Store metadata about drv outputs realisations

For each known realisation, store:
- its output
- its output path

This comes with a set of needed changes:

- New `realisations` module declaring the types needed for describing
  these mappings
- New `Store::registerDrvOutput` method registering all the needed informations
  about a derivation output (also replaces `LocalStore::linkDeriverToPath`)
- new `Store::queryRealisation` method to retrieve the informations for a
  derivations

This introcudes some redundancy on the remote-store side between
`wopQueryDerivationOutputMap` and `wopQueryRealisation`.
However we might need to keep both (regardless of backwards compat)
because we sometimes need to get some infos for all the outputs of a
derivation (where `wopQueryDerivationOutputMap` is handy), but all the
stores can't implement it − because listing all the outputs of a
derivation isn't really possible for binary caches where the server
doesn't allow to list a directory.

15 files changed, 246 insertions, 11 deletions

diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index a918b7208..085dc7ba1 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -443,6 +443,23 @@ 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::parse(*rawOutputInfo, outputInfoFilePath) };
+    } else {
+        return std::nullopt;
+    }
+}
+
+void BinaryCacheStore::registerDrvOutput(const Realisation& info) {
+    auto filePath = realisationsPrefix + "/" + info.id.to_string() + ".doi";
+    upsertFile(filePath, info.to_string(), "text/x-nix-derivertopath");
+}
+
 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..a0f10c33d 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -2094,6 +2094,20 @@ 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");
@@ -3393,7 +3407,10 @@ void DerivationGoal::registerOutputs()
 
     if (useDerivation || isCaFloating)
         for (auto & [outputName, newInfo] : infos)
-            worker.store.linkDeriverToPath(drvPathResolved, outputName, newInfo.path);
+            worker.store.registerDrvOutput(
+                DrvOutputId{drvPathResolved, outputName},
+                DrvOutputInfo{.outPath = newInfo.path,
+                              .resolvedDrv = drvPathResolved});
 }
 
 
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..418b3ab9c 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -597,13 +597,16 @@ 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);
+    // XXX: This ignores the references of the output because we can
+    // recompute them later from the drv and the references of the associated
+    // store path, but doing so is both inefficient and fragile.
+    return registerDrvOutput_(*state, queryValidPathId(*state, id.drvPath), id.outputName, info.outPath);
 }
 
-void LocalStore::linkDeriverToPath(State & state, uint64_t deriver, const string & outputName, const StorePath & output)
+void LocalStore::registerDrvOutput_(State & state, uint64_t deriver, const string & outputName, const StorePath & output)
 {
     retrySQLite<void>([&]() {
         state.stmts->AddDerivationOutput.use()
@@ -653,7 +656,7 @@ 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);
+                registerDrvOutput_(state, id, i.first, *i.second.second);
         }
     }
 
@@ -1612,5 +1615,13 @@ void LocalStore::createUser(const std::string & userName, uid_t userId)
     }
 }
 
-
+std::optional<const DrvOutputInfo> LocalStore::queryDrvOutputInfo(const DrvOutputId& id) {
+    auto outputPath = queryOutputPathOf(id.drvPath, id.outputName);
+    if (!(outputPath && isValidPath(*outputPath)))
+        return std::nullopt;
+    else
+        return {DrvOutputInfo{
+            .outPath = *outputPath,
+        }};
 }
+}  // namespace nix
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 332718af4..440411f01 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 DrvOutputId & outputId, const DrvOutputInfo & info) override;
+    void registerDrvOutput_(State & state, 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/realisation.cc b/src/libstore/realisation.cc
new file mode 100644
index 000000000..fcc1a3825
--- /dev/null
+++ b/src/libstore/realisation.cc
@@ -0,0 +1,72 @@
+#include "realisation.hh"
+#include "store-api.hh"
+
+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;
+}
+
+std::string Realisation::to_string() const {
+    std::string res;
+
+    res += "Id: " + id.to_string() + '\n';
+    res += "OutPath: " + std::string(outPath.to_string()) + '\n';
+
+    return res;
+}
+
+Realisation Realisation::parse(const std::string & s, const std::string & whence)
+{
+    // XXX: Copy-pasted from NarInfo::NarInfo. Should be factored out
+    auto corrupt = [&]() {
+        return Error("Drv output info file '%1%' is corrupt", whence);
+    };
+
+    std::optional<DrvOutput> id;
+    std::optional<StorePath> outPath;
+
+    size_t pos = 0;
+    while (pos < s.size()) {
+
+        size_t colon = s.find(':', pos);
+        if (colon == std::string::npos) throw corrupt();
+
+        std::string name(s, pos, colon - pos);
+
+        size_t eol = s.find('\n', colon + 2);
+        if (eol == std::string::npos) throw corrupt();
+
+        std::string value(s, colon + 2, eol - colon - 2);
+
+        if (name == "Id")
+            id = DrvOutput::parse(value);
+
+        if (name == "OutPath")
+            outPath = StorePath(value);
+
+        pos = eol + 1;
+    }
+
+    if (!outPath) corrupt();
+    if (!id) corrupt();
+    return Realisation {
+        .id = *id,
+        .outPath = *outPath,
+    };
+}
+
+} // namespace nix
diff --git a/src/libstore/realisation.hh b/src/libstore/realisation.hh
new file mode 100644
index 000000000..c573e1bb4
--- /dev/null
+++ b/src/libstore/realisation.hh
@@ -0,0 +1,34 @@
+#pragma once
+
+#include "path.hh"
+
+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;
+
+    std::string to_string() const;
+    static Realisation parse(const std::string & s, 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;