    BuildableForFromDrv<Outputs>
-> Buildable;
+>;
+
+typedef BuildableForFromDrv<std::set<std::string>> BuildableReqFromDrv;
+typedef BuildableFor<std::set<std::string>> BuildableReq;
+
+std::string to_string(const Store & store, const BuildableReq &);
+
+BuildableReq parseBuildableReq(const Store & store, std::string_view);
+
+typedef BuildableForFromDrv<std::map<std::string, std::optional<StorePath>>> BuildableFromDrv;
+typedef BuildableFor<std::map<std::string, std::optional<StorePath>>> Buildable;
 
 typedef std::vector<Buildable> Buildables;
 

src/libstore/daemon.cc

@@ -2,6 +2,7 @@
 #include "monitor-fd.hh"
 #include "worker-protocol.hh"
 #include "store-api.hh"
+#include "path-with-outputs.hh"
 #include "finally.hh"
 #include "affinity.hh"
 #include "archive.hh"
@@ -259,6 +260,18 @@ static void writeValidPathInfo(
     }
 }
 
+static std::vector<BuildableReq> readBuildableReqs(Store & store, unsigned int clientVersion, Source & from)
+{
+    std::vector<BuildableReq> reqs;
+    if (GET_PROTOCOL_MINOR(clientVersion) >= 29) {
+        reqs = worker_proto::read(store, from, Phantom<std::vector<BuildableReq>> {});
+    } else {
+        for (auto & s : readStrings<Strings>(from))
+            reqs.push_back(parsePathWithOutputs(store, s).toBuildableReq());
+    }
+    return reqs;
+}
+
 static void performOp(TunnelLogger * logger, ref<Store> store,
     TrustedFlag trusted, RecursiveFlag recursive, unsigned int clientVersion,
     Source & from, BufferedSink & to, unsigned int op)
@@ -493,9 +506,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopBuildPaths: {
-        std::vector<StorePathWithOutputs> drvs;
+        auto drvs = readBuildableReqs(*store, clientVersion, from);
-        for (auto & s : readStrings<Strings>(from))
-            drvs.push_back(parsePathWithOutputs(*store, s));
         BuildMode mode = bmNormal;
         if (GET_PROTOCOL_MINOR(clientVersion) >= 15) {
             mode = (BuildMode) readInt(from);
@@ -859,9 +870,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopQueryMissing: {
-        std::vector<StorePathWithOutputs> targets;
+        auto targets = readBuildableReqs(*store, clientVersion, from);
-        for (auto & s : readStrings<Strings>(from))
-            targets.push_back(parsePathWithOutputs(*store, s));
         logger->startWork();
         StorePathSet willBuild, willSubstitute, unknown;
         uint64_t downloadSize, narSize;

src/libstore/legacy-ssh-store.cc

@@ -3,6 +3,7 @@
 #include "remote-store.hh"
 #include "serve-protocol.hh"
 #include "store-api.hh"
+#include "path-with-outputs.hh"
 #include "worker-protocol.hh"
 #include "ssh.hh"
 #include "derivations.hh"
@@ -266,14 +267,23 @@ public:
         return status;
     }
 
-    void buildPaths(const std::vector<StorePathWithOutputs> & drvPaths, BuildMode buildMode) override
+    void buildPaths(const std::vector<BuildableReq> & drvPaths, BuildMode buildMode) override
     {
         auto conn(connections->get());
 
         conn->to << cmdBuildPaths;
         Strings ss;
-        for (auto & p : drvPaths)
+        for (auto & p : drvPaths) {
-            ss.push_back(p.to_string(*this));
+            auto sOrDrvPath = StorePathWithOutputs::tryFromBuildableReq(p);
+            std::visit(overloaded {
+                [&](StorePathWithOutputs s) {
+                    ss.push_back(s.to_string(*this));
+                },
+                [&](StorePath drvPath) {
+                    throw Error("wanted to fetch '%s' but the legacy ssh protocol doesn't support merely substituting drv files via the build paths command. It would build them instead. Try using ssh-ng://", printStorePath(drvPath));
+                },
+            }, sOrDrvPath);
+        }
         conn->to << ss;
 
         putBuildSettings(*conn);

src/libstore/misc.cc

@@ -117,7 +117,7 @@ std::optional<ContentAddress> getDerivationCA(const BasicDerivation & drv)
     return std::nullopt;
 }
 
