[release-branch.go1.21] runtime: profiling on Darwin cannot use blocking reads

On Darwin (and assume also on iOS but not sure), notetsleepg
cannot be called in a signal-handling context.  Avoid this
by disabling block reads on Darwin.

An alternate approach was to add "sigNote" with a pipe-based
implementation on Darwin, but that ultimately would have required
at least one more linkname between runtime and syscall to avoid
racing with fork and opening the pipe, so, not.

Fixes #62019.
Updates #61768.

Change-Id: I0e8dd4abf9a606a3ff73fc37c3bd75f55924e07e
Reviewed-on: https://go-review.googlesource.com/c/go/+/518836
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
(cherry picked from commit c6ee8e3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/519375
Reviewed-by: Austin Clements <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dr2chase

src/runtime/cpuprof.go

@@ -227,7 +227,11 @@ func runtime_pprof_readProfile() ([]uint64, []unsafe.Pointer, bool) {
 	lock(&cpuprof.lock)
 	log := cpuprof.log
 	unlock(&cpuprof.lock)
-	data, tags, eof := log.read(profBufBlocking)
+	readMode := profBufBlocking
+	if GOOS == "darwin" || GOOS == "ios" {
+		readMode = profBufNonBlocking // For #61768; on Darwin notes are not async-signal-safe.  See sigNoteSetup in os_darwin.go.
+	}
+	data, tags, eof := log.read(readMode)
 	if len(data) == 0 && eof {
 		lock(&cpuprof.lock)
 		cpuprof.log = nil

src/runtime/os_darwin.go

@@ -81,7 +81,7 @@ func semawakeup(mp *m) {
 // The read and write file descriptors used by the sigNote functions.
 var sigNoteRead, sigNoteWrite int32
 
-// sigNoteSetup initializes an async-signal-safe note.
+// sigNoteSetup initializes a single, there-can-only-be-one, async-signal-safe note.
 //
 // The current implementation of notes on Darwin is not async-signal-safe,
 // because the functions pthread_mutex_lock, pthread_cond_signal, and
@@ -93,6 +93,7 @@ var sigNoteRead, sigNoteWrite int32
 // not support timed waits but is async-signal-safe.
 func sigNoteSetup(*note) {
 	if sigNoteRead != 0 || sigNoteWrite != 0 {
+		// Generalizing this would require avoiding the pipe-fork-closeonexec race, which entangles syscall.
 		throw("duplicate sigNoteSetup")
 	}
 	var errno int32

src/runtime/profbuf.go

@@ -491,6 +491,7 @@ Read:
 		// Nothing to read right now.
 		// Return or sleep according to mode.
 		if mode == profBufNonBlocking {
+			// Necessary on Darwin, notetsleepg below does not work in signal handler, root cause of #61768.
 			return nil, nil, false
 		}
 		if !b.w.cas(bw, bw|profReaderSleeping) {