Check if d->attach() is successful

[michaelhubatka]

The SingleApplication constructor explicitly attaches itself to the shared memory and deletes the object QSharedMemory afterwards. This is to make sure that a 'dangling' shared memory which is not referenced anymore (due to a crashed application) is freed in the destructor.

I suggest to check the return value and the possible error when trying to attach, something like

d->memory->attach();
{
  switch(d->memory->error())
  {
    case QSharedMemory::NoError: // successfully attached, resource was already created
      d->memory->detach(); // should release the resource
      break;
    case QSharedMemory::NotFound: // resource is not available yet
      break;
    default:
      abortSafely(); // report error message
  }
}

https://github.com/itay-grudev/SingleApplication/blob/8c48163c4d3fbba603cfe8a5b94046c9dad71825/singleapplication.cpp#L75C25-L75C25

Background: We had the problem that our application didn't start anymore on the build server (Mac). The error message was

SingleApplication: Unable to create block.
SingleApplication:  7 "QSharedMemory::create: out of resources"

which is the message created when d->memory->create( sizeof( InstancesInfo ) ) fails. Unfortunately, we did not check the system resources with ipcs before rebooting the server.
After spending some time in the Qt documentation and the code base, I came to the conclusion that maybe already the attach() failed.
Because the memory itself is not a problem, I assume that the internal QSystemSemaphore of the QSharedMemory could not be acquired.

[itay-grudev]

We are aware of multiple issues with Qt's latest implementation of QSharedMemory and QSystemSemaphore and even the latter is not truly thread safe. We are working on an implementation that would rely on a different mechanism - QLocalSockets but the development is still in-progress.