proc: refresh cur thread/sel g after ContineOnce errors (#2081)

On platforms other than macOS this doesn't matter but on macOS a
segmentation fault will cause ContinueOnce to return an error, before
returning it we should still fix the current thread and selected
goroutine values.

Fixes #2078

_fixtures/issue2078.go

@@ -0,0 +1,10 @@
+package main
+
+func main() {
+	f(nil) //break
+	println("ok")
+}
+
+func f(x *int) {
+	println(*x)
+}

pkg/proc/proc_test.go

@@ -4809,3 +4809,27 @@ func TestStepIntoWrapperForEmbeddedPointer(t *testing.T) {
 
 	}
 }
+
+func TestRefreshCurThreadSelGAfterContinueOnceError(t *testing.T) {
+	// Issue #2078:
+	// Tests that on macOS/lldb the current thread/selected goroutine are
+	// refreshed after ContinueOnce returns an error due to a segmentation
+	// fault.
+
+	if runtime.GOOS != "darwin" && testBackend != "lldb" {
+		t.Skip("not applicable")
+	}
+
+	withTestProcess("issue2078", t, func(p *proc.Target, fixture protest.Fixture) {
+		setFileBreakpoint(p, t, fixture.Source, 4)
+		assertNoError(p.Continue(), t, "Continue() (first)")
+		if p.Continue() == nil {
+			t.Fatalf("Second continue did not return an error")
+		}
+		g := p.SelectedGoroutine()
+		if g.CurrentLoc.Line != 9 {
+			t.Fatalf("wrong current location %s:%d (expected :9)", g.CurrentLoc.File, g.CurrentLoc.Line)
+		}
+	})
+
+}

pkg/proc/target_exec.go

@@ -71,6 +71,17 @@ func (dbp *Target) Continue() error {
 		trapthread, stopReason, err := dbp.proc.ContinueOnce()
 		dbp.StopReason = stopReason
 		if err != nil {
+			// Attempt to refresh status of current thread/current goroutine, see
+			// Issue #2078.
+			// Errors are ignored because depending on why ContinueOnce failed this
+			// might very well not work.
+			if valid, _ := dbp.Valid(); valid {
+				if trapthread != nil {
+					_ = dbp.SwitchThread(trapthread.ThreadID())
+				} else if curth := dbp.CurrentThread(); curth != nil {
+					dbp.selectedGoroutine, _ = GetG(curth)
+				}
+			}
 			return err
 		}
 		if dbp.StopReason == StopLaunched {