Avoid deadlock in QGIS server

[m-kuhn]

In case of invalid map settings, renderJob.start() returns immediately and the finished() signal is emitted before the connection happens. This puts the server into an eternal loop.

QGIS/src/core/maprenderer/qgsmaprendererjob.cpp

Lines 139 to 148 in 32b1947

	void QgsMapRendererJob::start()
	{
	if ( mSettings.hasValidSettings() )
	startPrivate();
	else
	{
	mErrors.append( QgsMapRendererJob::Error( QString(), tr( "Invalid map settings" ) ) );
	emit finished();
	}
	}

[elpaso]

backport?

[m-kuhn]

likely, not yet verified

[m-kuhn]

Probably introduced in #43376
3.16 is not affected

[rldhont]

Thanks @m-kuhn

@dmarteau a review ?

[rldhont]

Same error in release3-16. Is backport to 3.16 possible ?

[m-kuhn]

How did you test?

[m-kuhn]

@rldhont still interested in a reprod steps, this could help to add a test

[pblottiere]

Thanks @m-kuhn! And +1 for adding a test.

[nyalldawson]

3.16 is EOL, so the discussion is moot now

[m-kuhn]

I would propose to merge this now.
We can still add a test later if we have a reprod feedback from @rldhont or someone else