Don't spawn a new engine if the pipe is busy

src/shared/Communication.cpp

 #include <Windows.h>
 #include <Lmcons.h>
 #include <Sddl.h>
 
 #include "Communication.h"
 
 namespace
 {
   const int bufferSize = 1024;
 
@@ skipping 30 matching lines @@
   }
   else
     hasType = false;
 }
 
 Communication::PipeConnectionError::PipeConnectionError()
     : std::runtime_error(AppendErrorCode("Unable to connect to a named pipe"))
 {
 }
 
+Communication::PipeBusyError::PipeBusyError()
+  : std::runtime_error("Timeout while trying to connect to a named pipe, pipe is busy")
+{
+}
+
 Communication::Pipe::Pipe(const std::wstring& pipeName, Communication::Pipe::Mode mode)
 {
   pipe = INVALID_HANDLE_VALUE;
   if (mode == MODE_CREATE)
   {
     SECURITY_ATTRIBUTES sa;
     memset(&sa, 0, sizeof(SECURITY_ATTRIBUTES));
     sa.nLength = sizeof(SECURITY_ATTRIBUTES);
 
     // Low mandatory label. See http://msdn.microsoft.com/en-us/library/bb625958.aspx
     LPCWSTR accessControlEntry = L"S:(ML;;NW;;;LW)";
     PSECURITY_DESCRIPTOR securitydescriptor;
     ConvertStringSecurityDescriptorToSecurityDescriptorW(accessControlEntry, SDDL_REVISION_1, &securitydescriptor, 0);
 
     sa.lpSecurityDescriptor = securitydescriptor;
     sa.bInheritHandle = TRUE;
 
     pipe = CreateNamedPipeW (pipeName.c_str(), PIPE_ACCESS_DUPLEX, PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
                                   PIPE_UNLIMITED_INSTANCES, bufferSize, bufferSize, 0, &sa);
     LocalFree(securitydescriptor);
   }
   else
   {
-    if (WaitNamedPipeW(pipeName.c_str(), 5000))
+    pipe = CreateFileW(pipeName.c_str(), GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);
+    if (pipe == INVALID_HANDLE_VALUE && GetLastError() == ERROR_PIPE_BUSY)
+    {
+      if (!WaitNamedPipeW(pipeName.c_str(), 5000))
+        throw PipeBusyError();
+
       pipe = CreateFileW(pipeName.c_str(), GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);
+    }
   }
 
   if (pipe == INVALID_HANDLE_VALUE)
     throw PipeConnectionError();
 
   DWORD pipeMode = PIPE_READMODE_MESSAGE | PIPE_WAIT;
   if (!SetNamedPipeHandleState(pipe, &pipeMode, 0, 0))
     throw std::runtime_error("SetNamedPipeHandleState failed: error " + GetLastError());
 
   if (mode == MODE_CREATE && !ConnectNamedPipe(pipe, 0))
@@ skipping 26 matching lines @@
   return Communication::InputBuffer(stream.str());
 }
 
 void Communication::Pipe::WriteMessage(Communication::OutputBuffer& message)
 {
   DWORD bytesWritten;
   std::string data = message.Get();
   if (!WriteFile(pipe, data.c_str(), data.length(), &bytesWritten, 0))
     throw std::runtime_error("Failed to write to pipe");
 }