a request context on the ipc server, cancelled by Close (V-638)

serveConn dispatched under context.Background(), so a dispatch in flight
during shutdown could not be told to stop and closeGrace could only
abandon it. The server now carries a context, Close cancels it, and each
conn derives its own so nothing outlives the connection.

A zero-value Server built outside Listen falls back to Background; two
wiring tests do that.

Client.Close read c.conn with no lock while roundtrip re-dialed and
dropped it, which -race caught on the new test. The conn field now has
its own mutex, held only across a read or an assignment, so Close and
the watchdog reach the connection without queueing behind the call they
are interrupting.

internal/ipc/server.go

@@ -30,6 +30,14 @@ type Server struct {
 	done   chan struct{}
 	accept sync.Mutex // guards wg.Add vs Close's wg.Wait sequence
 
+	// ctx — server-scoped, cancelled by Close, and the parent of every request
+	// context. serveConn dispatched under context.Background() until V-638, so
+	// a dispatch in flight during shutdown could not be told to stop and the
+	// closeGrace below could only abandon it. Cancelling gives a handler that
+	// respects its context the chance to return instead.
+	ctx    context.Context
+	cancel context.CancelFunc
+
 	// conns — every accepted connection still being served. Close needs these
 	// because closing the listener does nothing to a connection already
 	// accepted: serveConn is parked in readFrame waiting for a peer that may
@@ -208,11 +216,14 @@ func Listen(path string, api CoreAPI) (*Server, error) {
 	if err != nil {
 		return nil, err
 	}
+	ctx, cancel := context.WithCancel(context.Background())
 	s := &Server{
-		path: path,
-		addr: addr,
-		ln:   ln,
-		done: make(chan struct{}),
+		path:   path,
+		addr:   addr,
+		ln:     ln,
+		done:   make(chan struct{}),
+		ctx:    ctx,
+		cancel: cancel,
 	}
 	s.api.Store(api)
 	return s, nil
@@ -250,7 +261,10 @@ func (s *Server) Serve() error {
 
 func (s *Server) serveConn(c net.Conn) {
 	caller, callerOK := peerCaller(c)
-	ctx := context.Background()
+	// Derived from the server's, so Close cancels a dispatch in flight, and
+	// cancelled when this conn ends so nothing a handler spawned outlives it.
+	ctx, cancel := context.WithCancel(s.serverContext())
+	defer cancel()
 	if callerOK {
 		ctx = WithCaller(ctx, caller)
 	}
@@ -274,6 +288,15 @@ func (s *Server) serveConn(c net.Conn) {
 	}
 }
 
+// serverContext is s.ctx, or Background for a Server built as a zero value
+// rather than by Listen (the wiring tests do that).
+func (s *Server) serverContext() context.Context {
+	if s.ctx == nil {
+		return context.Background()
+	}
+	return s.ctx
+}
+
 func (s *Server) safeDispatch(ctx context.Context, req Request) (result json.RawMessage, err error) {
 	defer func() {
 		if r := recover(); r != nil {
@@ -720,6 +743,9 @@ func (s *Server) Close() error {
 	default:
 		close(s.done)
 	}
+	if s.cancel != nil {
+		s.cancel()
+	}
 	err := s.ln.Close()
 	// Closing the listener stops new connections; it does nothing to the ones
 	// already accepted. Close those too, or every serveConn parked in readFrame