panic: cannot access a TLS value during or after it is destroyed: AccessError

[mateon1]

This test case does not touch the network at all, which is why this panic is extremely confusing.

Minimal panicking testcase:

<style>
  menu { border:solid; }
</style>
<details open style="white-space:pre">
<menu style="display:inline; border-image:url()">
<span id="span">
<script>
  var span = document.getElementById("span");
  span.clientTop;
  span.textContent = "";
  window.gc();
  document.createEvent("");
</script>

panics:

cannot access a TLS value during or after it is destroyed: AccessError (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at libcore/result.rs:945)

Full backtrace

$ ../../rust/servo/mach run -dz -f seed.html
VMware, Inc.
softpipe
3.3 (Core Profile) Mesa 18.1.0-devel
ERROR 2018-08-18T17:11:25Z: script::dom::bindings::error: Error at :0:0 NotSupportedError: The operation is not supported.
cannot access a TLS value during or after it is destroyed: AccessError (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at libcore/result.rs:945)
stack backtrace:
   0:     0x556d09d0f0a5 - backtrace::backtrace::libunwind::trace::hc38180d489d66153
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace::h78a15d6884d51e5b
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x556d09d0ce6e - backtrace::capture::Backtrace::new::h9229778bab1a1cd5
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x556d026f3389 - servo::main::{{closure}}::he264d6b764632c29
                        at ports/servo/non_android_main.rs:122
   3:     0x556d09d49f73 - std::panicking::rust_panic_with_hook::h75f53ae1ba39714f
                        at libstd/panicking.rs:479
   4:     0x556d09d49b19 - std::panicking::continue_panic_fmt::h5f5c2db8ab083e0d
                        at libstd/panicking.rs:390
   5:     0x556d09d49a15 - rust_begin_unwind
                        at libstd/panicking.rs:325
   6:     0x556d09d6bbdb - core::panicking::panic_fmt::h28ab0e0b2f3bdb57
                        at libcore/panicking.rs:77
   7:     0x556d09c3440b - core::result::unwrap_failed::h23ba6210daaae459
                        at /checkout/src/libcore/macros.rs:26
   8:     0x556d0307bf35 - <core::result::Result<T, E>>::expect::hc37e68e270de6465
                        at /checkout/src/libcore/result.rs:809
   9:     0x556d04b65624 - <std::thread::local::LocalKey<T>>::with::h0d01692dea1dcec1
                        at /checkout/src/libstd/thread/local.rs:248
  10:     0x556d03f44a40 - <script::dom::bindings::root::Root<T>>::new::hd3e7ed29a7fedcf1
                        at components/script/dom/bindings/root.rs:68
  11:     0x556d03f70f91 - <script::dom::bindings::root::Root<script::dom::bindings::root::Dom<T>>>::from_ref::h92daf0e507c1b9cd
                        at components/script/dom/bindings/root.rs:155
  12:     0x556d043b1978 - script::dom::node::from_untrusted_node_address::h1335b68d41539f63
                        at components/script/dom/node.rs:1005
  13:     0x556d034e78a2 - script::dom::window::Window::force_reflow::hfbc60c32bc6fa4ac
                        at components/script/dom/window.rs:1397
  14:     0x556d034e86d8 - script::dom::window::Window::reflow::ha4c1149e1aed5ca7
                        at components/script/dom/window.rs:1443
  15:     0x556d04514d35 - script::dom::document::Document::finish_load::hb8f036cd4bc17c2e
                        at components/script/dom/document.rs:1621
  16:     0x556d039c139b - script::dom::servoparser::ServoParser::finish::h23f2eb563e28c3f4
                        at components/script/dom/servoparser/mod.rs:508
  17:     0x556d039c0d56 - script::dom::servoparser::ServoParser::do_parse_sync::h834c92cd40dc4db1
                        at components/script/dom/servoparser/mod.rs:442
  18:     0x556d044f6c20 - script::dom::servoparser::ServoParser::parse_sync::{{closure}}::ha8e190e695a5d558
                        at components/script/dom/servoparser/mod.rs:419
  19:     0x556d04061533 - profile_traits::time::profile::h732b888a7c346e33
                        at /shared/dev/rust/servo/components/profile_traits/time.rs:134
