Start factoring out the serve protocol for Hydra to share

Factor out `ServeProto::BasicClientConnection` for Hydra to share

- `queryValidPaths`: Hydra uses the lock argument differently than Nix,
  so we un-hard-code it.

- `buildDerivationRequest`: Just the request half, as Hydra does some
  things between requesting and responding.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/legacy-ssh-store.cc

@@ -22,45 +22,10 @@ std::string LegacySSHStoreConfig::doc()
 }
 
 
-struct LegacySSHStore::Connection
+struct LegacySSHStore::Connection : public ServeProto::BasicClientConnection
 {
     std::unique_ptr<SSHMaster::Connection> sshConn;
-    FdSink to;
-    FdSource from;
-    ServeProto::Version remoteVersion;
     bool good = true;
-
-    /**
-     * Coercion to `ServeProto::ReadConn`. This makes it easy to use the
-     * factored out serve protocol searlizers with a
-     * `LegacySSHStore::Connection`.
-     *
-     * The serve protocol connection types are unidirectional, unlike
-     * this type.
-     */
-    operator ServeProto::ReadConn ()
-    {
-        return ServeProto::ReadConn {
-            .from = from,
-            .version = remoteVersion,
-        };
-    }
-
-    /*
-     * Coercion to `ServeProto::WriteConn`. This makes it easy to use the
-     * factored out serve protocol searlizers with a
-     * `LegacySSHStore::Connection`.
-     *
-     * The serve protocol connection types are unidirectional, unlike
-     * this type.
-     */
-    operator ServeProto::WriteConn ()
-    {
-        return ServeProto::WriteConn {
-            .to = to,
-            .version = remoteVersion,
-        };
-    }
 };
 
 
@@ -232,16 +197,16 @@ void LegacySSHStore::narFromPath(const StorePath & path, Sink & sink)
 }
 
 
-void LegacySSHStore::putBuildSettings(Connection & conn)
+static ServeProto::BuildOptions buildSettings()
 {
-    ServeProto::write(*this, conn, ServeProto::BuildOptions {
+    return {
         .maxSilentTime = settings.maxSilentTime,
         .buildTimeout = settings.buildTimeout,
         .maxLogSize = settings.maxLogSize,
         .nrRepeats = 0, // buildRepeat hasn't worked for ages anyway
         .enforceDeterminism = 0,
         .keepFailed = settings.keepFailed,
-    });
+    };
 }
 
 
@@ -250,14 +215,7 @@ BuildResult LegacySSHStore::buildDerivation(const StorePath & drvPath, const Bas
 {
     auto conn(connections->get());
 
-    conn->to
-        << ServeProto::Command::BuildDerivation
-        << printStorePath(drvPath);
-    writeDerivation(conn->to, *this, drv);
-
-    putBuildSettings(*conn);
-
-    conn->to.flush();
+    conn->putBuildDerivationRequest(*this, drvPath, drv, buildSettings());
 
     return ServeProto::Serialise<BuildResult>::read(*this, *conn);
 }
@@ -288,7 +246,7 @@ void LegacySSHStore::buildPaths(const std::vector<DerivedPath> & drvPaths, Build
     }
     conn->to << ss;
 
-    putBuildSettings(*conn);
+    ServeProto::write(*this, *conn, buildSettings());
 
     conn->to.flush();
 
@@ -328,15 +286,8 @@ StorePathSet LegacySSHStore::queryValidPaths(const StorePathSet & paths,
     SubstituteFlag maybeSubstitute)
 {
     auto conn(connections->get());
-
-    conn->to
-        << ServeProto::Command::QueryValidPaths
-        << false // lock
-        << maybeSubstitute;
-    ServeProto::write(*this, *conn, paths);
-    conn->to.flush();
-
-    return ServeProto::Serialise<StorePathSet>::read(*this, *conn);
+    return conn->queryValidPaths(*this,
+        false, paths, maybeSubstitute);
 }