-void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
+void Store::queryMissing(const std::vector<BuildableReq> & targets,
     StorePathSet & willBuild_, StorePathSet & willSubstitute_, StorePathSet & unknown_,
     uint64_t & downloadSize_, uint64_t & narSize_)
 {
@@ -145,7 +145,7 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
 
     Sync<State> state_(State{{}, unknown_, willSubstitute_, willBuild_, downloadSize_, narSize_});
 
-    std::function<void(StorePathWithOutputs)> doPath;
+    std::function<void(BuildableReq)> doPath;
 
     auto mustBuildDrv = [&](const StorePath & drvPath, const Derivation & drv) {
         {
@@ -154,7 +154,7 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
         }
 
         for (auto & i : drv.inputDrvs)
-            pool.enqueue(std::bind(doPath, StorePathWithOutputs { i.first, i.second }));
+            pool.enqueue(std::bind(doPath, BuildableReqFromDrv { i.first, i.second }));
     };
 
     auto checkOutput = [&](
@@ -177,24 +177,25 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
                 drvState->outPaths.insert(outPath);
                 if (!drvState->left) {
                     for (auto & path : drvState->outPaths)
-                        pool.enqueue(std::bind(doPath, StorePathWithOutputs { path } ));
+                        pool.enqueue(std::bind(doPath, BuildableOpaque { path } ));
                 }
             }
         }
     };
 
-    doPath = [&](const StorePathWithOutputs & path) {
+    doPath = [&](const BuildableReq & req) {
 
         {
             auto state(state_.lock());
-            if (!state->done.insert(path.to_string(*this)).second) return;
+            if (!state->done.insert(to_string(*this, req)).second) return;
         }
 
-        if (path.path.isDerivation()) {
+        std::visit(overloaded {
-            if (!isValidPath(path.path)) {
+          [&](BuildableReqFromDrv bfd) {
+            if (!isValidPath(bfd.drvPath)) {
                 // FIXME: we could try to substitute the derivation.
                 auto state(state_.lock());
-                state->unknown.insert(path.path);
+                state->unknown.insert(bfd.drvPath);
                 return;
             }
 
@@ -202,52 +203,54 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
             /* true for regular derivations, and CA derivations for which we
                have a trust mapping for all wanted outputs. */
             auto knownOutputPaths = true;
-            for (auto & [outputName, pathOpt] : queryPartialDerivationOutputMap(path.path)) {
+            for (auto & [outputName, pathOpt] : queryPartialDerivationOutputMap(bfd.drvPath)) {
                 if (!pathOpt) {
                     knownOutputPaths = false;
                     break;
                 }
-                if (wantOutput(outputName, path.outputs) && !isValidPath(*pathOpt))
+                if (wantOutput(outputName, bfd.outputs) && !isValidPath(*pathOpt))
                     invalid.insert(*pathOpt);
             }
             if (knownOutputPaths && invalid.empty()) return;
 
-            auto drv = make_ref<Derivation>(derivationFromPath(path.path));
+            auto drv = make_ref<Derivation>(derivationFromPath(bfd.drvPath));
-            ParsedDerivation parsedDrv(StorePath(path.path), *drv);
+            ParsedDerivation parsedDrv(StorePath(bfd.drvPath), *drv);
 
             if (knownOutputPaths && settings.useSubstitutes && parsedDrv.substitutesAllowed()) {
                 auto drvState = make_ref<Sync<DrvState>>(DrvState(invalid.size()));
                 for (auto & output : invalid)
-                    pool.enqueue(std::bind(checkOutput, path.path, drv, output, drvState));
+                    pool.enqueue(std::bind(checkOutput, bfd.drvPath, drv, output, drvState));
             } else
-                mustBuildDrv(path.path, *drv);
+                mustBuildDrv(bfd.drvPath, *drv);
 