  20:     0x556d039c0993 - script::dom::servoparser::ServoParser::parse_sync::h047d1cb54787997b
                        at components/script/dom/servoparser/mod.rs:416
  21:     0x556d039c46a5 - <script::dom::servoparser::ParserContext as net_traits::FetchResponseListener>::process_response_eof::h51e5d93138da5fef
                        at components/script/dom/servoparser/mod.rs:745
  22:     0x556d03d07f9d - script::script_thread::ScriptThread::handle_fetch_eof::h85faca36fb139f76
                        at components/script/script_thread.rs:2605
  23:     0x556d03cebced - script::script_thread::ScriptThread::handle_msg_from_constellation::h21b96db506a978b7
                        at components/script/script_thread.rs:1309
  24:     0x556d03ed4b9c - script::script_thread::ScriptThread::handle_msgs::{{closure}}::h8bad1fee06535610
                        at components/script/script_thread.rs:1111
  25:     0x556d03ed8167 - script::script_thread::ScriptThread::profile_event::hf1c778ce5c3dcccc
                        at components/script/script_thread.rs:1286
  26:     0x556d03ce7ef5 - script::script_thread::ScriptThread::handle_msgs::hda35bf02bfbaf6b0
                        at components/script/script_thread.rs:1105
  27:     0x556d03ce5156 - script::script_thread::ScriptThread::start::h56f336c7b8763b1a
                        at components/script/script_thread.rs:937
  28:     0x556d03ed1a70 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::{{closure}}::h48d2d1b326024676
                        at components/script/script_thread.rs:596
  29:     0x556d0499fecb - profile_traits::mem::ProfilerChan::run_with_memory_reporting::hb57e41f249b238c5
                        at /shared/dev/rust/servo/components/profile_traits/mem.rs:72
  30:     0x556d03ed21d8 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::he6630a4a80bfce57
                        at components/script/script_thread.rs:595
  31:     0x556d057a57c6 - std::sys_common::backtrace::__rust_begin_short_backtrace::hf65c7e18c41d0180
                        at /checkout/src/libstd/sys_common/backtrace.rs:136
  32:     0x556d04f7de94 - std::thread::Builder::spawn::{{closure}}::{{closure}}::h14287f4e715a0bb6
                        at /checkout/src/libstd/thread/mod.rs:409
  33:     0x556d057230c4 - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::hac3ff334a47d5c2c
                        at /checkout/src/libstd/panic.rs:313
  34:     0x556d0591dcc9 - std::panicking::try::do_call::h1859f1dc50402b13
                        at /checkout/src/libstd/panicking.rs:310
  35:     0x556d09d5b7a9 - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:106
  36:     0x556d058e7830 - std::panicking::try::he25c140619154546
                        at /checkout/src/libstd/panicking.rs:289
  37:     0x556d0577d646 - std::panic::catch_unwind::he6ebfcbf7caad1a9
                        at /checkout/src/libstd/panic.rs:392
  38:     0x556d04f7c852 - std::thread::Builder::spawn::{{closure}}::h5dc199d6a5476ee2
                        at /checkout/src/libstd/thread/mod.rs:408
  39:     0x556d04f7e3f4 - <F as alloc::boxed::FnBox<A>>::call_box::hbff3be30e9f83d91
                        at /checkout/src/liballoc/boxed.rs:640
  40:     0x556d09d3486a - <alloc::boxed::Box<(dyn alloc::boxed::FnBox<A, Output$u3d$R$GT$$u20$$u2b$$u20$$u27$a$RP$$GT$$u20$as$u20$core..ops..function..FnOnce$LT$A$GT$$GT$::call_once::h3757e4bb61b80a30
                        at /checkout/src/liballoc/boxed.rs:650
                         - std::sys_common::thread::start_thread::h9710965ece577cf4
                        at libstd/sys_common/thread.rs:24
  41:     0x556d09d32af5 - std::sys::unix::thread::Thread::new::thread_start::h661e40e1cea4ad5b
                        at libstd/sys/unix/thread.rs:90
  42:     0x7f39a3a2bf29 - start_thread
  43:     0x7f39a24d2ede - __clone
  44:                0x0 - <unknown>
ERROR 2018-08-18T17:11:27Z: servo: cannot access a TLS value during or after it is destroyed: AccessError
Pipeline failed in hard-fail mode.  Crashing!
Servo exited with return value 1

