[Patch] Fix gethwnd race

[Brian Ford]

On Thu, 6 May 2004, Christopher Faylor wrote:

> Thanks, but, I see that you're using busy loops.  I use those in places
> where I have no choice but to do so or when the potential for a race is
> unlikely.
>
> I don't think that this is really a situation that qualifies for either.
> It seems like a muto is a cleaner choice here.

Sorry for the delay; my free time has been in short supply lately.

I can't seem to make a muto fit this situation cleanly since it would have
to be acquired and released by the same thread.  Maybe I missed something
obvious, but I think the attached patch addresses the root of your
objection to the previous patch.

2004-05-13  Brian Ford  <ford@vss.fsi.com>

	* window.cc (window_started): Make NO_COPY.
	(gethwnd): Fix initialization race.
	(window_init): New function to initialize window_started.
	* winsup.h (window_init): Prototype it.
	* dcrt0.cc (dll_crt0_1): Call it.

-- 
Brian Ford
Senior Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained pilot...
-------------- next part --------------
Index: window.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/window.cc,v
retrieving revision 1.30
diff -u -p -r1.30 window.cc
--- window.cc	9 Feb 2004 04:04:24 -0000	1.30
+++ window.cc	13 May 2004 16:34:33 -0000
@@ -73,7 +73,13 @@ WndProc (HWND hwnd, UINT uMsg, WPARAM wP
     }
 }
 
-static HANDLE window_started;
+static NO_COPY HANDLE window_started;
+
+void
+window_init ()
+{
+    window_started = CreateEvent (&sec_none_nih, TRUE, FALSE, NULL);
+}
 
 static DWORD WINAPI
 Winmain (VOID *)
@@ -126,18 +132,29 @@ Winmain (VOID *)
 HWND __stdcall
 gethwnd ()
 {
-  if (ourhwnd != NULL)
-    return ourhwnd;
+  static NO_COPY long window_waiters;
+  long waiters = InterlockedIncrement (&window_waiters);
+  HWND hwnd = ourhwnd;
+
+  if (hwnd == NULL)
+    {
+      if (waiters == 1)
+	{
+	  cygthread *h = new cygthread (Winmain, NULL, "win");
+	  h->zap_h ();
+	}
+
+      WaitForSingleObject (window_started, INFINITE);
+      hwnd = ourhwnd;
+    }
+
+  HANDLE ws;
 
-  cygthread *h;
+  if (InterlockedDecrement (&window_waiters) == 0 && hwnd
+      && (ws = (HANDLE) InterlockedExchange ((long *) &window_started, 0)))
+    CloseHandle (ws);
 
-  window_started = CreateEvent (&sec_none_nih, TRUE, FALSE, NULL);
-  h = new cygthread (Winmain, NULL, "win");
-  h->SetThreadPriority (THREAD_PRIORITY_HIGHEST);
-  WaitForSingleObject (window_started, INFINITE);
-  CloseHandle (window_started);
-  h->zap_h ();
-  return ourhwnd;
+  return hwnd;
 }
 
 extern "C" int
Index: winsup.h
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/winsup.h,v
retrieving revision 1.143
diff -u -p -r1.143 winsup.h
--- winsup.h	3 May 2004 11:53:07 -0000	1.143
+++ winsup.h	13 May 2004 16:34:33 -0000
@@ -218,6 +218,7 @@ void events_terminate (void);
 void __stdcall close_all_files ();
 
 /* Invisible window initialization/termination. */
+void window_init (void);
 HWND __stdcall gethwnd (void);
 /* Check if running in a visible window station. */
 extern bool has_visible_window_station (void);
Index: dcrt0.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/dcrt0.cc,v
retrieving revision 1.218
diff -u -p -r1.218 dcrt0.cc
--- dcrt0.cc	12 Mar 2004 03:09:28 -0000	1.218
+++ dcrt0.cc	13 May 2004 16:34:33 -0000
@@ -799,6 +799,9 @@ dll_crt0_1 (char *)
   /* Connect to tty. */
   tty_init ();
 
+  /* Initialize hidden window for itimers/sockets. */
+  window_init ();
+
   if (!__argc)
     {
       char *line = GetCommandLineA ();