-        } else {
+          },
+          [&](BuildableOpaque bo) {
 
-            if (isValidPath(path.path)) return;
+            if (isValidPath(bo.path)) return;
 
             SubstitutablePathInfos infos;
-            querySubstitutablePathInfos({{path.path, std::nullopt}}, infos);
+            querySubstitutablePathInfos({{bo.path, std::nullopt}}, infos);
 
             if (infos.empty()) {
                 auto state(state_.lock());
-                state->unknown.insert(path.path);
+                state->unknown.insert(bo.path);
                 return;
             }
 
-            auto info = infos.find(path.path);
+            auto info = infos.find(bo.path);
             assert(info != infos.end());
 
             {
                 auto state(state_.lock());
-                state->willSubstitute.insert(path.path);
+                state->willSubstitute.insert(bo.path);
                 state->downloadSize += info->second.downloadSize;
                 state->narSize += info->second.narSize;
             }
 
             for (auto & ref : info->second.references)
-                pool.enqueue(std::bind(doPath, StorePathWithOutputs { ref }));
+                pool.enqueue(std::bind(doPath, BuildableOpaque { ref }));
-        }
+          },
+        }, req);
     };
 
     for (auto & path : targets)

src/libstore/path-with-outputs.cc

@@ -1,3 +1,4 @@
+#include "path-with-outputs.hh"
 #include "store-api.hh"
 
 namespace nix {
@@ -10,6 +11,40 @@ std::string StorePathWithOutputs::to_string(const Store & store) const
 }
 
 
+BuildableReq StorePathWithOutputs::toBuildableReq() const
+{
+    if (!outputs.empty() || path.isDerivation())
+        return BuildableReqFromDrv { path, outputs };
+    else
+        return BuildableOpaque { path };
+}
+
+
+std::vector<BuildableReq> toBuildableReqs(const std::vector<StorePathWithOutputs> ss)
+{
+	std::vector<BuildableReq> reqs;
+	for (auto & s : ss) reqs.push_back(s.toBuildableReq());
+	return reqs;
+}
+
+
+std::variant<StorePathWithOutputs, StorePath> StorePathWithOutputs::tryFromBuildableReq(const BuildableReq & p)
+{
+    return std::visit(overloaded {
+        [&](BuildableOpaque bo) -> std::variant<StorePathWithOutputs, StorePath> {
+            if (bo.path.isDerivation()) {
+                // drv path gets interpreted as "build", not "get drv file itself"
+                return bo.path;
+            }
+            return StorePathWithOutputs { bo.path };
+        },
+        [&](BuildableReqFromDrv bfd) -> std::variant<StorePathWithOutputs, StorePath> {
+            return StorePathWithOutputs { bfd.drvPath, bfd.outputs };
+        },
+    }, p);
+}
+
+
 std::pair<std::string_view, StringSet> parsePathWithOutputs(std::string_view s)
 {
     size_t n = s.find("!");

src/libstore/path-with-outputs.hh

@@ -1,6 +1,9 @@
 #pragma once
 
+#include <variant>
+
 #include "path.hh"
+#include "buildable.hh"
 
 namespace nix {
 
@@ -10,8 +13,14 @@ struct StorePathWithOutputs
     std::set<std::string> outputs;
 
     std::string to_string(const Store & store) const;
+
+    BuildableReq toBuildableReq() const;
+
+    static std::variant<StorePathWithOutputs, StorePath> tryFromBuildableReq(const BuildableReq &);
 };
 
+std::vector<BuildableReq> toBuildableReqs(const std::vector<StorePathWithOutputs>);
+
 std::pair<std::string_view, StringSet> parsePathWithOutputs(std::string_view s);
 
 class Store;

src/libstore/remote-store.cc

@@ -1,5 +1,6 @@
 #include "serialise.hh"
 #include "util.hh"
+#include "path-with-outputs.hh"
 #include "remote-fs-accessor.hh"
 #include "remote-store.hh"
 #include "worker-protocol.hh"
@@ -50,6 +51,19 @@ void write(const Store & store, Sink & out, const ContentAddress & ca)
     out << renderContentAddress(ca);
 }
 
