runtime: avoid CreateThread panic when exiting process

On Windows, CreateThread occasionally fails with ERROR_ACCESS_DENIED.
We're not sure why this is, but the Wine source code suggests that
this can happen when there's a concurrent CreateThread and ExitProcess
in the same process.

Fix this by setting a flag right before calling ExitProcess and
halting if CreateThread fails and this flag is set.

Updates #18253 (might fix it, but we're not sure this is the issue and
can't reproduce it on demand).

Change-Id: I1945b989e73a16cf28a35bf2613ffab07577ed4e
Reviewed-on: https://go-review.googlesource.com/34616
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_windows.go

@@ -332,8 +332,12 @@ func goenvs() {
 	stdcall1(_FreeEnvironmentStringsW, uintptr(strings))
 }
 
+// exiting is set to non-zero when the process is exiting.
+var exiting uint32
+
 //go:nosplit
 func exit(code int32) {
+	atomic.Store(&exiting, 1)
 	stdcall1(_ExitProcess, uintptr(code))
 }
 
@@ -519,6 +523,14 @@ func newosproc(mp *m, stk unsafe.Pointer) {
 		_STACK_SIZE_PARAM_IS_A_RESERVATION, 0)
 
 	if thandle == 0 {
+		if atomic.Load(&exiting) != 0 {
+			// CreateThread may fail if called
+			// concurrently with ExitProcess. If this
+			// happens, just freeze this thread and let
+			// the process exit. See issue #18253.
+			lock(&deadlock)
+			lock(&deadlock)
+		}
 		print("runtime: failed to create new OS thread (have ", mcount(), " already; errno=", getlasterror(), ")\n")
 		throw("runtime.newosproc")
 	}