Terminate the engine if another instance is already running

src/engine/main.cpp

Index: src/engine/main.cpp
===================================================================
--- a/src/engine/main.cpp
+++ b/src/engine/main.cpp
@@ -187,12 +187,11 @@
 
 int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
 {
-  // TODO: Attempt to create the pipe first, and exit immediately if this
-  //       fails. Since multiple instances of the engine could be running,
-  //       this may need named mutices to avoid race conditions.
-  //       Note that as soon as the pipe is created first, we can reduce the
-  //       client timeout after CreateProcess(), but should increase the one
-  //       in WaitNamedPipe().
+  if (Communication::PipeExists(Communication::pipeName))
+  {
+    DebugLastError("Named pipe exists, another engine instance appears to be running");
+    return 1;
+  }

[Wladimir Palant, 2013/06/07 06:16:08]

That's causing a race condition. It can take several seconds for this engine
instance to create its own pipe - in the meantime additional engine instances
are free to start.

If it doesn't look like we can avoid race conditions with pipes then we should
definitely use mutexes.

[Felix Dahlke, 2013/06/07 06:37:22]

Hm, you're right, this actually shouldn't work. Makes me wonder why it does... I
spawned about 5 engine instances in parallel several times, four of those always
exited after a few ms. Never saw two engines running after this was added.

However, I guess named mutexes are better after all. Even if we create the pipe
right away and the filterEngine later, we could in theory have a race condition
here.

 
   int argc;
   LPWSTR* argv = CommandLineToArgvW(GetCommandLineW(), &argc);