Thanks @boerni.
In our testing, it definitely causes an exception if you call completion more than once, but it seems to be a little different - "No such file or directory" instead of "Invalid Argument".
Terminating app due to uncaught exception 'NSFileHandleOperationException', reason: '* -[NSConcreteFileHandle fileDescriptor]: No such file or directory'
terminating with uncaught exception of type NSException
Having said that, I'll see if I can find any ways that ours could possibly call completion more than once and see if that helps.
Topic:
App & System Services
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General
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