A similar stack trace is obtained:      

==2146== Conditional jump or move depends on uninitialised value(s)
==2146==    at 0x4CA625A: composite_traps (cairo/src/cairo-xlib-render-compositor.c:1888)
==2146==    by 0x4C8E5DA: composite_traps (cairo/src/cairo-traps-compositor.c:1098)
==2146==    by 0x4C8E5DA: composite_mask_clip (cairo/src/cairo-traps-compositor.c:2035)
==2146==    by 0x4C8EB05: create_composite_mask (cairo/src/cairo-traps-compositor.c:477)
==2146==    by 0x4C8FAB4: clip_and_composite_with_mask (cairo/src/cairo-traps-compositor.c:548)
==2146==    by 0x4C8FAB4: clip_and_composite (cairo/src/cairo-traps-compositor.c:1039)
==2146==    by 0x4C8FE41: _cairo_traps_compositor_mask (cairo/src/cairo-traps-compositor.c:2102)
==2146==    by 0x4C38D78: _cairo_compositor_mask (cairo/src/cairo-compositor.c:106)
==2146==    by 0x4C7EE1B: _cairo_surface_mask (cairo/src/cairo-surface.c:2166)
==2146==    by 0x4C85125: _cairo_surface_wrapper_mask (cairo/src/cairo-surface-wrapper.c:206)
==2146==    by 0x4C70A60: _cairo_recording_surface_replay_internal (cairo/src/cairo-recording-surface.c:1754)
==2146==    by 0x4C71D48: _cairo_recording_surface_replay_with_clip (cairo/src/cairo-recording-surface.c:2013)
==2146==    by 0x4C901E5: composite_aligned_boxes (cairo/src/cairo-traps-compositor.c:1242)
==2146==    by 0x4C901E5: clip_and_composite_boxes.part.13 (cairo/src/cairo-traps-compositor.c:1792)
==2146==    by 0x4C9058C: clip_and_composite_boxes (cairo/src/cairo-traps-compositor.c:1742)
==2146==    by 0x4C90BD1: _cairo_traps_compositor_paint (cairo/src/cairo-traps-compositor.c:2063)
==2146==    by 0x4C38CE8: _cairo_compositor_paint (cairo/src/cairo-compositor.c:65)
==2146==    by 0x4C7EB40: _cairo_surface_paint (cairo/src/cairo-surface.c:2117)
==2146==    by 0x4C4062E: _cairo_gstate_paint (cairo/src/cairo-gstate.c:1067)
==2146==    by 0x4C33BC4: cairo_paint (cairo/src/cairo.c:2003)
==2146==    by 0x43C154: record_replay (cairo/test/record2x.c:377)
==2146==    by 0x40E5F1: cairo_test_for_target (cairo/test/cairo-test.c:929)
==2146==    by 0x40E5F1: _cairo_test_context_run_for_target (cairo/test/cairo-test.c:1532)
==2146==    by 0x40BA53: _cairo_test_runner_draw (cairo/test/cairo-test-runner.c:255)
==2146==    by 0x40BA53: main (cairo/test/cairo-test-runner.c:937)
==2146==  Uninitialised value was created by a stack allocation
==2146==    at 0x4CA5EAD: composite_traps (cairo/src/cairo-xlib-render-compositor.c:1818)


running:

( cd test; DISPLAY=:2 CAIRO_TEST_TARGET=xlib valgrind --track-origins=yes .libs/cairo-test-suite -f record2x-paint-alpha-clip-mask )

it happens when in 'composite_traps'  traps->num_traps == 0

in that case returning CAIRO_STATUS_SUCCESS at line 1829

http://cgit.freedesktop.org/cairo/tree/src/cairo-xlib-render-compositor.c#n1829

silences valgrind.

(In reply to Massimo from comment #1)
> it happens when in 'composite_traps'  traps->num_traps == 0
> 
> in that case returning CAIRO_STATUS_SUCCESS at line 1829
> 
> http://cgit.freedesktop.org/cairo/tree/src/cairo-xlib-render-compositor.
> c#n1829
> 
> silences valgrind.

So, is that the right thing to do?

CCing some people who have committed patches recently. Proposed fix is at the bottom of comment #1.

(In reply to Michael Catanzaro from comment #3)
> Proposed fix is at the bottom of comment #1.

This two-line fix really needs to be landed, it's been polluting attempts to run valgrind on applications that use cairo for years.

This is the right fix.  I'll create a merge request in a second.

Merge request at https://gitlab.com/cairo/cairo/merge_requests/2

Note that the CI pipeline fails because the test suite fails... for other reasons.

Pushed to commit 5454b85d4bf2f7bea454c940d90255a15517fa3b.