Bug#931921: clutter's autopkgtests hang when ran with a libglib2.0-0 built with gcc-9

[Simon McVittie]

Control: reassign -1 clutter-1.0-tests
Control: severity -1 serious

We were trying so hard to solve this in either gcc-9 or libglib2.0-0
that we didn't consider whether it could be a clutter bug. (It is.)

On Fri, 12 Jul 2019 at 11:16:53 +0100, Iain Lane wrote:
> Here's the bit of code.
> 
>   https://sources.debian.org/src/clutter-1.0/1.26.2+dfsg-10/tests/conform/actor-offscreen-redirect.c/#L172
> 
> It's adding some stuff to a main loop and expecting it to finish when a
> particular signal handler is called.
...
> Things which make it work again
> 
>   - Building glib2.0 w/gcc-9 -O1 (and -O0)
>   - Building w/gcc-8

This appears to have been because building gtestutils.c with different
optimizations results in different junk being left on the stack afterwards.
When running the clutter test under valgrind, we get:

# Start of actor tests
# Start of offscreen tests
==13864== Conditional jump or move depends on uninitialised value(s)
==13864==    at 0x10AD7C: actor_offscreen_redirect (actor-offscreen-redirect.c:331)
==13864==    by 0x10AD7C: actor_offscreen_redirect (actor-offscreen-redirect.c:299)
==13864==    by 0x492F889: clutter_test_func_wrapper (clutter-test-utils.c:138)
==13864==    by 0x4B6F3BD: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.6000.6)

This is the variable 'data' here:

> static void
> actor_offscreen_redirect (void)
> {
>   Data data;
>
>   ... data.was_painted is never initialized ...
>
>   while (!data.was_painted)
>     g_main_context_iteration (NULL, FALSE);
> }

It seems that data.was_painted was intended to be initialized to FALSE
(all-zeroes), but this never actually happened.

If the uninitialized value of data.was_painted happens to be nonzero,
this results in basically the entire test being skipped - we never enter
the main loop, and never have the opportunity for the test to hang while
waiting for a paint signal that will never happen.

Adding some debug code to hexdump the contents of the data struct reveals
that gcc-9 -O1, or gcc-9 -O2 with -fno-tree-pre, fairly reliably fills
data.was_painted with a nonzero value, so most of the test is effectively
never run. gcc-9 -O2 fills it with zeroes, so the test runs. The paint
signal never happens (at least in my testing) and the test hangs.

The attached is probably a good starting point for someone who has some
sort of understanding of Clutter to start to investigate this.

    smcv
-------------- next part --------------
A non-text attachment was scrubbed...
Name: actor-offscreen-redirect.diff
Type: text/x-diff
Size: 1827 bytes
Desc: not available
URL: <http://alioth-lists.debian.net/pipermail/pkg-gnome-maintainers/attachments/20190820/a519e235/attachment.diff>