Unity8 deadlock when trying to screenshot without a current gst registry

[errors.ubuntu.com bug bridge]

The Ubuntu Error Tracker has been receiving reports about a problem regarding unity8. This problem was most recently seen with version 8.11+16.04.20151126-0ubuntu2, the problem page at https://errors.ubuntu.com/problem/811233d22c3b1dcb08f9f596e5dbf9d0e23e4220 contains more details.

Inspecting gst-clutter causes it to try and connect to the display server. When it's unity8 that triggered the registry update, registry generation blocks while gst-clutter tries to connect back to unity8. That results in a deadlock, because we're unable to answer the authorization challenge because we're blocked by the registry generation.

This is a bug in either gst-clutter or libclutter (filed upstream), but we should try and make sure that we don't block on this either.

To reproduce:
- log in to unity8
- rm ~/.cache/gstreamer-1.0/registry.x86_64.bin
- press PrtScr to get a screenshot
- deadlock

To recover:
- run gst-inspect-1.0 somewhere

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Michał Sawicz (saviq)]

Just managed to find that the culprit is actually the clutter plugin (gstreamer1.0-clutter-3.0), uninstalling that (sudo dpkg -r --force-depends gstreamer1.0-clutter-3.0) makes the session start again.

[Michał Sawicz (saviq)]

There are suspect code blocks like the following:

#if defined (CLUTTER_WINDOWING_X11)
  /* Required by some GStreamer element like VA */
  XInitThreads ();
#endif

But that doesn't seem to be the problem, even if looks really shady (AFAICT basically means that if clutter was built with x11 support, it will call XInitThreads(), even if you're not actually running under X11).

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Iain Lane (laney)]

Will just pointed me to this. Any chance that someone could forward this bug to upstream, or link the bug if it's already been done?

[Albert Astals Cid (aacid)]

> Will just pointed me to this. Any chance that someone could forward this bug to upstream, or link the bug if it's already been done?

Posted at https://bugzilla.gnome.org/show_bug.cgi?id=780193

[Albert Astals Cid (aacid)]

Looking at the backtrace i get nowadays http://paste.ubuntu.com/24195519/

Is this even still a bug? I can only reproduce it if i am on a VT and run the commands, then it gets stuck but if i loggin into a clean unity8 session having cleaned the file, it's all fine.

I'm confused now :S

[Michał Sawicz (saviq)]

There was a change in when we trigger the registry (as in we don't on startup), but I've been getting the same hangs when trying to take a screenshot in unity8 (which triggers a sound).

[Michał Sawicz (saviq)]

Starting unity8 without the registry present and taking a screenshot results in: http://paste.ubuntu.com/24214544/

[Michał Sawicz (saviq)]

And gst-plugin-scanner hangs with: http://pastebin.ubuntu.com/24214612/

[Gerry Boland (gerboland)]

Would be good if QtMir didn't authenticate processes on the GUI thread, but on the relevant Mir thread. This should be possible.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Launchpad Janitor (janitor)]

This bug was fixed in the package qtmir - 0.5.1+17.04.20170328-0ubuntu1

---------------
qtmir (0.5.1+17.04.20170328-0ubuntu1) zesty; urgency=medium

  [ Alan Griffiths ]
  * MirAL will be correcting another spelling error shortly. Avoid an
    FTBFS when that happens.
  * Compatibility with lp:mir and lp:~alan-griffiths/miral/1.3

  [ Albert Astals Cid ]
  * Remove empty property and move invokable
  * Run ApplicationManager::authorizeSession in the calling thread (LP:
    #1525285)

  [ Andreas Pokorny ]
  * Report and consume exception that may occur on constructing a keymap
    (LP: #1557634)

  [ Daniel d'Andrada ]
  * Ensure the window that got a key down also gets the corresponding
    key up (LP: #1671072)

  [ Gerry Boland ]
  * Don't let a child window go offscreen because its parent moved
  * Add env var equivalent to desktop_file_hint

  [ Lukáš Tinkl ]
  * Raise (activate) windows when an app focus is requested (LP:
    #1672337)

 -- Michał Sawicz <email address hidden> Tue, 28 Mar 2017 17:14:50 +0000