---

Note: If you do not run servo with the -f (hard fail) flag, Servo outputs this panic message forever in a loop:

called `Result::unwrap()` on an `Err` value: () (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(2) }, at libcore/result.rs:945)
stack backtrace:
   0:     0x55b263b220a5 - backtrace::backtrace::libunwind::trace::hc38180d489d66153
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace::h78a15d6884d51e5b
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x55b263b1fe6e - backtrace::capture::Backtrace::new::h9229778bab1a1cd5
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x55b25c506389 - servo::main::{{closure}}::he264d6b764632c29
                        at ports/servo/non_android_main.rs:122
   3:     0x55b263b5cf73 - std::panicking::rust_panic_with_hook::h75f53ae1ba39714f
                        at libstd/panicking.rs:479
   4:     0x55b263b5cb19 - std::panicking::continue_panic_fmt::h5f5c2db8ab083e0d
                        at libstd/panicking.rs:390
   5:     0x55b263b5ca15 - rust_begin_unwind
                        at libstd/panicking.rs:325
   6:     0x55b263b7ebdb - core::panicking::panic_fmt::h28ab0e0b2f3bdb57
                        at libcore/panicking.rs:77
   7:     0x55b260410e3b - core::result::unwrap_failed::had299b03fe1df828
                        at /checkout/src/libcore/macros.rs:26
   8:     0x55b25cea43d2 - <core::result::Result<T, E>>::unwrap::h68a086a9ab580b25
                        at /checkout/src/libcore/result.rs:782
   9:     0x55b25e1d6136 - script::script_runtime::new_rt_and_cx::h2c890130c5f4e1ce
                        at components/script/script_runtime.rs:147
  10:     0x55b25daf64fe - script::script_thread::ScriptThread::new::hd453f7994bd4d073
                        at components/script/script_thread.rs:838
  11:     0x55b25dce4d36 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::he6630a4a80bfce57
                        at components/script/script_thread.rs:579
  12:     0x55b25f5b87c6 - std::sys_common::backtrace::__rust_begin_short_backtrace::hf65c7e18c41d0180
                        at /checkout/src/libstd/sys_common/backtrace.rs:136
  13:     0x55b25ed90e94 - std::thread::Builder::spawn::{{closure}}::{{closure}}::h14287f4e715a0bb6
                        at /checkout/src/libstd/thread/mod.rs:409
  14:     0x55b25f5360c4 - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::hac3ff334a47d5c2c
                        at /checkout/src/libstd/panic.rs:313
  15:     0x55b25f730cc9 - std::panicking::try::do_call::h1859f1dc50402b13
                        at /checkout/src/libstd/panicking.rs:310
  16:     0x55b263b6e7a9 - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:106
  17:     0x55b25f6fa830 - std::panicking::try::he25c140619154546
                        at /checkout/src/libstd/panicking.rs:289
  18:     0x55b25f590646 - std::panic::catch_unwind::he6ebfcbf7caad1a9
                        at /checkout/src/libstd/panic.rs:392
  19:     0x55b25ed8f852 - std::thread::Builder::spawn::{{closure}}::h5dc199d6a5476ee2
                        at /checkout/src/libstd/thread/mod.rs:408
  20:     0x55b25ed913f4 - <F as alloc::boxed::FnBox<A>>::call_box::hbff3be30e9f83d91
                        at /checkout/src/liballoc/boxed.rs:640
  21:     0x55b263b4786a - <alloc::boxed::Box<(dyn alloc::boxed::FnBox<A, Output$u3d$R$GT$$u20$$u2b$$u20$$u27$a$RP$$GT$$u20$as$u20$core..ops..function..FnOnce$LT$A$GT$$GT$::call_once::h3757e4bb61b80a30
                        at /checkout/src/liballoc/boxed.rs:650