+
+BuildableReq read(const Store & store, Source & from, Phantom<BuildableReq> _)
+{
+    auto s = readString(from);
+    return parseBuildableReq(store, s);
+}
+
+void write(const Store & store, Sink & out, const BuildableReq & req)
+{
+    out << to_string(store, req);
+}
+
+
 Realisation read(const Store & store, Source & from, Phantom<Realisation> _)
 {
     std::string rawInput = readString(from);
@@ -58,8 +72,12 @@ Realisation read(const Store & store, Source & from, Phantom<Realisation> _)
         "remote-protocol"
     );
 }
+
 void write(const Store & store, Sink & out, const Realisation & realisation)
-{ out << realisation.toJSON().dump(); }
+{
+    out << realisation.toJSON().dump();
+}
+
 
 DrvOutput read(const Store & store, Source & from, Phantom<DrvOutput> _)
 {
@@ -652,16 +670,36 @@ std::optional<const Realisation> RemoteStore::queryRealisation(const DrvOutput &
     return {Realisation{.id = id, .outPath = *outPaths.begin()}};
 }
 
+static void writeBuildableReqs(RemoteStore & store, ConnectionHandle & conn, const std::vector<BuildableReq> & reqs)
+{
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 29) {
+        worker_proto::write(store, conn->to, reqs);
+    } else {
+        Strings ss;
+        for (auto & p : reqs) {
+            auto sOrDrvPath = StorePathWithOutputs::tryFromBuildableReq(p);
+            std::visit(overloaded {
+                [&](StorePathWithOutputs s) {
+                    ss.push_back(s.to_string(store));
+                },
+                [&](StorePath drvPath) {
+                    throw Error("trying to request '%s', but daemon protocol %d.%d is too old (< 1.29) to request a derivation file",
+                        store.printStorePath(drvPath),
+                        GET_PROTOCOL_MAJOR(conn->daemonVersion),
+                        GET_PROTOCOL_MINOR(conn->daemonVersion));
+                },
+            }, sOrDrvPath);
+        }
+        conn->to << ss;
+    }
+}
 
-void RemoteStore::buildPaths(const std::vector<StorePathWithOutputs> & drvPaths, BuildMode buildMode)
+void RemoteStore::buildPaths(const std::vector<BuildableReq> & drvPaths, BuildMode buildMode)
 {
     auto conn(getConnection());
     conn->to << wopBuildPaths;
     assert(GET_PROTOCOL_MINOR(conn->daemonVersion) >= 13);
-    Strings ss;
+    writeBuildableReqs(*this, conn, drvPaths);
-    for (auto & p : drvPaths)
-        ss.push_back(p.to_string(*this));
-    conn->to << ss;
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 15)
         conn->to << buildMode;
     else
@@ -800,7 +838,7 @@ void RemoteStore::addSignatures(const StorePath & storePath, const StringSet & s
 }
 
 
-void RemoteStore::queryMissing(const std::vector<StorePathWithOutputs> & targets,
+void RemoteStore::queryMissing(const std::vector<BuildableReq> & targets,
     StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
     uint64_t & downloadSize, uint64_t & narSize)
 {
@@ -811,10 +849,7 @@ void RemoteStore::queryMissing(const std::vector<StorePathWithOutputs> & targets
             // to prevent a deadlock.
             goto fallback;
         conn->to << wopQueryMissing;
-        Strings ss;
+        writeBuildableReqs(*this, conn, targets);
-        for (auto & p : targets)
-            ss.push_back(p.to_string(*this));
-        conn->to << ss;
         conn.processStderr();
         willBuild = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
         willSubstitute = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});

src/libstore/remote-store.hh

@@ -85,7 +85,7 @@ public:
 
     std::optional<const Realisation> queryRealisation(const DrvOutput &) override;
 
-    void buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode) override;
+    void buildPaths(const std::vector<BuildableReq> & paths, BuildMode buildMode) override;
 
