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

@@ -40,45 +40,10 @@ 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 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,
-            };
-        }
     };
 
     std::string host;
@@ -402,15 +367,8 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
         SubstituteFlag maybeSubstitute = NoSubstitute) override
     {
         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);
     }
 
     void connect() override

src/libstore/serve-protocol-impl.cc

@@ -0,0 +1,20 @@
+#include "serve-protocol-impl.hh"
+
+namespace nix {
+
+StorePathSet ServeProto::BasicClientConnection::queryValidPaths(
+    const Store & store,
+    bool lock, const StorePathSet & paths,
+    SubstituteFlag maybeSubstitute)
+{
+    to
+        << ServeProto::Command::QueryValidPaths
+        << lock
+        << maybeSubstitute;
+    write(store, *this, paths);
+    to.flush();
+
+    return Serialise<StorePathSet>::read(store, *this);
+}
+
+}

src/libstore/serve-protocol-impl.hh

@@ -10,6 +10,7 @@
 
 #include "serve-protocol.hh"
 #include "length-prefixed-protocol-helper.hh"
+#include "store-api.hh"
 
 namespace nix {
 
@@ -56,4 +57,48 @@ struct ServeProto::Serialise
 
 /* protocol-specific templates */
 
+struct ServeProto::BasicClientConnection
+{
+    FdSink to;
+    FdSource from;
+    ServeProto::Version remoteVersion;
+
+    /**
+     * 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,
+        };
+    }
+
+    StorePathSet queryValidPaths(
+        const Store & remoteStore,
+        bool lock, const StorePathSet & paths,
+        SubstituteFlag maybeSubstitute);
+};
+
 }

src/libstore/serve-protocol.hh

@@ -58,6 +58,13 @@ struct ServeProto
         Version version;
     };
 
+    /**
+     * Stripped down serialization logic suitable for sharing with Hydra.
+     *
+     * @todo remove once Hydra uses Store abstraction consistently.
+     */
+    struct BasicClientConnection;
+
     /**
      * Data type for canonical pairs of serialisers for the serve protocol.
      *