Intermittent timeout in /css21_dev/html4/border-left-applies-to-012.htm and /referrer-policy/origin/meta-referrer/same-origin/http-https/iframe-tag/generic.swap-origin-redirect.http.html (panic in FontCacheThread::get_font_instance)

[jdm]

  ▶ TIMEOUT [expected PASS] /css21_dev/html4/border-left-applies-to-012.htm
  │ 
  │ VMware, Inc.
  │ softpipe
  │ 3.3 (Core Profile) Mesa 17.3.0-devel
  │ failed to receive response to font request: Io(Error { repr: Os { code: 104, message: "Connection reset by peer" } }) (thread LayoutThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(NonZero(NonZero(1))) }, at /checkout/src/libcore/result.rs:906)
  │ stack backtrace:
  │    0:     0x7fe93b2dd2cc - backtrace::backtrace::trace::h3049d3b23f5565ad
  │    1:     0x7fe93b2dd302 - backtrace::capture::Backtrace::new::h48230b597774155d
  │    2:     0x7fe93a1132e5 - servo::main::{{closure}}::h56bd67e7de3b8245
  │    3:     0x7fe93c48a196 - std::panicking::rust_panic_with_hook
  │                         at /checkout/src/libstd/panicking.rs:578
  │    4:     0x7fe93c48a024 - std::panicking::begin_panic<alloc::string::String>
  │                         at /checkout/src/libstd/panicking.rs:538
  │    5:     0x7fe93c489f29 - std::panicking::begin_panic_fmt
  │                         at /checkout/src/libstd/panicking.rs:522
  │    6:     0x7fe93c489eba - std::panicking::rust_begin_panic
  │                         at /checkout/src/libstd/panicking.rs:498
  │    7:     0x7fe93c4997b0 - core::panicking::panic_fmt
  │                         at /checkout/src/libcore/panicking.rs:71
  │    8:     0x7fe93b6bf771 - core::result::unwrap_failed::h6e890c3e80e1df3f
  │    9:     0x7fe93b6d85fd - gfx::font_cache_thread::FontCacheThread::get_font_instance::h21d39ca48940c8ac
  │   10:     0x7fe93b6d8f67 - gfx::font_context::FontContext::create_layout_font::h0c91b511177d6ad6
  │   11:     0x7fe93b6d97b8 - gfx::font_context::FontContext::layout_font_group_for_style::h23103247c7eb49d2
  │   12:     0x7fe93a53d411 - layout::text::TextRunScanner::scan_for_runs::hd778676c6e422b92
  │   13:     0x7fe93a4516e2 - <layout::construct::FlowConstructor<'a, ConcreteThreadSafeLayoutNode>>::build_flow_for_block_starting_with_fragments::h0e4eecaa31f2e0fa
  │   14:     0x7fe93a4501c4 - <layout::construct::FlowConstructor<'a, ConcreteThreadSafeLayoutNode>>::build_flow_for_block_like::hfa047d4cfdcc4f27
  │   15:     0x7fe93a44f7bd - <layout::construct::FlowConstructor<'a, ConcreteThreadSafeLayoutNode>>::build_flow_for_block::h5dbbd690367eca8c
  │   16:     0x7fe93a3c6b02 - <layout::construct::FlowConstructor<'a, ConcreteThreadSafeLayoutNode> as layout::traversal::PostorderNodeMutTraversal<ConcreteThreadSafeLayoutNode>>::process::hb68738cf76e60a09
  │   17:     0x7fe93a426d31 - style::traversal::DomTraversal::handle_postorder_traversal::h4e7a67f49f911525
  │   18:     0x7fe93a48cff5 - layout_thread::LayoutThread::handle_reflow::ha5b68ae18da89834
  │   19:     0x7fe93a47d270 - layout_thread::LayoutThread::handle_request_helper::hef20e9fe132e2b97
  │   20:     0x7fe93a478b90 - <layout_thread::LayoutThread as layout_traits::LayoutThreadFactory>::create::{{closure}}::hc3fa2ba307caca8a
  │   21:     0x7fe93a3db3f5 - std::sys_common::backtrace::__rust_begin_short_backtrace::h93b0014a8f30f78e
  │   22:     0x7fe93a3e0375 - std::panicking::try::do_call::h5279d24fdcb663c5
  │   23:     0x7fe93c4912dc - panic_unwind::__rust_maybe_catch_panic
  │                         at /checkout/src/libpanic_unwind/lib.rs:99
  │   24:     0x7fe93a413092 - <F as alloc::boxed::FnBox<A>>::call_box::hec997d9b454dd6f3
  │   25:     0x7fe93c488eab - alloc::boxed::{{impl}}::call_once<(),()>
  │                         at /checkout/src/liballoc/boxed.rs:772
  │                          - std::sys_common::thread::start_thread
  │                         at /checkout/src/libstd/sys_common/thread.rs:24
