gateway: wrap ExecProcessServer Send calls with a mutex

According to gRPC ClientStream interface documentation it isn't safe to call
SendMsg on the same stream in different goroutines, which we do in
LLBBridgeServer implementation. The generated bindings don't provide
a mechanism to avoid this scenario. This change wraps LLBBridgeServer
implementation with a custom locking behavior:
https://pkg.go.dev/google.golang.org/[email protected]#ClientStream.SendMsg

Signed-off-by: Matias Insaurralde <[email protected]>

Author: matiasinsaurralde

frontend/gateway/gateway.go

@@ -1176,7 +1176,21 @@ func (w *outputWriter) Write(msg []byte) (int, error) {
 	return len(msg), stack.Enable(err)
 }
 
+type execProcessServerThreadSafe struct {
+	pb.LLBBridge_ExecProcessServer
+	sendMu sync.Mutex
+}
+
+func (w *execProcessServerThreadSafe) Send(m *pb.ExecMessage) error {
+	w.sendMu.Lock()
+	defer w.sendMu.Unlock()
+	return w.LLBBridge_ExecProcessServer.Send(m)
+}
+
 func (lbf *llbBridgeForwarder) ExecProcess(srv pb.LLBBridge_ExecProcessServer) error {
+	srv = &execProcessServerThreadSafe{
+		LLBBridge_ExecProcessServer: srv,
+	}
 	eg, ctx := errgroup.WithContext(srv.Context())
 
 	msgs := make(chan *pb.ExecMessage)