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
         BuildMode buildMode) override;
@@ -108,7 +108,7 @@ public:
 
     void addSignatures(const StorePath & storePath, const StringSet & sigs) override;
 
-    void queryMissing(const std::vector<StorePathWithOutputs> & targets,
+    void queryMissing(const std::vector<BuildableReq> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
         uint64_t & downloadSize, uint64_t & narSize) override;
 

src/libstore/store-api.cc

@@ -529,10 +529,10 @@ void Store::queryPathInfo(const StorePath & storePath,
 
 void Store::substitutePaths(const StorePathSet & paths)
 {
-    std::vector<StorePathWithOutputs> paths2;
+    std::vector<BuildableReq> paths2;
     for (auto & path : paths)
         if (!path.isDerivation())
-            paths2.push_back({path});
+            paths2.push_back(BuildableOpaque{path});
     uint64_t downloadSize, narSize;
     StorePathSet willBuild, willSubstitute, unknown;
     queryMissing(paths2,
@@ -540,8 +540,8 @@ void Store::substitutePaths(const StorePathSet & paths)
 
     if (!willSubstitute.empty())
         try {
-            std::vector<StorePathWithOutputs> subs;
+            std::vector<BuildableReq> subs;
-            for (auto & p : willSubstitute) subs.push_back({p});
+            for (auto & p : willSubstitute) subs.push_back(BuildableOpaque{p});
             buildPaths(subs);
         } catch (Error & e) {
             logWarning(e.info());

src/libstore/store-api.hh

@@ -2,7 +2,7 @@
 
 #include "realisation.hh"
 #include "path.hh"
-#include "path-with-outputs.hh"
+#include "buildable.hh"
 #include "hash.hh"
 #include "content-address.hh"
 #include "serialise.hh"
@@ -494,7 +494,7 @@ public:
        recursively building any sub-derivations. For inputs that are
        not derivations, substitute them. */
     virtual void buildPaths(
-        const std::vector<StorePathWithOutputs> & paths,
+        const std::vector<BuildableReq> & paths,
         BuildMode buildMode = bmNormal);
 
     /* Build a single non-materialized derivation (i.e. not from an
@@ -656,7 +656,7 @@ public:
     /* Given a set of paths that are to be built, return the set of
        derivations that will be built, and the set of output paths
        that will be substituted. */
-    virtual void queryMissing(const std::vector<StorePathWithOutputs> & targets,
+    virtual void queryMissing(const std::vector<BuildableReq> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
         uint64_t & downloadSize, uint64_t & narSize);
 

src/libstore/worker-protocol.hh

@@ -86,9 +86,11 @@ namespace worker_proto {
 MAKE_WORKER_PROTO(, std::string);
 MAKE_WORKER_PROTO(, StorePath);
 MAKE_WORKER_PROTO(, ContentAddress);
+MAKE_WORKER_PROTO(, BuildableReq);
 MAKE_WORKER_PROTO(, Realisation);
 MAKE_WORKER_PROTO(, DrvOutput);
 
+MAKE_WORKER_PROTO(template<typename T>, std::vector<T>);
 MAKE_WORKER_PROTO(template<typename T>, std::set<T>);
 
 #define X_ template<typename K, typename V>
@@ -113,6 +115,26 @@ MAKE_WORKER_PROTO(X_, Y_);
 MAKE_WORKER_PROTO(, std::optional<StorePath>);
 MAKE_WORKER_PROTO(, std::optional<ContentAddress>);
 
+template<typename T>
+std::vector<T> read(const Store & store, Source & from, Phantom<std::vector<T>> _)
+{
+    std::vector<T> resSet;
+    auto size = readNum<size_t>(from);
+    while (size--) {
+        resSet.push_back(read(store, from, Phantom<T> {}));
+    }
+    return resSet;
+}
+
+template<typename T>
+void write(const Store & store, Sink & out, const std::vector<T> & resSet)
+{
+    out << resSet.size();
+    for (auto & key : resSet) {
+        write(store, out, key);
+    }
+}
+
 template<typename T>
 std::set<T> read(const Store & store, Source & from, Phantom<std::set<T>> _)
 {

src/nix-build/nix-build.cc

@@ -12,6 +12,7 @@
 #include "affinity.hh"
 #include "util.hh"
 #include "shared.hh"
+#include "path-with-outputs.hh"
 #include "eval.hh"
 #include "eval-inline.hh"
 #include "get-drvs.hh"
@@ -321,7 +322,8 @@ static void main_nix_build(int argc, char * * argv)
 
     state->printStats();
 
