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/+/206078
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 25 insertions, 10 deletions

diff --git a/src/runtime/export_unix_test.go b/src/runtime/export_unix_test.go
index 375513337e..621488eaba 100644
--- a/src/runtime/export_unix_test.go
+++ b/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
-	mID  int64
+	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
 }