proc/linux: do not route signals to threads while stopping (#1752)

* proc/linux: do not route signals to threads while stopping

While we are trying to stop the process we should not route signals
sent to threads because that will result in threads being resumed.
Also keep better track of which threads are stopped.

This fixes an incompatibility with Go 1.14, which sends a lot of
signals to its threads to implement non-cooperative preemption,
resulting in Delve hanging waiting for an already-stopped thread to
stop.

In principle however this bug has nothing to do with Go 1.14 and could
manifest in any instance of high signal pressure.

* Makefile: discard stderr of "go list"

In module mode "go" will print messages about downloading modules to
stderr, we shouldn't confuse them for the real command output.

pkg/proc/native/proc_linux.go

@@ -319,6 +319,16 @@ func (dbp *Process) trapWaitInternal(pid int, halt bool) (*Thread, error) {
 		}
 
 		// TODO(dp) alert user about unexpected signals here.
+		if halt && !th.os.running {
+			// We are trying to stop the process, queue this signal to be delivered
+			// to the thread when we resume.
+			// Do not do this for threads that were running because we sent them a
+			// STOP signal and we need to observe it so we don't mistakenly deliver
+			// it later.
+			th.os.delayedSignal = int(status.StopSignal())
+			th.os.running = false
+			return th, nil
+		} else {
 			if err := th.resumeWithSig(int(status.StopSignal())); err != nil {
 				if err == sys.ESRCH {
 					dbp.postExit()
@@ -328,6 +338,7 @@ func (dbp *Process) trapWaitInternal(pid int, halt bool) (*Thread, error) {
 			}
 		}
 	}
+}
 
 func status(pid int, comm string) rune {
 	f, err := os.Open(fmt.Sprintf("/proc/%d/stat", pid))
@@ -429,6 +440,9 @@ func (dbp *Process) stop(trapthread *Thread) (err error) {
 			if err := th.stop(); err != nil {
 				return dbp.exitGuard(err)
 			}
+		} else {
+			// Thread is already in a trace stop but we didn't get the notification yet.
+			th.os.running = false
 		}
 	}
 

pkg/proc/native/threads_linux.go

@@ -16,6 +16,7 @@ type WaitStatus sys.WaitStatus
 // OSSpecificDetails hold Linux specific
 // process details.
 type OSSpecificDetails struct {
+	delayedSignal int
 	registers     sys.PtraceRegs
 	running       bool
 }
@@ -37,7 +38,9 @@ func (t *Thread) Stopped() bool {
 }
 
 func (t *Thread) resume() error {
-	return t.resumeWithSig(0)
+	sig := t.os.delayedSignal
+	t.os.delayedSignal = 0
+	return t.resumeWithSig(sig)
 }
 
 func (t *Thread) resumeWithSig(sig int) (err error) {

pkg/proc/proc_test.go

@@ -953,7 +953,7 @@ func TestStacktraceGoroutine(t *testing.T) {
 					if locations[i].Call.Fn != nil {
 						name = locations[i].Call.Fn.Name
 					}
-					t.Logf("\t%s:%d %s (%#x)\n", locations[i].Call.File, locations[i].Call.Line, name, locations[i].Current.PC)
+					t.Logf("\t%s:%d %s (%#x) %x %v\n", locations[i].Call.File, locations[i].Call.Line, name, locations[i].Current.PC, locations[i].FrameOffset(), locations[i].SystemStack)
 				}
 			}
 		}

scripts/make.go

@@ -187,7 +187,7 @@ func quotemaybe(args []string) []string {
 func getoutput(cmd string, args ...interface{}) string {
 	x := exec.Command(cmd, strflatten(args)...)
 	x.Env = os.Environ()
-	out, err := x.CombinedOutput()
+	out, err := x.Output()
 	if err != nil {
 		log.Fatal(err)
 	}