runtime: use a pipe to wake up signal_recv on Darwin

The implementation of semaphores, and therefore notes, used on Darwin
is not async-signal-safe. The runtime has one case where a note needs
to be woken up from a signal handler: the call to notewakeup in sigsend.
That notewakeup call is only called on a single note, and it doesn't
need the full functionality of notes: nothing ever does a timed wait on it.
So change that one note to use a different implementation on Darwin,
based on a pipe. This lets the wakeup code use the write call, which is
async-signal-safe.

Fixes #31264

Change-Id: If705072d7a961dd908ea9d639c8d12b222c64806
Reviewed-on: https://go-review.googlesource.com/c/go/+/184169
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/sigqueue.go

@@ -105,6 +105,10 @@ Send:
 			break Send
 		case sigReceiving:
 			if atomic.Cas(&sig.state, sigReceiving, sigIdle) {
+				if GOOS == "darwin" {
+					sigNoteWakeup(&sig.note)
+					break Send
+				}
 				notewakeup(&sig.note)
 				break Send
 			}
@@ -136,6 +140,10 @@ func signal_recv() uint32 {
 				throw("signal_recv: inconsistent state")
 			case sigIdle:
 				if atomic.Cas(&sig.state, sigIdle, sigReceiving) {
+					if GOOS == "darwin" {
+						sigNoteSleep(&sig.note)
+						break Receive
+					}
 					notetsleepg(&sig.note, -1)
 					noteclear(&sig.note)
 					break Receive
@@ -188,6 +196,10 @@ func signal_enable(s uint32) {
 		// to use for initialization. It does not pass
 		// signal information in m.
 		sig.inuse = true // enable reception of signals; cannot disable
+		if GOOS == "darwin" {
+			sigNoteSetup(&sig.note)
+			return
+		}
 		noteclear(&sig.note)
 		return
 	}