-    auto buildPaths = [&](const std::vector<StorePathWithOutputs> & paths) {
+    auto buildPaths = [&](const std::vector<StorePathWithOutputs> & paths0) {
+        auto paths = toBuildableReqs(paths0);
         /* Note: we do this even when !printMissing to efficiently
            fetch binary cache data. */
         uint64_t downloadSize, narSize;

src/nix-env/nix-env.cc

@@ -6,6 +6,7 @@
 #include "globals.hh"
 #include "names.hh"
 #include "profiles.hh"
+#include "path-with-outputs.hh"
 #include "shared.hh"
 #include "store-api.hh"
 #include "local-fs-store.hh"
@@ -418,13 +419,13 @@ static void queryInstSources(EvalState & state,
 
 static void printMissing(EvalState & state, DrvInfos & elems)
 {
-    std::vector<StorePathWithOutputs> targets;
+    std::vector<BuildableReq> targets;
     for (auto & i : elems) {
         Path drvPath = i.queryDrvPath();
         if (drvPath != "")
-            targets.push_back({state.store->parseStorePath(drvPath)});
+            targets.push_back(BuildableReqFromDrv{state.store->parseStorePath(drvPath)});
         else
-            targets.push_back({state.store->parseStorePath(i.queryOutPath())});
+            targets.push_back(BuildableOpaque{state.store->parseStorePath(i.queryOutPath())});
     }
 
     printMissing(state.store, targets);
@@ -693,17 +694,18 @@ static void opSet(Globals & globals, Strings opFlags, Strings opArgs)
     if (globals.forceName != "")
         drv.setName(globals.forceName);
 
-    if (drv.queryDrvPath() != "") {
+    std::vector<BuildableReq> paths {
-        std::vector<StorePathWithOutputs> paths{{globals.state->store->parseStorePath(drv.queryDrvPath())}};
+        (drv.queryDrvPath() != "")
+        ? (BuildableReq) (BuildableReqFromDrv {
+                globals.state->store->parseStorePath(drv.queryDrvPath())
+            })
+        : (BuildableReq) (BuildableOpaque {
+                globals.state->store->parseStorePath(drv.queryOutPath())
+            }),
+    };
     printMissing(globals.state->store, paths);
     if (globals.dryRun) return;
     globals.state->store->buildPaths(paths, globals.state->repair ? bmRepair : bmNormal);
-    } else {
-        printMissing(globals.state->store,
-            {{globals.state->store->parseStorePath(drv.queryOutPath())}});
-        if (globals.dryRun) return;
-        globals.state->store->ensurePath(globals.state->store->parseStorePath(drv.queryOutPath()));
-    }
 
     debug(format("switching to new user environment"));
     Path generation = createGeneration(

src/nix-env/user-env.cc

@@ -2,6 +2,7 @@
 #include "util.hh"
 #include "derivations.hh"
 #include "store-api.hh"
+#include "path-with-outputs.hh"
 #include "local-fs-store.hh"
 #include "globals.hh"
 #include "shared.hh"
@@ -41,7 +42,9 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
             drvsToBuild.push_back({state.store->parseStorePath(i.queryDrvPath())});
 
     debug(format("building user environment dependencies"));
-    state.store->buildPaths(drvsToBuild, state.repair ? bmRepair : bmNormal);
+    state.store->buildPaths(
+        toBuildableReqs(drvsToBuild),
+        state.repair ? bmRepair : bmNormal);
 
