runtime: use pipe rather than note in TestSignalM

At least on Darwin notewakeup is not async-signal-safe.

Fixes #35276

Change-Id: I1d7523715e8e77dbd7f21d9b1ed131e52d46cc41
Reviewed-on: https://go-review.googlesource.com/c/go/+/206078Reviewed-by: Austin Clements <austin@google.com>

src/runtime/crash_unix_test.go

@@ -18,6 +18,7 @@ import (
 	"sync"
 	"syscall"
 	"testing"
+	"time"
 	"unsafe"
 )
 
@@ -308,20 +309,45 @@ func TestSignalDuringExec(t *testing.T) {
 }
 
 func TestSignalM(t *testing.T) {
+	r, w, errno := runtime.Pipe()
+	if errno != 0 {
+		t.Fatal(syscall.Errno(errno))
+	}
+	defer func() {
+		runtime.Close(r)
+		runtime.Close(w)
+	}()
+	runtime.Closeonexec(r)
+	runtime.Closeonexec(w)
+
 	var want, got int64
 	var wg sync.WaitGroup
 	ready := make(chan *runtime.M)
 	wg.Add(1)
 	go func() {
 		runtime.LockOSThread()
-		want, got = runtime.WaitForSigusr1(func(mp *runtime.M) {
+		var errno int32
+		want, got = runtime.WaitForSigusr1(r, w, func(mp *runtime.M) {
 			ready <- mp
-		}, 1e9)
+		})
+		if errno != 0 {
+			t.Error(syscall.Errno(errno))
+		}
 		runtime.UnlockOSThread()
 		wg.Done()
 	}()
 	waitingM := <-ready
 	runtime.SendSigusr1(waitingM)
+
+	timer := time.AfterFunc(time.Second, func() {
+		// Write 1 to tell WaitForSigusr1 that we timed out.
+		bw := byte(1)
+		if n := runtime.Write(uintptr(w), unsafe.Pointer(&bw), 1); n != 1 {
+			t.Errorf("pipe write failed: %d", n)
+		}
+	})
+	defer timer.Stop()
+
 	wg.Wait()
 	if got == -1 {
 		t.Fatal("signalM signal not received")

src/runtime/export_unix_test.go

@@ -6,6 +6,8 @@
 
 package runtime
 
+import "unsafe"
+
 var NonblockingPipe = nonblockingPipe
 var Pipe = pipe
 var SetNonblock = setNonblock
@@ -26,33 +28,45 @@ func Sigisblocked(i int) bool {
 type M = m
 
 var waitForSigusr1 struct {
-	park note
+	rdpipe int32
+	wrpipe int32
 	mID    int64
 }
 
 // WaitForSigusr1 blocks until a SIGUSR1 is received. It calls ready
 // when it is set up to receive SIGUSR1. The ready function should
-// cause a SIGUSR1 to be sent.
+// cause a SIGUSR1 to be sent. The r and w arguments are a pipe that
+// the signal handler can use to report when the signal is received.
 //
 // Once SIGUSR1 is received, it returns the ID of the current M and
-// the ID of the M the SIGUSR1 was received on. If no SIGUSR1 is
-// received for timeoutNS nanoseconds, it returns -1.
-func WaitForSigusr1(ready func(mp *M), timeoutNS int64) (int64, int64) {
+// the ID of the M the SIGUSR1 was received on. If the caller writes
+// a non-zero byte to w, WaitForSigusr1 returns immediately with -1, -1.
+func WaitForSigusr1(r, w int32, ready func(mp *M)) (int64, int64) {
 	lockOSThread()
 	// Make sure we can receive SIGUSR1.
 	unblocksig(_SIGUSR1)
 
+	waitForSigusr1.rdpipe = r
+	waitForSigusr1.wrpipe = w
+
 	mp := getg().m
 	testSigusr1 = waitForSigusr1Callback
 	ready(mp)
-	ok := notetsleepg(&waitForSigusr1.park, timeoutNS)
-	noteclear(&waitForSigusr1.park)
+
+	// Wait for the signal. We use a pipe rather than a note
+	// because write is always async-signal-safe.
+	entersyscallblock()
+	var b byte
+	read(waitForSigusr1.rdpipe, noescape(unsafe.Pointer(&b)), 1)
+	exitsyscall()
+
 	gotM := waitForSigusr1.mID
 	testSigusr1 = nil
 
 	unlockOSThread()
 
-	if !ok {
+	if b != 0 {
+		// timeout signal from caller
 		return -1, -1
 	}
 	return mp.id, gotM
@@ -69,7 +83,8 @@ func waitForSigusr1Callback(gp *g) bool {
 	} else {
 		waitForSigusr1.mID = gp.m.id
 	}
-	notewakeup(&waitForSigusr1.park)
+	b := byte(0)
+	write(uintptr(waitForSigusr1.wrpipe), noescape(unsafe.Pointer(&b)), 1)
 	return true
 }