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/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;
-        for (auto & [_, status] : initialOutputs) {
+        StringSet missingOutputs;
+        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>);