                         - std::sys_common::thread::start_thread::h9710965ece577cf4
                        at libstd/sys_common/thread.rs:24
  22:     0x55b263b45af5 - std::sys::unix::thread::Thread::new::thread_start::h661e40e1cea4ad5b
                        at libstd/sys/unix/thread.rs:90
  23:     0x7f3422466f29 - start_thread
  24:     0x7f3420f0dede - __clone
  25:                0x0 - <unknown>
ERROR 2018-08-18T17:17:49Z: servo: called `Result::unwrap()` on an `Err` value: ()
called `Result::unwrap()` on an `Err` value: () (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(3) }, at libcore/result.rs:945)
stack backtrace:
   0:     0x55b263b220a5 - backtrace::backtrace::libunwind::trace::hc38180d489d66153
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace::h78a15d6884d51e5b
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x55b263b1fe6e - backtrace::capture::Backtrace::new::h9229778bab1a1cd5
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x55b25c506389 - servo::main::{{closure}}::he264d6b764632c29
                        at ports/servo/non_android_main.rs:122
   3:     0x55b263b5cf73 - std::panicking::rust_panic_with_hook::h75f53ae1ba39714f
                        at libstd/panicking.rs:479
   4:     0x55b263b5cb19 - std::panicking::continue_panic_fmt::h5f5c2db8ab083e0d
                        at libstd/panicking.rs:390
   5:     0x55b263b5ca15 - rust_begin_unwind
                        at libstd/panicking.rs:325
   6:     0x55b263b7ebdb - core::panicking::panic_fmt::h28ab0e0b2f3bdb57
                        at libcore/panicking.rs:77
   7:     0x55b260410e3b - core::result::unwrap_failed::had299b03fe1df828
                        at /checkout/src/libcore/macros.rs:26
   8:     0x55b25cea43d2 - <core::result::Result<T, E>>::unwrap::h68a086a9ab580b25
                        at /checkout/src/libcore/result.rs:782
   9:     0x55b25e1d6136 - script::script_runtime::new_rt_and_cx::h2c890130c5f4e1ce
                        at components/script/script_runtime.rs:147
  10:     0x55b25daf64fe - script::script_thread::ScriptThread::new::hd453f7994bd4d073
                        at components/script/script_thread.rs:838
...
  23:     0x7f3422466f29 - start_thread
  24:     0x7f3420f0dede - __clone
  25:                0x0 - <unknown>
ERROR 2018-08-18T17:17:49Z: servo: called `Result::unwrap()` on an `Err` value: ()
called `Result::unwrap()` on an `Err` value: () (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(4) }, at libcore/result.rs:945)
stack backtrace:
   0:     0x55b263b220a5 - backtrace::backtrace::libunwind::trace::hc38180d489d66153
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
...
ERROR 2018-08-18T17:17:49Z: servo: called `Result::unwrap()` on an `Err` value: ()
called `Result::unwrap()` on an `Err` value: () (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(5) }, at libcore/result.rs:945)
stack backtrace:
   0:     0x55b263b220a5 - backtrace::backtrace::libunwind::trace::hc38180d489d66153
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace::h78a15d6884d51e5b
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
...
ERROR 2018-08-18T17:17:49Z: servo: called `Result::unwrap()` on an `Err` value: ()
called `Result::unwrap()` on an `Err` value: () (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(6) }, at libcore/result.rs:945)
stack backtrace:
   0:     0x55b263b220a5 - backtrace::backtrace::libunwind::trace::hc38180d489d66153
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53

[jdm]

In this case TLS refers to thread-local storage.

