Warning: XShm get_pixels_ptr XImage is NULL

[alas]

During the testing for #849 (osx client 0.16.0 r11304 against fedora 21 0.16.0 r11357 server) ... after about 4 hours of an overnight test, I got the following warning/traceback on the server (also saw with 0.16.0 r11392 windows client against 0.16.0 r11366 fedora 21 server after about 4 hours of playing sound/video).

2015-12-14 22:19:58,994 Warning: XShm get_pixels_ptr XImage is NULL
2015-12-14 22:19:58,996 error processing encode queue: failed to get pixels from XShmImageWrapper(BGRX: 85, 137, 480, 360)
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_source.py", line 1335, in encode_from_queue
    self.make_data_packet_cb(*item)
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_source.py", line 1364, in make_data_packet_cb
    packet = self.make_data_packet(damage_time, process_damage_time, wid, image, coding, sequence, options, flush)
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_source.py", line 1707, in make_data_packet
    ret = encoder(coding, image, options)
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_video_source.py", line 1310, in video_encode
    ret = ve.compress_image(csc_image, quality, speed, options)
  File "encoder.pyx", line 593, in xpra.codecs.enc_x264.encoder.Encoder.compress_image (xpra/codecs/enc_x264/encoder.c:5709)
AssertionError: failed to get pixels from XShmImageWrapper(BGRX: 85, 137, 480, 360)
2015-12-14 22:28:59,061 Warning: XShm get_pixels_ptr XImage is NULL
2015-12-14 22:28:59,071 error processing encode queue: failed to get pixels from XShmImageWrapper(BGRX: 85, 137, 480, 360)
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_source.py", line 1335, in encode_from_queue
    self.make_data_packet_cb(*item)
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_source.py", line 1364, in make_data_packet_cb
    packet = self.make_data_packet(damage_time, process_damage_time, wid, image, coding, sequence, options, flush)
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_source.py", line 1707, in make_data_packet
    ret = encoder(coding, image, options)
  File "/usr/lib64/python2.7/site-packages/xpra/server/window/window_video_source.py", line 1310, in video_encode
    ret = ve.compress_image(csc_image, quality, speed, options)
  File "encoder.pyx", line 593, in xpra.codecs.enc_x264.encoder.Encoder.compress_image (xpra/codecs/enc_x264/encoder.c:5709)
AssertionError: failed to get pixels from XShmImageWrapper(BGRX: 85, 137, 480, 360)

Wasn't actually present during the tests, but in the case of the osx client test, didn't see a sound source stopping message for another 2 hours after the message - so I don't know if it produced any observable effects.

[Antoine Martin]

I have seen this same trace twice, and I had recorded it - just not in a ticket.
This can only happen if we free() the xshm image before we use it, this should never happen - and I don't see how it can happen...
The fact that it is so hard to reproduce makes it harder to debug.

[Antoine Martin]

We create the XShmImageWrapper in process_damage_region, which runs in the UI thread.

The XImage is freed and set to NULL by calling the Cython function free_image on the image wrapper, this can only be called from XImageWrapper.free().

This is only called from 2 places:

• immediately if the operation has been cancelled - unlikely to be a problem
• via free_image_wrapper, which is called from:
  • cancel_damage for av-sync delayed images
  • encode_from_queue when av-sync is enabled
  • make_data_packet_cb after the compression (via a UI thread callback since this runs in the encode thread)

There is also some similar code in the server's root overlay paint code (#988), but this is not enabled by default and looks straightforward as it does it all in the UI thread.

The av-sync stuff looks suspicious (#835).

[Antoine Martin]

Huge bug found in av-sync: r11462 fixes this (will backport), r11463 tries to ensure this does not break again.

I am closing this because it must be quite hard to hit this problem.
We can re-open it if it happens again.

[Antoine Martin]

(more descriptive bug title)

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1054