[Interest] Weird issue: Debugger won't start when qInstallMsgHandler is used

[Mike Nolan]

I have a weird issue that I finally tracked down.

I am using Qt Creator (2.8.1). When I was in debug mode, and hit the Start
Debugging button, the application would never actually load. Finally, I
traced it down to qInstallMsgHandler — if I commented out that line then
the application would again load and debugging would start.

The code looked like this:

  static void msgHandler(QtMsgType type, const char *msg) {
    Q_UNUSED(type);
    Q_UNUSED(msgHandler);
    ((MyApp*)qApp)->emitMessageHandled(type, msg);
  }

  MyApp::MyApp(int & argc, char **argv) : QApplication(argc, argv) {
    qInstallMsgHandler(msgHandler);
  }

(There’s a few things to explain here: I’m calling emitMessageHandled,
rather than emitting the signal directly, because we’re in a static
function. That signal is caught by a QML file that is loaded later on,
which then has a logging object that can write to a file, add to a queue
in memory, etc.)

What I found — the weird part — is that I could work around the issue by
adding something like this:
  static void msgHandler(QtMsgType type, const char *msg) {
    Q_UNUSED(type);
    Q_UNUSED(msgHandler);

    // Workaround
    if (strstr(msg, "QDeclarativeDebugServer") > 0) {
      printf("%s\n", msg);
      fflush(stdout);
    }
    // End workaround

    ((MyApp*)qApp)->emitMessageHandled(type, msg);
  }

Basically, it seems as though the QDeclarativeDebugServer must have its
message outputted or it won’t continue. Since the QML file isn’t loaded at
the time it wants to print (and it handles the output), I force the output
here, enabling it to continue.

I cannot understand why this is, however. Is this a bug in
QDeclarativeDebugServer or am I maybe breaking a rule about using
qInstallMsgHandler? I have a slightly better workaround (always printing
if the QML file hasn’t loaded yet) but I’d like to understand this better.

-- Version info --
Qt Creator: 2.8.1
Qt: 4.8.4