Webrender: assertion failure in clear_target: DEPTH_WRITEMASK is 0

[staktrace]

Filing for https://bugzilla.mozilla.org/show_bug.cgi?id=1432819, seems to be an intermittent failure in mozilla-central CI. The panic is coming from this line - apparently both arguments are 0 so the debug_assert_ne is failing.

[kvark]

Good, means we have a logic bug and now my new checks panic on it. Hope it's not too annoying for Gecko. I'll look into it.

[kvark]

Hmm, scanning the code it seems pretty solid on enforcing the invariants about depth clearing. Could we get a more detailed @staktrace (no pun!)?

[staktrace]

The log pointed to in the bug has a stacktrace which isn't properly symbolicated. I pulled the xul.sym file for the build (it's in the target.crashreporter-symbols-full.zip artifact that's uploaded for the build job corresponding to the test job that failed) and did some symbolication-by-hand for the addresses. I got the xul.dll base address from the c6b19016-5e9e-4177-8bee-9530b869c841.extra artifact on the test job itself. Below is the stacktrace with the frames I symbolicated. I doubt it's very useful though:

stack backtrace:
0x7ff9923f4941 - <unknown>
0x7ff9924079ac - <unknown>
0x7ff992407695 - <unknown>
0x7ff992408033 - <unknown>
0x7ff992407ea8 - <unknown>
0x7ff992407d81 - <unknown> // std::panicking::begin_panic_fmt
0x7ff9929df8ea - <unknown> // webrender::device::Device::clear_target
0x7ff992a8aca7 - <unknown> // webrender::renderer::{{impl}}::render_impl::{{closure}}
0x7ff992a87c54 - <unknown> // webrender::renderer::Renderer::render_impl
0x7ff992a877f1 - <unknown> // webrender::renderer::Renderer::render
0x7ff992397011 - <unknown> // webrender_bindings::bindings::wr_renderer_render
0x7ff993f2986e - mozilla_dump_image // mozilla::wr::RendererOGL::Render()
0x7ff993f2b1c1 - mozilla_dump_image // mozilla::wr::RenderThread::UpdateAndRender(mozilla::wr::WrWindowId)
0x7ff993f27d70 - mozilla_dump_image
0x7ff993f29c8a - mozilla_dump_image
0x7ff993372138 - <unknown>
0x7ff99336bb46 - <unknown>
0x7ff99336c2f9 - <unknown>
0x7ff993371eff - <unknown>
0x7ff993372025 - <unknown>
0x7ff993371c35 - <unknown>
0x7ff99337328c - <unknown>
0x7ff99335d789 - <unknown>

I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1432886 to make the symbolication better by default.

Edited to fill in a couple of the mozilla_dump_image frames as well, those aren't really in mozilla_dump_image.

[kvark]

Thanks!

seems to be an intermittent failure in mozilla-central CI

The fact it's an intermittent is worrying though. The assertion is pretty deterministic.
Might be a sign of a memory corruption affecting either WR or the driver.

[staktrace]

Apparently it was on a broken machine, https://bugzilla.mozilla.org/show_bug.cgi?id=1432819#c2