debugger: report error when switching goroutine is impossible (#2424)

Due to variable shadowing the SwitchGoroutine command never failed.

service/dap/server.go

@@ -1023,14 +1023,16 @@ func stoppedGoroutineID(state *api.DebuggerState) (id int) {
 
 func (s *Server) doStepCommand(command string, threadId int) {
 	// Use SwitchGoroutine to change the current goroutine.
-	state, err := s.debugger.Command(&api.DebuggerCommand{Name: api.SwitchGoroutine, GoroutineID: threadId}, nil)
+	_, err := s.debugger.Command(&api.DebuggerCommand{Name: api.SwitchGoroutine, GoroutineID: threadId}, nil)
 	if err != nil {
-		s.log.Errorf("Error switching goroutines while stepping: %e", err)
+		s.log.Errorf("Error switching goroutines while stepping: %v", err)
 		// If we encounter an error, we will have to send a stopped event
 		// since we already sent the step response.
 		stopped := &dap.StoppedEvent{Event: *newEvent("stopped")}
 		stopped.Body.AllThreadsStopped = true
-		if state != nil {
+		if state, err := s.debugger.State(false); err != nil {
+			s.log.Errorf("Error retrieving state: %e", err)
+		} else {
 			stopped.Body.ThreadId = stoppedGoroutineID(state)
 		}
 		stopped.Body.Reason = "error"

service/dap/server_test.go

@@ -2202,9 +2202,12 @@ func TestNextAndStep(t *testing.T) {
 					client.ExpectStepInResponse(t)
 					expectStop("main.inlineThis", 5)
 
-					client.NextRequest(-10000 /*this is ignored*/)
+					client.NextRequest(-1000)
 					client.ExpectNextResponse(t)
-					expectStop("main.inlineThis", 6)
+					if se := client.ExpectStoppedEvent(t); se.Body.Reason != "error" || se.Body.Text != "unknown goroutine -1000" {
+						t.Errorf("got %#v, want Reaspon=\"error\", Text=\"unknown goroutine -1000\"", se)
+					}
+					handleStop(t, client, 1, "main.inlineThis", 5)
 				},
 				disconnect: false,
 			}})

service/debugger/debugger.go

@@ -1095,7 +1095,8 @@ func (d *Debugger) Command(command *api.DebuggerCommand, resumeNotify chan struc
 		withBreakpointInfo = false
 	case api.SwitchGoroutine:
 		d.log.Debugf("switching to goroutine %d", command.GoroutineID)
-		g, err := proc.FindGoroutine(d.target, command.GoroutineID)
+		var g *proc.G
+		g, err = proc.FindGoroutine(d.target, command.GoroutineID)
 		if err == nil {
 			err = d.target.SwitchGoroutine(g)
 		}