[mateon1]

Using this testcase on commit 0c14880 (2018-03-29) triggers a different panic:

mateon@ceres:/shared/dev/rust/servo
$ setarch x86_64 -R ./mach run -dz -f ../../fuzz/domato/class/md5-48fbe03233183a4b6f368272ff827d34/test.html
VMware, Inc.
softpipe
3.3 (Core Profile) Mesa 17.4.0-devel
DomRefCell<T> already mutably borrowed: BorrowError (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(NonZero(NonZero(1))) }, at libcore/result.rs:916)
stack backtrace:
   0:     0x55555dbce3b4 - backtrace::backtrace::libunwind::trace::ha3cdbcef574b9dc2
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace::he9053b6d49715d88
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x55555dbc5b3c - backtrace::capture::Backtrace::new::h69c0fdf670d7d4cd
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x5555559bb6af - servo::main::{{closure}}::h149bf00d246e8067
                        at ports/servo/main.rs:147
   3:     0x55555dbe1785 - std::panicking::rust_panic_with_hook::haa00f5a9417cd684
                        at libstd/panicking.rs:577
   4:     0x55555dbe160e - std::panicking::begin_panic::ha956683198499384
                        at libstd/panicking.rs:537
   5:     0x55555dbe150a - std::panicking::begin_panic_fmt::h587e116719f14631
                        at libstd/panicking.rs:521
   6:     0x55555dbe14a2 - rust_begin_unwind
                        at libstd/panicking.rs:497
   7:     0x55555dc165b0 - core::panicking::panic_fmt::h1d64949939b0af2f
                        at libcore/panicking.rs:71
   8:     0x5555585da05b - core::result::unwrap_failed::h917895849b5ea964
                        at /checkout/src/libcore/macros.rs:23
   9:     0x55555856c1d4 - <core::result::Result<T, E>>::expect::h3513eddb45d34412
                        at /checkout/src/libcore/result.rs:809
  10:     0x55555821cea5 - <script::dom::bindings::cell::DomRefCell<T>>::borrow::h418c462ac1dccf9d
                        at components/script/dom/bindings/cell.rs:70
  11:     0x55555822b4e8 - <script::dom::bindings::cell::DomRefCell<T> as script::dom::bindings::trace::JSTraceable>::trace::h7830bb80b625cc00
                        at components/script/dom/bindings/trace.rs:214
  12:     0x555557724e28 - <script::script_thread::ScriptThread as script::dom::bindings::trace::JSTraceable>::trace::h93a121a546be527f
                        at components/script/script_thread.rs:388
  13:     0x5555579479d1 - script::script_thread::trace_thread::{{closure}}::h4845b8448d43b03c
                        at components/script/script_thread.rs:137
  14:     0x555558971de2 - <std::thread::local::LocalKey<T>>::try_with::h1633b97245259a3c
                        at /checkout/src/libstd/thread/local.rs:377
  15:     0x5555588d92dc - <std::thread::local::LocalKey<T>>::with::h184d6c69503898df
                        at /checkout/src/libstd/thread/local.rs:288
  16:     0x5555577035b3 - script::script_thread::trace_thread::hd63efec5f9c575be
                        at components/script/script_thread.rs:134
  17:     0x555557e07b6a - script::script_runtime::trace_rust_roots::h547efb880cb2d521
                        at components/script/script_runtime.rs:449
  18:     0x55555d7b3c7e - _ZN2js2gc9GCRuntime11markRuntimeEP8JSTracerNS1_18TraceOrMarkRuntimeERNS_26AutoLockForExclusiveAccessE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/gc/RootMarking.cpp:343
  19:     0x55555d327c0d - _ZN2js2gc9GCRuntime14beginMarkPhaseEN2JS8gcreason6ReasonERNS_26AutoLockForExclusiveAccessE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsgc.cpp:3907
