runtime: handle windows callback on non-go thread

Adds an extra M in mstartm0 and accounts for it in checkdead. This allows Windows callbacks created with syscall.NewCallback and syscall.NewCallbackCDecl to be called on a non-Go thread.

Fixes #6751

Change-Id: I57626bc009a6370b9ca0827ab64b14b01dec39d4
GitHub-Last-Rev: d429e3eed923640edab580bdb47fcb81e75dbfe8
GitHub-Pull-Request: golang/go#25575
Reviewed-on: https://go-review.googlesource.com/114802Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -1293,7 +1293,9 @@ func mstart1() {
 //go:yeswritebarrierrec
 func mstartm0() {
 	// Create an extra M for callbacks on threads not created by Go.
-	if iscgo && !cgoHasExtraM {
+	// An extra M is also needed on Windows for callbacks created by
+	// syscall.NewCallback. See issue #6751 for details.
+	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
 		cgoHasExtraM = true
 		newextram()
 	}
@@ -1618,8 +1620,12 @@ func allocm(_p_ *p, fn func()) *m {
 // put the m back on the list.
 //go:nosplit
 func needm(x byte) {
-	if iscgo && !cgoHasExtraM {
+	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
 		// Can happen if C/C++ code calls Go from a global ctor.
+		// Can also happen on Windows if a global ctor uses a
+		// callback created by syscall.NewCallback. See issue #6751
+		// for details.
+		//
 		// Can not throw, because scheduler is not initialized yet.
 		write(2, unsafe.Pointer(&earlycgocallback[0]), int32(len(earlycgocallback)))
 		exit(1)
@@ -4215,8 +4221,17 @@ func checkdead() {
 		return
 	}
 
+	// If we are not running under cgo, but we have an extra M then account
+	// for it. (It is possible to have an extra M on Windows without cgo to
+	// accommodate callbacks created by syscall.NewCallback. See issue #6751
+	// for details.)
+	var run0 int32
+	if !iscgo && cgoHasExtraM {
+		run0 = 1
+	}
+
 	run := mcount() - sched.nmidle - sched.nmidlelocked - sched.nmsys
-	if run > 0 {
+	if run > run0 {
 		return
 	}
 	if run < 0 {

src/runtime/syscall_windows_test.go

@@ -251,8 +251,6 @@ func TestBlockingCallback(t *testing.T) {
 }
 
 func TestCallbackInAnotherThread(t *testing.T) {
-	t.Skip("Skipping failing test (see golang.org/issue/6751 for details)")
-
 	d := GetDLL(t, "kernel32.dll")
 
 	f := func(p uintptr) uintptr {