Found while fuzzing m-c 20210821-2374a282fbbb (--enable-debug --enable-fuzzing)

Hit MOZ_CRASH(BUG: Unable to find glyph key cache!) at gfx/wr/webrender/src/glyph_cache.rs:118

#0 0x7f3465d4f0e5 in MOZ_Crash /builds/worker/workspace/obj-build/dist/include/mozilla/Assertions.h:256:3
#1 0x7f3465d4f0e5 in RustMozCrash src/mozglue/static/rust/wrappers.cpp:18:3
#2 0x7f3465d4f064 in mozglue_static::panic_hook::h63b3c2e6144e67e9 src/mozglue/static/rust/lib.rs:91:9
#3 0x7f3465d4eadb in core::ops::function::Fn::call::h0d4763c52fdc30fd /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/ops/function.rs:70:5
#4 0x7f3466b1a218 in std::panicking::rust_panic_with_hook::h7ee9e1a2d0f8975a /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panicking.rs:626:17
#5 0x7f3466b19cc6 in std::panicking::begin_panic_handler::_$u7b$$u7b$closure$u7d$$u7d$::h8ab3b4491718b2c7 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panicking.rs:519:13
#6 0x7f3466b1603b in std::sys_common::backtrace::__rust_end_short_backtrace::hd489062ffa586a9f /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/sys_common/backtrace.rs:141:18
#7 0x7f3466b19c28 in rust_begin_unwind /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panicking.rs:515:5
#8 0x7f345ccf5070 in core::panicking::panic_fmt::hca6330e3e14086b4 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/panicking.rs:92:14
#9 0x7f345ccf4f62 in core::option::expect_failed::hb6407502861d5eec /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/option.rs:1243:5
#10 0x7f3465699617 in core::option::Option$LT$T$GT$::expect::h505d71038fdee358 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/option.rs:351:21
#11 0x7f3465699617 in webrender::glyph_cache::GlyphCache::get_glyph_key_cache_for_font_mut::ha5c72efef0a0b938 src/gfx/wr/webrender/src/glyph_cache.rs:116:9
#12 0x7f34656a1280 in webrender::glyph_rasterizer::GlyphRasterizer::resolve_glyphs::hb5ca80ff83d62504 src/gfx/wr/webrender/src/glyph_rasterizer/mod.rs:277:35
#13 0x7f34657a8722 in webrender::resource_cache::ResourceCache::block_until_all_resources_added::h8fe775b2a8e29806 src/gfx/wr/webrender/src/resource_cache.rs:1244:9
#14 0x7f346569339a in webrender::frame_builder::FrameBuilder::build_layer_screen_rects_and_cull_layers::hb39b12fc61671736 src/gfx/wr/webrender/src/frame_builder.rs:523:13
#15 0x7f346569339a in webrender::frame_builder::FrameBuilder::build::h617610d843e065aa src/gfx/wr/webrender/src/frame_builder.rs:579:9
#16 0x7f346570fcd5 in webrender::render_backend::Document::build_frame::hef19963e71a5916c src/gfx/wr/webrender/src/render_backend.rs:580:25
#17 0x7f3465724bbc in webrender::render_backend::RenderBackend::update_document::h5ae9c92b46a4f16d src/gfx/wr/webrender/src/render_backend.rs:1480:41
#18 0x7f346571b17c in webrender::render_backend::RenderBackend::prepare_transactions::hef8b514b26ff2b10 src/gfx/wr/webrender/src/render_backend.rs:1332:28
#19 0x7f346571b17c in webrender::render_backend::RenderBackend::process_api_msg::hfcd4003ad23f6df7 src/gfx/wr/webrender/src/render_backend.rs:1190:17
#20 0x7f34655322a9 in webrender::render_backend::RenderBackend::run::h1db72915240b83f0 src/gfx/wr/webrender/src/render_backend.rs:849:21
#21 0x7f34655322a9 in webrender::renderer::Renderer::new::_$u7b$$u7b$closure$u7d$$u7d$::h6456e7d80ba99d9d src/gfx/wr/webrender/src/renderer/mod.rs:1314:13
#22 0x7f34655322a9 in std::sys_common::backtrace::__rust_begin_short_backtrace::h83408d0ff5189749 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/sys_common/backtrace.rs:125:18
#23 0x7f34655525ae in std::thread::Builder::spawn_unchecked::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::h11e1fb5338bea221 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/thread/mod.rs:481:17
#24 0x7f34655525ae in _$LT$std..panic..AssertUnwindSafe$LT$F$GT$$u20$as$u20$core..ops..function..FnOnce$LT$$LP$$RP$$GT$$GT$::call_once::h895ace1efedb02bb /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panic.rs:347:9
#25 0x7f34655525ae in std::panicking::try::do_call::hdb6a851d6f5de4d4 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panicking.rs:401:40
#26 0x7f34655525ae in std::panicking::try::h7faca17d40865a23 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panicking.rs:365:19
#27 0x7f34655525ae in std::panic::catch_unwind::h10041b3a70424ee9 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panic.rs:434:14
#28 0x7f34655525ae in std::thread::Builder::spawn_unchecked::_$u7b$$u7b$closure$u7d$$u7d$::he046eb7f60f9c9c4 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/thread/mod.rs:480:30
#29 0x7f34655525ae in core::ops::function::FnOnce::call_once$u7b$$u7b$vtable.shim$u7d$$u7d$::h57a47164ace8616b /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/ops/function.rs:227:5
#30 0x7f3466b26736 in _$LT$alloc..boxed..Box$LT$F$C$A$GT$$u20$as$u20$core..ops..function..FnOnce$LT$Args$GT$$GT$::call_once::h7ece6cfefaff1005 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/alloc/src/boxed.rs:1575:9
#31 0x7f3466b26736 in _$LT$alloc..boxed..Box$LT$F$C$A$GT$$u20$as$u20$core..ops..function..FnOnce$LT$Args$GT$$GT$::call_once::hb8b48e55c21f193e /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/alloc/src/boxed.rs:1575:9
#32 0x7f3466b26736 in std::sys::unix::thread::Thread::new::thread_start::h8c7c4450dba62914 /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/sys/unix/thread.rs:71:17
#33 0x7f3472be8608 in start_thread /build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477:8
#34 0x7f34727b0292 in clone /build/glibc-eX1tMB/glibc-2.31/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:95

A Pernosco session is available here: https://pernos.co/debug/bHnzzm_VJpqobFbC4_vE7Q/index.html

Bugmon Analysis
Verified bug as reproducible on mozilla-central 20210908213905-a72096bff057.
The bug appears to have been introduced in the following build range:

Start: 57328f12e67aafad12fd1f062fddf48b41120a4f (20210614004220)
End: e77eb14241b9e712ddda1e8c1cc21ef455377e3c (20210614070416)
Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=57328f12e67aafad12fd1f062fddf48b41120a4f&tochange=e77eb14241b9e712ddda1e8c1cc21ef455377e3c

Bugmon Analysis
Unable to reproduce bug 1729829 using build mozilla-central 20210821212131-2374a282fbbb. Without a baseline, bugmon is unable to analyze this bug.
Removing bugmon keyword as no further action possible. Please review the bug and re-add the keyword for further analysis.

Since the crash volume is low (less than 5 per week), the severity is downgraded to S3. Feel free to change it back if you think the bug is still critical.

For more information, please visit auto_nag documentation.