In the real_download() method of the ExternalFD class:
Special-case output to stdout, when filename=='-', because otherwise
the call to os.path.getsize() raises
OSError: [Errno 2] No such file or directory: '-'
which in turn gets masked by a try block and re-raised as
UnavailableVideoError().
And even if getsize had succeeded, we would fail on renaming, and then
we would also fail on reporting progress as report_progress() expects
to have file size information.
So report download completion with self.to_screen() just as
report_progress() would in the noprogress==True case.
You might ask whether it's an abstraction violation for external.py to
have to know and think about '-' as a filename, but unfortunately
we're there already. _call_downloader() thinks about it, and also
real_download() calls common.py's temp_name() which special-cases the
'-' filename to preserve it instead of returnign a temp file. So we
already worry about this case at this abstraction level, as well as
both below and above. Perhaps that should change, but that's for
another day.
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