Bare minimum ssh-ng:// extensions for Hydra

This is not as comprehensive as #10748, but I am also interested in
figuring out whether all those additions are in fact necessary.

This is bare minimum needed for
NixOS/hydra#1445, which has notable gaps but
nevertheless reimplements enough with `ssh-ng://` to past all tests.

I would like to merge this change as definitely necessary, and unclear
whether sufficient. Then I would iterate on the corresponding Hydra PR
until it seems potentially correct, seeing what, if any, further Nix API
changes are necessary.

src/libstore/remote-store.cc

@@ -767,7 +767,22 @@ BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicD
     auto conn(getConnection());
     conn->putBuildDerivationRequest(*this, &conn.daemonException, drvPath, drv, buildMode);
     conn.processStderr();
-    return WorkerProto::Serialise<BuildResult>::read(*this, *conn);
+    return conn->getBuildDerivationResponse(*this, &conn.daemonException);
+}
+
+
+std::function<BuildResult()> RemoteStore::buildDerivationAsync(
+    const StorePath & drvPath, const BasicDerivation & drv,
+    BuildMode buildMode)
+{
+    // Until we have C++23 std::move_only_function
+    auto conn = std::make_shared<ConnectionHandle>(getConnection());
+    (*conn)->putBuildDerivationRequest(*this, &conn->daemonException, drvPath, drv, buildMode);
+    conn->processStderr();
+
+    return [this,conn]() -> BuildResult {
+        return (*conn)->getBuildDerivationResponse(*this, &conn->daemonException);
+    };
 }
 
 

src/libstore/remote-store.hh

@@ -122,6 +122,16 @@ public:
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
         BuildMode buildMode) override;
 
+    /**
+     * Note, the returned function must only be called once, or we'll
+     * try to read from the connection twice.
+     *
+     * @todo Use C++23 `std::move_only_function`.
+     */
+    std::function<BuildResult()> buildDerivationAsync(
+        const StorePath & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode);
+
     void ensurePath(const StorePath & path) override;
 
     void addTempRoot(const StorePath & path) override;

src/libstore/ssh-store.cc

@@ -209,7 +209,7 @@ ref<RemoteStore::Connection> SSHStore::openConnection()
     }
     command.insert(command.end(),
         extraRemoteProgramArgs.begin(), extraRemoteProgramArgs.end());
-    conn->sshConn = master.startCommand(std::move(command));
+    conn->sshConn = master.startCommand(std::move(command), std::list{extraSshArgs});
     conn->to = FdSink(conn->sshConn->in.get());
     conn->from = FdSource(conn->sshConn->out.get());
     return conn;

src/libstore/ssh-store.hh

@@ -18,6 +18,11 @@ struct SSHStoreConfig : virtual RemoteStoreConfig, virtual CommonSSHStoreConfig
     const Setting<Strings> remoteProgram{
         this, {"nix-daemon"}, "remote-program", "Path to the `nix-daemon` executable on the remote machine."};
 
+    /**
+     * Hack for hydra
+     */
+    Strings extraSshArgs = {};
+
     const std::string name() override
     {
         return "Experimental SSH Store";