     /* Construct the whole top level derivation. */
     StorePathSet references;
@@ -136,7 +139,9 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     debug("building user environment");
     std::vector<StorePathWithOutputs> topLevelDrvs;
     topLevelDrvs.push_back({topLevelDrv});
-    state.store->buildPaths(topLevelDrvs, state.repair ? bmRepair : bmNormal);
+    state.store->buildPaths(
+        toBuildableReqs(topLevelDrvs),
+        state.repair ? bmRepair : bmNormal);
 
     /* Switch the current user environment to the output path. */
     auto store2 = state.store.dynamic_pointer_cast<LocalFSStore>();

src/nix-store/nix-store.cc

@@ -10,6 +10,7 @@
 #include "worker-protocol.hh"
 #include "graphml.hh"
 #include "legacy.hh"
+#include "path-with-outputs.hh"
 
 #include <iostream>
 #include <algorithm>
@@ -62,7 +63,7 @@ static PathSet realisePath(StorePathWithOutputs path, bool build = true)
     auto store2 = std::dynamic_pointer_cast<LocalFSStore>(store);
 
     if (path.path.isDerivation()) {
-        if (build) store->buildPaths({path});
+        if (build) store->buildPaths({path.toBuildableReq()});
         auto outputPaths = store->queryDerivationOutputMap(path.path);
         Derivation drv = store->derivationFromPath(path.path);
         rootNr++;
@@ -132,7 +133,9 @@ static void opRealise(Strings opFlags, Strings opArgs)
 
     uint64_t downloadSize, narSize;
     StorePathSet willBuild, willSubstitute, unknown;
-    store->queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
+    store->queryMissing(
+        toBuildableReqs(paths),
+        willBuild, willSubstitute, unknown, downloadSize, narSize);
 
     if (ignoreUnknown) {
         std::vector<StorePathWithOutputs> paths2;
@@ -148,7 +151,7 @@ static void opRealise(Strings opFlags, Strings opArgs)
     if (dryRun) return;
 
     /* Build all paths at the same time to exploit parallelism. */
-    store->buildPaths(paths, buildMode);
+    store->buildPaths(toBuildableReqs(paths), buildMode);
 
     if (!ignoreUnknown)
         for (auto & i : paths) {
@@ -879,7 +882,7 @@ static void opServe(Strings opFlags, Strings opArgs)
 
                 try {
                     MonitorFdHup monitor(in.fd);
-                    store->buildPaths(paths);
+                    store->buildPaths(toBuildableReqs(paths));
                     out << 0;
                 } catch (Error & e) {
                     assert(e.status);

src/nix/bundle.cc

@@ -70,7 +70,7 @@ struct CmdBundle : InstallableCommand
         auto evalState = getEvalState();
 
         auto app = installable->toApp(*evalState);
-        store->buildPaths(app.context);
+        store->buildPaths(toBuildableReqs(app.context));
 
         auto [bundlerFlakeRef, bundlerName] = parseFlakeRefWithFragment(bundler, absPath("."));
         const flake::LockFlags lockFlags{ .writeLockFile = false };
@@ -110,7 +110,7 @@ struct CmdBundle : InstallableCommand
 
         StorePath outPath = store->parseStorePath(evalState->coerceToPath(*attr2->pos, *attr2->value, context2));
 
-        store->buildPaths({{drvPath}});
+        store->buildPaths({ BuildableReqFromDrv { drvPath } });
 
         auto outPathS = store->printStorePath(outPath);
 

src/nix/develop.cc

@@ -3,6 +3,7 @@
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "path-with-outputs.hh"
 #include "derivations.hh"
 #include "affinity.hh"
 #include "progress-bar.hh"
@@ -159,7 +160,7 @@ StorePath getDerivationEnvironment(ref<Store> store, const StorePath & drvPath)
     auto shellDrvPath = writeDerivation(*store, drv);
 
