Trac #33045: ffmpeg tests time out

{{{
sage: a.show(iterations=3)    # optional -- ImageMagick ## line 781 ##
sage: a.show(delay=50)        # optional -- ImageMagick ## line 785 ##
sage: a.show(format="ogg")         # optional -- ffmpeg ## line 789 ##
sage: a.show(format="webm")        # optional -- ffmpeg ## line 790 ##
sage: a.show(format="mp4")         # optional -- ffmpeg ## line 791 ##
------------------------------------------------------------------------
/var/lib/buildbot/slave/sage_git/build/local/var/lib/sage/venv-
python3.9.9/lib/python3.9/site-
packages/cysignals/signals.cpython-39-x86_64-linux-
gnu.so(+0x6c68)[0x7f3cd264cc68]
/var/lib/buildbot/slave/sage_git/build/local/var/lib/sage/venv-
python3.9.9/lib/python3.9/site-
packages/cysignals/signals.cpython-39-x86_64-linux-
gnu.so(+0x6e48)[0x7f3cd264ce48]
/var/lib/buildbot/slave/sage_git/build/local/var/lib/sage/venv-
python3.9.9/lib/python3.9/site-
packages/cysignals/signals.cpython-39-x86_64-linux-
gnu.so(+0x94ba)[0x7f3cd264f4ba]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x11390)[0x7f3cda990390]
/lib/x86_64-linux-gnu/libpthread.so.0(waitpid+0x6b)[0x7f3cda98ff7b]
[...]
----------------------------------------------------------------------
sage -t --long --random-seed=192258908271772461060599147744190497953
src/sage/plot/animate.py  # Timed out
----------------------------------------------------------------------
}}}

These tests were originally included in !SageMath 11 years ago in
#11170.

URL: https://trac.sagemath.org/33045
Reported by: vbraun
Ticket author(s): Sébastien Labbé
Reviewer(s): Matthias Koeppe

src/sage/plot/animate.py

@@ -969,7 +969,9 @@ def ffmpeg(self, savefile=None, show_path=False, output_format=None,
         # ...', need to come before the input file names, while
         # some options, like '-pix_fmt rgb24', need to come
         # afterwards.  Hence 'early_options' and 'ffmpeg_options'
-        cmd = 'cd "%s"; sage-native-execute ffmpeg -y -f image2 %s -i %s %s %s' % (pngdir, early_options, pngs, ffmpeg_options, savefile)
+        # The `-nostdin` is needed to avoid the command to hang, see
+        # https://stackoverflow.com/questions/16523746/ffmpeg-hangs-when-run-in-background
+        cmd = 'cd "%s"; sage-native-execute ffmpeg -nostdin -y -f image2 %s -i %s %s %s' % (pngdir, early_options, pngs, ffmpeg_options, savefile)
         from subprocess import check_call, CalledProcessError, PIPE
         try:
             if sage.misc.verbose.get_verbose() > 0: