runtime: add test for signalM

For #10958, #24543.

Change-Id: Ib009a83fe02bc623894f4908fe8f6b266382ba95
Reviewed-on: https://go-review.googlesource.com/c/go/+/201404
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/export_unix_test.go

@@ -17,3 +17,42 @@ func Sigisblocked(i int) bool {
 	sigprocmask(_SIG_SETMASK, nil, &sigmask)
 	return sigismember(&sigmask, i)
 }
+
+type M = m
+
+var waitForSigusr1 struct {
+	park note
+	mp   *m
+}
+
+// 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.
+//
+// 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) {
+	mp := getg().m
+	testSigusr1 = func(gp *g) bool {
+		waitForSigusr1.mp = gp.m
+		notewakeup(&waitForSigusr1.park)
+		return true
+	}
+	ready(mp)
+	ok := notetsleepg(&waitForSigusr1.park, timeoutNS)
+	noteclear(&waitForSigusr1.park)
+	gotM := waitForSigusr1.mp
+	waitForSigusr1.mp = nil
+	testSigusr1 = nil
+
+	if !ok {
+		return -1, -1
+	}
+	return mp.id, gotM.id
+}
+
+// SendSigusr1 sends SIGUSR1 to mp.
+func SendSigusr1(mp *M) {
+	signalM(mp, _SIGUSR1)
+}

src/runtime/preemptm_test.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package runtime_test
+
+import (
+	"runtime"
+	"sync"
+	"testing"
+)
+
+func TestPreemptM(t *testing.T) {
+	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) {
+			ready <- mp
+		}, 1e9)
+		runtime.UnlockOSThread()
+		wg.Done()
+	}()
+	runtime.SendSigusr1(<-ready)
+	wg.Wait()
+	if got == -1 {
+		t.Fatal("preemptM signal not received")
+	} else if want != got {
+		t.Fatalf("signal sent to M %d, but received on M %d", want, got)
+	}
+}

src/runtime/signal_unix.go

@@ -412,10 +412,11 @@ func adjustSignalStack(sig uint32, mp *m, gsigStack *gsignalStack) bool {
 // GOTRACEBACK=crash when a signal is received.
 var crashing int32
 
-// testSigtrap is used by the runtime tests. If non-nil, it is called
-// on SIGTRAP. If it returns true, the normal behavior on SIGTRAP is
-// suppressed.
+// testSigtrap and testSigusr1 are used by the runtime tests. If
+// non-nil, it is called on SIGTRAP/SIGUSR1. If it returns true, the
+// normal behavior on this signal is suppressed.
 var testSigtrap func(info *siginfo, ctxt *sigctxt, gp *g) bool
+var testSigusr1 func(gp *g) bool
 
 // sighandler is invoked when a signal occurs. The global g will be
 // set to a gsignal goroutine and we will be running on the alternate
@@ -441,6 +442,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		return
 	}
 
+	if sig == _SIGUSR1 && testSigusr1 != nil && testSigusr1(gp) {
+		return
+	}
+
 	flags := int32(_SigThrow)
 	if sig < uint32(len(sigtable)) {
 		flags = sigtable[sig].flags