     /* Build the derivation. */
-    store->buildPaths({{shellDrvPath}});
+    store->buildPaths({BuildableReqFromDrv{shellDrvPath}});
 
     for (auto & [_0, outputAndOptPath] : drv.outputsAndOptPaths(*store)) {
         auto & [_1, optPath] = outputAndOptPath;

src/nix/flake.cc

@@ -7,6 +7,7 @@
 #include "get-drvs.hh"
 #include "store-api.hh"
 #include "derivations.hh"
+#include "path-with-outputs.hh"
 #include "attr-path.hh"
 #include "fetchers.hh"
 #include "registry.hh"
@@ -292,7 +293,7 @@ struct CmdFlakeCheck : FlakeCommand
             }
         };
 
-        std::vector<StorePathWithOutputs> drvPaths;
+        std::vector<BuildableReq> drvPaths;
 
         auto checkApp = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
@@ -461,7 +462,7 @@ struct CmdFlakeCheck : FlakeCommand
                                         fmt("%s.%s.%s", name, attr.name, attr2.name),
                                         *attr2.value, *attr2.pos);
                                     if ((std::string) attr.name == settings.thisSystem.get())
-                                        drvPaths.push_back({drvPath});
+                                        drvPaths.push_back(BuildableReqFromDrv{drvPath});
                                 }
                             }
                         }

src/nix/profile.cc

@@ -233,7 +233,7 @@ struct CmdProfileInstall : InstallablesCommand, MixDefaultProfile
     {
         ProfileManifest manifest(*getEvalState(), *profile);
 
-        std::vector<StorePathWithOutputs> pathsToBuild;
+        std::vector<BuildableReq> pathsToBuild;
 
         for (auto & installable : installables) {
             if (auto installable2 = std::dynamic_pointer_cast<InstallableFlake>(installable)) {
@@ -249,7 +249,7 @@ struct CmdProfileInstall : InstallablesCommand, MixDefaultProfile
                     attrPath,
                 };
 
-                pathsToBuild.push_back({drv.drvPath, StringSet{drv.outputName}});
+                pathsToBuild.push_back(BuildableReqFromDrv{drv.drvPath, StringSet{drv.outputName}});
 
                 manifest.elements.emplace_back(std::move(element));
             } else {
@@ -260,12 +260,15 @@ struct CmdProfileInstall : InstallablesCommand, MixDefaultProfile
 
                     std::visit(overloaded {
                         [&](BuildableOpaque bo) {
-                            pathsToBuild.push_back({bo.path, {}});
+                            pathsToBuild.push_back(bo);
                             element.storePaths.insert(bo.path);
                         },
                         [&](BuildableFromDrv bfd) {
+                            // TODO: Why are we querying if we know the output
+                            // names already? Is it just to figure out what the
+                            // default one is?
                             for (auto & output : store->queryDerivationOutputMap(bfd.drvPath)) {
-                                pathsToBuild.push_back({bfd.drvPath, {output.first}});
+                                pathsToBuild.push_back(BuildableReqFromDrv{bfd.drvPath, {output.first}});
                                 element.storePaths.insert(output.second);
                             }
                         },
@@ -388,7 +391,7 @@ struct CmdProfileUpgrade : virtual SourceExprCommand, MixDefaultProfile, MixProf
         auto matchers = getMatchers(store);
 
         // FIXME: code duplication
-        std::vector<StorePathWithOutputs> pathsToBuild;
+        std::vector<BuildableReq> pathsToBuild;
 
         for (size_t i = 0; i < manifest.elements.size(); ++i) {
             auto & element(manifest.elements[i]);
@@ -423,7 +426,7 @@ struct CmdProfileUpgrade : virtual SourceExprCommand, MixDefaultProfile, MixProf
                     attrPath,
                 };
 
-                pathsToBuild.push_back({drv.drvPath, StringSet{"out"}}); // FIXME
+                pathsToBuild.push_back(BuildableReqFromDrv{drv.drvPath, {"out"}}); // FIXME
             }
         }
 

src/nix/run.cc

@@ -182,7 +182,7 @@ struct CmdRun : InstallableCommand, RunCommon
 
         auto app = installable->toApp(*state);
 
-        state->store->buildPaths(app.context);
+        state->store->buildPaths(toBuildableReqs(app.context));
 
         Strings allArgs{app.program};
         for (auto & i : args) allArgs.push_back(i);