Start factoring out the serve protocol for Hydra to share

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

The parent commit is chosen such that this should work for 2.6 and
master alike.

src/libstore/legacy-ssh-store.cc

@@ -2,6 +2,7 @@
 #include "pool.hh"
 #include "remote-store.hh"
 #include "serve-protocol.hh"
+#include "serve-protocol-impl.hh"
 #include "store-api.hh"
 #include "path-with-outputs.hh"
 #include "worker-protocol.hh"
@@ -31,12 +32,9 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
     // the documentation
     const Setting<int> logFD{(StoreConfig*) this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"};
 
-    struct Connection
+    struct Connection : public serve_proto::BasicConnection
     {
         std::unique_ptr<SSHMaster::Connection> sshConn;
-        FdSink to;
-        FdSource from;
-        int remoteVersion;
         bool good = true;
     };
 
@@ -353,15 +351,8 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
         SubstituteFlag maybeSubstitute = NoSubstitute) override
     {
         auto conn(connections->get());
-
-        conn->to
-            << cmdQueryValidPaths
-            << false // lock
-            << maybeSubstitute;
-        worker_proto::write(*this, conn->to, paths);
-        conn->to.flush();
-
-        return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+        return serve_proto::queryValidPaths(*conn, *this,
+            false, paths, maybeSubstitute);
     }
 
     void connect() override

src/libstore/serve-protocol-impl.cc

@@ -0,0 +1,24 @@
+#include "serve-protocol-impl.hh"
+
+#include "remote-store.hh"
+
+namespace nix {
+namespace serve_proto {
+
+StorePathSet queryValidPaths(
+    BasicConnection & conn, const Store & remoteStore,
+    bool lock, const StorePathSet & paths,
+    SubstituteFlag maybeSubstitute)
+{
+    conn.to
+        << cmdQueryValidPaths
+        << lock
+        << maybeSubstitute;
+    worker_proto::write(remoteStore, conn.to, paths);
+    conn.to.flush();
+
+    return worker_proto::read(remoteStore, conn.from, Phantom<StorePathSet> {});
+}
+
+}
+}

src/libstore/serve-protocol-impl.hh

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "serialise.hh"
+
+#include "store-api.hh"
+#include "serve-protocol.hh"
+#include "worker-protocol.hh"
+
+namespace nix {
+namespace serve_proto {
+
+struct BasicConnection
+{
+    FdSink to;
+    FdSource from;
+    int remoteVersion;
+};
+
+StorePathSet queryValidPaths(
+    BasicConnection & conn, const Store & remoteStore,
+    bool lock, const StorePathSet & paths,
+    SubstituteFlag maybeSubstitute);
+
+}
+}