Make the build-hook work with ca derivations

- Pass it the name of the outputs rather than their output paths (as
  these don't exist for ca derivations)
- Get the built output paths from the remote builder
- Register the new received realisations

src/build-remote/build-remote.cc

@@ -248,7 +248,7 @@ connected:
         std::cerr << "# accept\n" << storeUri << "\n";
 
         auto inputs = readStrings<PathSet>(source);
-        auto outputs = readStrings<PathSet>(source);
+        auto wantedOutputs = readStrings<StringSet>(source);
 
         AutoCloseFD uploadLock = openLockFile(currentLoad + "/" + escapeUri(storeUri) + ".upload-lock", true);
 
@@ -273,6 +273,7 @@ connected:
         uploadLock = -1;
 
         auto drv = store->readDerivation(*drvPath);
+        auto outputHashes = staticOutputHashes(*store, drv);
         drv.inputSrcs = store->parseStorePathSet(inputs);
 
         auto result = sshStore->buildDerivation(*drvPath, drv);
@@ -280,16 +281,35 @@ connected:
         if (!result.success())
             throw Error("build of '%s' on '%s' failed: %s", store->printStorePath(*drvPath), storeUri, result.errorMsg);
 
-        StorePathSet missing;
+        std::set<Realisation> missingRealisations;
-        for (auto & path : outputs)
+        StorePathSet missingPaths;
-            if (!store->isValidPath(store->parseStorePath(path))) missing.insert(store->parseStorePath(path));
+        if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+            for (auto & outputName : wantedOutputs) {
+                auto thisOutputHash = outputHashes.at(outputName);
+                auto thisOutputId = DrvOutput{ thisOutputHash, outputName };
+                if (!store->queryRealisation(thisOutputId)) {
+                    notice("Missing output %s", outputName);
+                    assert(result.builtOutputs.count(thisOutputId));
+                    auto newRealisation = result.builtOutputs.at(thisOutputId);
+                    missingRealisations.insert(newRealisation);
+                    missingPaths.insert(newRealisation.outPath);
+                }
+            }
+        } else {
+            auto outputPaths = drv.outputsAndOptPaths(*store);
+            for (auto & [outputName, hopefullyOutputPath] : outputPaths) {
+                assert(hopefullyOutputPath.second);
+                if (!store->isValidPath(*hopefullyOutputPath.second))
+                    missingPaths.insert(*hopefullyOutputPath.second);
+            }
+        }
 
-        if (!missing.empty()) {
+        if (!missingPaths.empty()) {
             Activity act(*logger, lvlTalkative, actUnknown, fmt("copying outputs from '%s'", storeUri));
             if (auto localStore = store.dynamic_pointer_cast<LocalStore>())
-                for (auto & i : missing)
+                for (auto & path : missingPaths)
-                    localStore->locksHeld.insert(store->printStorePath(i)); /* FIXME: ugly */
+                    localStore->locksHeld.insert(store->printStorePath(path)); /* FIXME: ugly */
-            copyPaths(ref<Store>(sshStore), store, missing, NoRepair, NoCheckSigs, NoSubstitute);
+            copyPaths(ref<Store>(sshStore), store, missingPaths, NoRepair, NoCheckSigs, NoSubstitute);
         }
 
         return 0;

src/libstore/build/derivation-goal.cc

@@ -1159,13 +1159,14 @@ HookReply DerivationGoal::tryBuildHook()
     /* Tell the hooks the missing outputs that have to be copied back
        from the remote system. */
     {
-        StorePathSet missingPaths;
+        StringSet missingOutputs;
-        for (auto & [_, status] : initialOutputs) {
+        for (auto & [outputName, status] : initialOutputs) {
             if (!status.known) continue;
             if (buildMode != bmCheck && status.known->isValid()) continue;
-            missingPaths.insert(status.known->path);
+            missingOutputs.insert(outputName);
+            /* missingPaths.insert(status.known->path); */
         }
-        worker_proto::write(worker.store, hook->sink, missingPaths);
+        worker_proto::write(worker.store, hook->sink, missingOutputs);
     }
 
     hook->sink = FdSink();

src/libstore/realisation.hh

@@ -33,6 +33,8 @@ struct Realisation {
     GENERATE_CMP(Realisation, me->id, me->outPath);
 };
 
+typedef std::map<DrvOutput, Realisation> DrvOutputs;
+
 struct OpaquePath {
     StorePath path;
 

src/libstore/remote-store.cc

@@ -12,6 +12,7 @@
 #include "logging.hh"
 #include "callback.hh"
 #include "filetransfer.hh"
+#include <nlohmann/json.hpp>
 
 namespace nix {
 
@@ -49,6 +50,21 @@ void write(const Store & store, Sink & out, const ContentAddress & ca)
     out << renderContentAddress(ca);
 }
 
+Realisation read(const Store & store, Source & from, Phantom<Realisation> _)
+{
+    std::string rawInput = readString(from);
+    return Realisation::fromJSON(
+        nlohmann::json::parse(rawInput),
+        "remote-protocol"
+    );
+}
+void write(const Store & store, Sink & out, const Realisation & realisation)
+{ out << realisation.toJSON().dump(); }
+
+DrvOutput read(const Store & store, Source & from, Phantom<DrvOutput> _)
+{ return DrvOutput::parse(readString(from)); }
+void write(const Store & store, Sink & out, const DrvOutput & drvOutput)
+{ out << drvOutput.to_string(); }
 
 std::optional<StorePath> read(const Store & store, Source & from, Phantom<std::optional<StorePath>> _)
 {

src/libstore/store-api.hh

@@ -162,6 +162,8 @@ struct BuildResult
        non-determinism.) */
     bool isNonDeterministic = false;
 
+    DrvOutputs builtOutputs;
+
     /* The start/stop times of the build (or one of the rounds, if it
        was repeated). */
     time_t startTime = 0, stopTime = 0;

src/libstore/worker-protocol.hh

@@ -86,6 +86,8 @@ namespace worker_proto {
 MAKE_WORKER_PROTO(, std::string);
 MAKE_WORKER_PROTO(, StorePath);
 MAKE_WORKER_PROTO(, ContentAddress);
+MAKE_WORKER_PROTO(, Realisation);
+MAKE_WORKER_PROTO(, DrvOutput);
 
 MAKE_WORKER_PROTO(template<typename T>, std::set<T>);