occasional FSEventStreamFlushSync(): failed assertion '(SInt64)last_id > 0LL on macOS

I am building GNU Octave against Qt 5.12.5 on macOS 10.14.6.

Sometimes, when running this Qt-enabled Octave, I will see this message in the Command Window:

2018-06-21 09:18 octave-gui[48030] (FSEvents.framework) FSEventStreamFlushSync(): failed assertion '(SInt64)last_id > 0LL'

I believe this is coming from Qt's Filesystem Watcher thing that watches for changes in directories, in qfilesystemwatcher_fsevents.mm.

I also suspect that this is not a bug in Qt itself, but may be an underlying OS bug in macOS. (Why is it casting something to a signed int and requiring that to be greater than 0?). Unfortunately, since it's Qt interacting with this part of macOS, it may be up to Qt to do a workaround.

There appear to be no actual consequences to this warning - file system changes still seem to produce notifications. But this is a disturbing warning message to get, especially for an end user that's not familiar with the system.

I'm currently working around this by patching Qt to not call FSEventStreamFlushSync(), which gets rid of the warning messages, but I don't know what the consequences of doing that are.

Bug report against Octave.app here: https://github.com/octave-app/octave-app/issues/62.

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