service/dap: limit the number of goroutines to return from a threads request (#2595)

This adds a cap and a log message if there are many goroutines. This will help
prevent the debugger from freezing, but does not yet address making sure the
interesting goroutines are the ones that are returned.

Updates golang/vscode-go#129

service/dap/server.go

@@ -191,6 +191,8 @@ const (
 	// what is presented. A common use case of a call injection is to
 	// stringify complex data conveniently.
 	maxStringLenInCallRetVars = 1 << 10 // 1024
+	// Max number of goroutines that we will return.
+	maxGoroutines = 1 << 10
 )
 
 // NewServer creates a new DAP Server. It takes an opened Listener
@@ -1456,7 +1458,7 @@ func (s *Server) onThreadsRequest(request *dap.ThreadsRequest) {
 		return
 	}
 
-	gs, _, err := s.debugger.Goroutines(0, 0)
+	gs, next, err := s.debugger.Goroutines(0, maxGoroutines)
 	if err != nil {
 		switch err.(type) {
 		case proc.ErrProcessExited:
@@ -1469,6 +1471,10 @@ func (s *Server) onThreadsRequest(request *dap.ThreadsRequest) {
 		return
 	}
 
+	if next >= 0 {
+		s.logToConsole(fmt.Sprintf("too many goroutines, only loaded %d", len(gs)))
+	}
+
 	threads := make([]dap.Thread, len(gs))
 	if len(threads) == 0 {
 		// Depending on the debug session stage, goroutines information