service/debugger: evaluate breakpoint vars on g-less threads (#3759)

Use a thread scope to evaluate breakpoint variables if the current
thread does not have an associated goroutine.

Fixes #3758

cmd/dlv/dlv_test.go

@@ -1082,7 +1082,7 @@ func TestTraceBreakpointExists(t *testing.T) {
 	// We always set breakpoints on some runtime functions at startup, so this would return with
 	// a breakpoints exists error.
 	// TODO: Perhaps we shouldn't be setting these default breakpoints in trace mode, however.
-	cmd := exec.Command(dlvbin, "trace", "--output", filepath.Join(t.TempDir(), "__debug"), filepath.Join(fixtures, "issue573.go"), "runtime.*")
+	cmd := exec.Command(dlvbin, "trace", "--output", filepath.Join(t.TempDir(), "__debug"), filepath.Join(fixtures, "issue573.go"), "runtime.*panic")
 	rdr, err := cmd.StderrPipe()
 	assertNoError(err, t, "stderr pipe")
 	defer rdr.Close()

service/debugger/debugger.go

@@ -1388,7 +1388,13 @@ func (d *Debugger) collectBreakpointInformation(apiThread *api.Thread, thread pr
 
 	s, err := proc.GoroutineScope(tgt, thread)
 	if err != nil {
-		return err
+		var errNoGoroutine proc.ErrNoGoroutine
+		if errors.As(err, &errNoGoroutine) {
+			s, err = proc.ThreadScope(tgt, thread)
+		}
+		if err != nil {
+			return err
+		}
 	}
 
 	if len(bp.Variables) > 0 {

service/test/integration2_test.go

@@ -3153,3 +3153,17 @@ func TestNextInstruction(t *testing.T) {
 		}
 	})
 }
+
+func TestBreakpointVariablesWithoutG(t *testing.T) {
+	// Tests that evaluating variables on a breakpoint that is hit on a thread
+	// without a goroutine does not cause an error.
+	withTestClient2("math", t, func(c service.Client) {
+		_, err := c.CreateBreakpoint(&api.Breakpoint{
+			FunctionName: "runtime.mallocgc",
+			LoadArgs:     &normalLoadConfig,
+		})
+		assertNoError(err, t, "CreateBreakpoint")
+		state := <-c.Continue()
+		assertNoError(state.Err, t, "Continue()")
+	})
+}