Factor our `ServeProto::BasicServerConnection::handshake`

We'll need this for unit testing.

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

src/libstore/serve-protocol-impl.cc

@@ -22,6 +22,18 @@ ServeProto::Version ServeProto::BasicClientConnection::handshake(
     return remoteVersion;
 }
 
+ServeProto::Version ServeProto::BasicServerConnection::handshake(
+    BufferedSink & to,
+    Source & from,
+    ServeProto::Version localVersion)
+{
+    unsigned int magic = readInt(from);
+    if (magic != SERVE_MAGIC_1) throw Error("protocol mismatch");
+    to << SERVE_MAGIC_2 << localVersion;
+    to.flush();
+    return readInt(from);
+}
+
 
 StorePathSet ServeProto::BasicClientConnection::queryValidPaths(
     const Store & store,

src/libstore/serve-protocol-impl.hh

@@ -132,4 +132,26 @@ struct ServeProto::BasicClientConnection
         const ServeProto::BuildOptions & options);
 };
 
+struct ServeProto::BasicServerConnection
+{
+    /**
+     * Establishes connection, negotiating version.
+     *
+     * @return the version provided by the other side of the
+     * connection.
+     *
+     * @param to Taken by reference to allow for various error handling
+     * mechanisms.
+     *
+     * @param from Taken by reference to allow for various error
+     * handling mechanisms.
+     *
+     * @param localVersion Our version which is sent over
+     */
+    static ServeProto::Version handshake(
+        BufferedSink & to,
+        Source & from,
+        ServeProto::Version localVersion);
+};
+
 }

src/libstore/serve-protocol.hh

@@ -65,6 +65,7 @@ struct ServeProto
      * @todo remove once Hydra uses Store abstraction consistently.
      */
     struct BasicClientConnection;
+    struct BasicServerConnection;
 
     /**
      * Data type for canonical pairs of serialisers for the serve protocol.

src/nix-store/nix-store.cc

@@ -828,11 +828,9 @@ static void opServe(Strings opFlags, Strings opArgs)
     FdSink out(STDOUT_FILENO);
 
     /* Exchange the greeting. */
-    unsigned int magic = readInt(in);
-    if (magic != SERVE_MAGIC_1) throw Error("protocol mismatch");
-    out << SERVE_MAGIC_2 << SERVE_PROTOCOL_VERSION;
-    out.flush();
-    ServeProto::Version clientVersion = readInt(in);
+    ServeProto::Version clientVersion =
+        ServeProto::BasicServerConnection::handshake(
+            out, in, SERVE_PROTOCOL_VERSION);
 
     ServeProto::ReadConn rconn {
         .from = in,