  │                          - std::sys::imp::thread::{{impl}}::new::thread_start
  │                         at /checkout/src/libstd/sys/unix/thread.rs:90
  │   26:     0x7fe9381db183 - start_thread
  │   27:     0x7fe9378e5ffc - clone
  │   28:                0x0 - <unknown>
  │ ERROR:servo: failed to receive response to font request: Io(Error { repr: Os { code: 104, message: "Connection reset by peer" } })
  │ assertion failed: !self.Document().needs_reflow() ||
  │     (!for_display && self.Document().needs_paint()) ||
  │     self.window_size.get().is_none() || self.suppress_reflow.get() (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(NonZero(NonZero(1))) }, at /home/servo/buildbot/slave/linux-rel-css/build/components/script/dom/window.rs:1329)
  │ stack backtrace:
  │    0:     0x7fe93b2dd2cc - backtrace::backtrace::trace::h3049d3b23f5565ad
  │    1:     0x7fe93b2dd302 - backtrace::capture::Backtrace::new::h48230b597774155d
  │    2:     0x7fe93a1132e5 - servo::main::{{closure}}::h56bd67e7de3b8245
  │    3:     0x7fe93c48a196 - std::panicking::rust_panic_with_hook
  │                         at /checkout/src/libstd/panicking.rs:578
  │    4:     0x7fe93a617d6a - std::panicking::begin_panic::h43bb9a8708c4756b
  │    5:     0x7fe93adab677 - script::dom::window::Window::reflow::h69f04f5700930dc4
  │    6:     0x7fe93abc0737 - script::dom::document::Document::finish_load::h982c2f0e07730443
  │    7:     0x7fe93ad1a711 - script::dom::servoparser::ServoParser::do_parse_sync::h79fb973507b6b716
  │    8:     0x7fe93ad19ea6 - script::dom::servoparser::ServoParser::parse_sync::h515a34fa9361a048
  │    9:     0x7fe93ad1ed0b - <script::dom::servoparser::ParserContext as net_traits::FetchResponseListener>::process_response_eof::ha8bbe87528059937
  │   10:     0x7fe93ae01beb - script::script_thread::ScriptThread::handle_msg_from_constellation::h1d79dd554db1374c
  │   11:     0x7fe93adfbb9f - script::script_thread::ScriptThread::handle_msgs::{{closure}}::hb6feafa9f4657a26
  │   12:     0x7fe93adf6e04 - script::script_thread::ScriptThread::handle_msgs::he4ea694ff271836b
  │   13:     0x7fe93adf128b - script::script_thread::ScriptThread::start::h408b35f8f36475ba
  │   14:     0x7fe93a616162 - std::sys_common::backtrace::__rust_begin_short_backtrace::he5c0ff541073bde6
  │   15:     0x7fe93a7cf647 - std::panicking::try::do_call::hc709de172a33c040
  │   16:     0x7fe93c4912dc - panic_unwind::__rust_maybe_catch_panic
  │                         at /checkout/src/libpanic_unwind/lib.rs:99
  │   17:     0x7fe93a970292 - <F as alloc::boxed::FnBox<A>>::call_box::ha9b1cf18b639c0ae
  │   18:     0x7fe93c488eab - alloc::boxed::{{impl}}::call_once<(),()>
  │                         at /checkout/src/liballoc/boxed.rs:772
  │                          - std::sys_common::thread::start_thread
  │                         at /checkout/src/libstd/sys_common/thread.rs:24
  │                          - std::sys::imp::thread::{{impl}}::new::thread_start
  │                         at /checkout/src/libstd/sys/unix/thread.rs:90
  │   19:     0x7fe9381db183 - start_thread
  │   20:     0x7fe9378e5ffc - clone
  │   21:                0x0 - <unknown>
  │ ERROR:servo: assertion failed: !self.Document().needs_reflow() ||
  │     (!for_display && self.Document().needs_paint()) ||
  └     self.window_size.get().is_none() || self.suppress_reflow.get()

[jdm]

I suspect this occurs during shutdown when there is a layout in process. The pipeline is closed, but the constellation does not wait for any notification that the close message has been received, and goes ahead and closes the font cache thread. When the ongoing layout tries to communicate with the font cache thread, we panic.

[jdm]

The other possibility is that this is yet another instance of the underlying problem behind #13480.