Called `Result::unwrap()` on an `Err` value: "SendError(..)" in Window::force_reflow on shutdown

[Ms2ger]

stack backtrace:
   0:     0x7fcff704add1 - backtrace::backtrace::libunwind::trace
                        at backtrace-0.2.3/src/backtrace/mod.rs:82
                         - backtrace::backtrace::trace<closure>
                        at backtrace-0.2.3/src/backtrace/mod.rs:70
   1:     0x7fcff704ae98 - backtrace::capture::{{impl}}::new
                        at backtrace-0.2.3/src/lib.rs:96
   2:     0x7fcff1d341eb - servo::main::{{closure}}
                        at SERVO/ports/servo/main.rs:122
   3:     0x7fcff86e63f8 - std::panicking::rust_panic_with_hook
                        at RUST/src/libstd/panicking.rs:556
   4:     0x7fcff86e62c4 - std::panicking::begin_panic<collections::string::String>
                        at RUST/src/libstd/panicking.rs:517
   5:     0x7fcff86e61e9 - std::panicking::begin_panic_fmt
                        at RUST/src/libstd/panicking.rs:501
   6:     0x7fcff86e6177 - std::panicking::rust_begin_panic
                        at RUST/src/libstd/panicking.rs:477
   7:     0x7fcff87137bd - core::panicking::panic_fmt
                        at RUST/src/libcore/panicking.rs:69
   8:     0x7fcff30aab75 - core::result::unwrap_failed<std::sync::mpsc::SendError<script_layout_interface::message::Msg>>
                        at RUST/src/libcore/macros.rs:29
   9:     0x7fcff2fec103 - core::result::{{impl}}::unwrap<(),std::sync::mpsc::SendError<script_layout_interface::message::Msg>>
                        at RUST/src/libcore/result.rs:745
  10:     0x7fcff33c146a - script::dom::window::{{impl}}::force_reflow
                        at SERVO/components/script/dom/window.rs:1119
  11:     0x7fcff33c1f03 - script::dom::window::{{impl}}::reflow
                        at SERVO/components/script/dom/window.rs:1170
  12:     0x7fcff33c383f - script::dom::window::{{impl}}::resolved_style_query
                        at SERVO/components/script/dom/window.rs:1330
  13:     0x7fcff4f442fb - script::dom::cssstyledeclaration::{{impl}}::get_computed_style
                        at SERVO/components/script/dom/cssstyledeclaration.rs:139
  14:     0x7fcff4f44416 - script::dom::cssstyledeclaration::{{impl}}::get_property_value
                        at SERVO/components/script/dom/cssstyledeclaration.rs:147
  15:     0x7fcff522f8fc - script::dom::cssstyledeclaration::{{impl}}::Display
                        at SERVO/components/script/dom/cssstyledeclaration.rs:430
  16:     0x7fcff5036db0 - script::script_thread::{{impl}}::handle_transition_event
                        at SERVO/components/script/script_thread.rs:1646
  17:     0x7fcff5026a98 - script::script_thread::{{impl}}::handle_msg_from_constellation
                        at SERVO/components/script/script_thread.rs:1004
  18:     0x7fcff50245aa - script::script_thread::{{impl}}::handle_msgs::{{closure}}
                        at SERVO/components/script/script_thread.rs:868
  19:     0x7fcff50253a1 - script::script_thread::{{impl}}::profile_event<closure,core::option::Option<bool>>
                        at SERVO/components/script/script_thread.rs:963
  20:     0x7fcff5022d6a - script::script_thread::{{impl}}::handle_msgs
                        at SERVO/components/script/script_thread.rs:862
  21:     0x7fcff5021355 - script::script_thread::{{impl}}::start
                        at SERVO/components/script/script_thread.rs:720
  22:     0x7fcff501daa4 - script::script_thread::{{impl}}::create::{{closure}}::{{closure}}
                        at SERVO/components/script/script_thread.rs:556
  23:     0x7fcff4615cb7 - profile_traits::mem::{{impl}}::run_with_memory_reporting<closure,fn(profile_traits::mem::ReportsChan) -> script::script_runtime::CommonScriptMsg,script::script_runtime::CommonScriptMsg,std::sync::mpsc::Sender<script::script_thread::MainThreadScriptMsg>>
                        at SERVO/components/profile_traits/mem.rs:63
  24:     0x7fcff501e158 - script::script_thread::{{impl}}::create::{{closure}}
                        at SERVO/components/script/script_thread.rs:555
  25:     0x7fcff4e6a37a - std::panic::{{impl}}::call_once<(),closure>
                        at RUST/src/libstd/panic.rs:296
  26:     0x7fcff4a05149 - std::panicking::try::do_call<std::panic::AssertUnwindSafe<closure>,()>
                        at RUST/src/libstd/panicking.rs:460
  27:     0x7fcff86ed37a - panic_unwind::__rust_maybe_catch_panic
                        at RUST/src/libpanic_unwind/lib.rs:98
  28:     0x7fcff4997623 - std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
                        at RUST/src/libstd/panicking.rs:436
  29:     0x7fcff490f5c4 - std::panic::catch_unwind<std::panic::AssertUnwindSafe<closure>,()>
                        at RUST/src/libstd/panic.rs:361
  30:     0x7fcff4921b9b - std::thread::{{impl}}::spawn::{{closure}}<closure,()>
                        at RUST/src/libstd/thread/mod.rs:357
  31:     0x7fcff4ca1832 - alloc::boxed::{{impl}}::call_box<(),closure>
                        at RUST/src/liballoc/boxed.rs:605
  32:     0x7fcff86e5084 - alloc::boxed::{{impl}}::call_once<(),()>
                        at RUST/src/liballoc/boxed.rs:615
                         - std::sys_common::thread::start_thread
                        at RUST/src/libstd/sys_common/thread.rs:21
                         - std::sys::imp::thread::{{impl}}::new::thread_start
                        at RUST/src/libstd/sys/unix/thread.rs:84
  33:     0x7fcfef85d183 - start_thread
  34:     0x7fcfef37437c - clone
  35:                0x0 - <unknown>

That's

        self.layout_chan.send(Msg::Reflow(reflow)).unwrap();

[jdm]

Usually this would occur when the layout thread panics first.

[jdm]

Oh wait, this might happen when a pipeline is closed but a transition event is received afterwards.

[jdm]

The transition event should really include a PipelineId so we can detect if it's targeting a closed pipeline.

[nox]

Detecting closed pipeline sounds like we should ping @asajeffrey.

[asajeffrey]

Could just have script thread ignore transition events for pipelines that aren't fully active? Or we could check in script thread that the pipeline's not been closed.

[jdm]

Right, the point being that there is no pipeline present in the message from which to draw such a conclusion yet.

[asajeffrey]

@jdm the window is still alive though, so we can query it to see if it has been discarded, since it knows its own pipeline id and we could look that up in the documents collection.