Terminate the engine if another instance is already running

src/engine/main.cpp

 #include <AdblockPlus.h>
 #include <vector>
 #include <Windows.h>
 
 #include "../shared/AutoHandle.h"
 #include "../shared/Communication.h"
 #include "../shared/Utils.h"
 #include "../shared/Version.h"
 #include "Debug.h"
 
@@ skipping 169 matching lines @@
 
   AdblockPlus::JsEnginePtr jsEngine = AdblockPlus::JsEngine::New(appInfo);
   std::string dataPath = ToUtf8String(GetAppDataPath());
   dynamic_cast<AdblockPlus::DefaultFileSystem*>(jsEngine->GetFileSystem().get())->SetBasePath(dataPath);
   std::auto_ptr<AdblockPlus::FilterEngine> filterEngine(new AdblockPlus::FilterEngine(jsEngine));
   return filterEngine;
 }
 
 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);
   std::wstring locale(argc >= 1 ? argv[0] : L"");
   LocalFree(argv);
 
   filterEngine = CreateFilterEngine(locale);
 
   for (;;)
   {
@@ skipping 13 matching lines @@
     }
     catch (std::runtime_error e)
     {
       DebugException(e);
       return 1;
     }
   }
 
   return 0;
 }