Under certain circumstances Picard can crash with a stack overflow when loading a larger amount of files.

I can only reliable reproduce this on one system (KDE Neon, Python 3.6.9, Qt 5.14.2, PyQt 5.14.1) with a certain set of files, but in theory it can happen everywhere.

The basic recursion happening is this:

1. After a file's load method has finished the callback tagger._file_loaded is set as a callback. This callback is not called immediately, but is put on the event queue via a ProxyToMainEvent by the picard.util.thread module.
2. Under certain circumstances many of those ProxyToMainEvent(tagger._file_loaded) events can pile up
3. When invoked tagger._file_loaded calls tagger.move_files, which in return calls QCoreApplication.processEvents
4. QCoreApplication.processEvents processes the next event in the queue by calling tagger.even. If this again is a ProxyToMainEvent(tagger._file_loaded) this again calls tagger.move_files and QCoreApplication.processEvents. We get a recursion

With proper timing and enough files to be loaded this can get into a situation where the maximum stack depth gets used up.

File "./picard/tagger.py", line 402 in event
File "./picard/util/thread.py", line 47 in run
File "./picard/file.py", line 218 in _loading_finished
File "./picard/tagger.py", line 456 in _file_loaded
File "./picard/tagger.py", line 473 in move_files
File "./picard/tagger.py", line 402 in event
File "./picard/util/thread.py", line 47 in run
File "./picard/file.py", line 218 in _loading_finished
File "./picard/tagger.py", line 456 in _file_loaded
File "./picard/tagger.py", line 473 in move_files
File "./picard/tagger.py", line 402 in event
File "./picard/util/thread.py", line 47 in run
File "./picard/file.py", line 218 in _loading_finished
...

See crash.log for full output.