runtime: handle windows callback on non-go thread

- add comments as per reviewer

Author: billziss-gh

src/runtime/proc.go

@@ -1293,6 +1293,8 @@ func mstart1() {
 //go:yeswritebarrierrec
 func mstartm0() {
 	// Create an extra M for callbacks on threads not created by Go.
+	// An extra M is also needed on Windows for callbacks created by
+	// syscall.NewCallback.
 	if (iscgo || GOOS == "windows") && !cgoHasExtraM {
 		cgoHasExtraM = true
 		newextram()
@@ -1620,6 +1622,9 @@ func allocm(_p_ *p, fn func()) *m {
 func needm(x byte) {
 	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 using syscall.NewCallback.
+		//
 		// Can not throw, because scheduler is not initialized yet.
 		write(2, unsafe.Pointer(&earlycgocallback[0]), int32(len(earlycgocallback)))
 		exit(1)
@@ -4215,6 +4220,9 @@ 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.)
 	var run0 int32
 	if !iscgo && cgoHasExtraM {
 		run0 = 1