  20:     0x55555d32eb04 - _ZN2js2gc9GCRuntime23incrementalCollectSliceERNS_11SliceBudgetEN2JS8gcreason6ReasonERNS_26AutoLockForExclusiveAccessE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsgc.cpp:5889
  21:     0x55555d32f866 - _ZN2js2gc9GCRuntime7gcCycleEbRNS_11SliceBudgetEN2JS8gcreason6ReasonE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsgc.cpp:6174
  22:     0x55555d32fda7 - _ZN2js2gc9GCRuntime7collectEbNS_11SliceBudgetEN2JS8gcreason6ReasonE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsgc.cpp:6287
  23:     0x55555d330136 - _ZN2js2gc9GCRuntime2gcE18JSGCInvocationKindN2JS8gcreason6ReasonE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsgc.cpp:6354
  24:     0x55555d26cc1a - _Z5JS_GCP9JSRuntime
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsapi.cpp:1340
  25:     0x555556e2f1be - <script::dom::window::Window as script::dom::bindings::codegen::Bindings::WindowBinding::WindowBinding::WindowMethods>::Gc::ha132059ce4977831
                        at components/script/dom/window.rs:808
  26:     0x5555574c449b - script::dom::bindings::codegen::Bindings::WindowBinding::WindowBinding::gc::{{closure}}::hc11d0dc10cb39e51
                        at /shared/dev/rust/servo/target/debug/build/script-6b6718ab84e0c242/out/Bindings/WindowBinding.rs:2689
  27:     0x55555695db9c - core::ops::function::FnOnce::call_once::he833c1f0be25326e
                        at /checkout/src/libcore/ops/function.rs:223
  28:     0x5555577602ba - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::hcc4acff55a01076f
                        at /checkout/src/libstd/panic.rs:293
  29:     0x555557216894 - std::panicking::try::do_call::he3d9e4c9eca054cb
                        at /checkout/src/libstd/panicking.rs:479
  30:     0x55555dc0ad6e - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:102
  31:     0x5555571276a5 - std::panicking::try::h93f0deef19367698
                        at /checkout/src/libstd/panicking.rs:458
  32:     0x555557778221 - std::panic::catch_unwind::h21264e20826f8456
                        at /checkout/src/libstd/panic.rs:358
  33:     0x555556ab2b1e - mozjs::panic::wrap_panic::h593b98c26c549d20
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs-0.6.0/src/panic.rs:22
  34:     0x55555812169a - script::dom::bindings::codegen::Bindings::WindowBinding::WindowBinding::gc::hf01804d015bbd90f
                        at /shared/dev/rust/servo/target/debug/build/script-6b6718ab84e0c242/out/Bindings/WindowBinding.rs:2685
  35:     0x55555cec53be - CallJitMethodOp
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs-0.6.0/src/jsglue.cpp:560
  36:     0x55555729bb39 - script::dom::bindings::utils::generic_call::hc2459cd225ca9a0a
                        at components/script/dom/bindings/utils.rs:458
  37:     0x55555729bbab - script::dom::bindings::utils::generic_method::h74985f73157def27
                        at components/script/dom/bindings/utils.rs:466
  38:     0x55555d52f0e1 - _ZN2js12CallJSNativeEP9JSContextPFbS1_jPN2JS5ValueEERKNS2_8CallArgsE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jscntxtinlines.h:232
  39:     0x55555d4fe761 - _ZN2js23InternalCallOrConstructEP9JSContextRKN2JS8CallArgsENS_14MaybeConstructE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:453
  40:     0x55555d4fea88 - InternalCall
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:498
  41:     0x55555d4feab2 - _ZN2js13CallFromStackEP9JSContextRKN2JS8CallArgsE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:504
  42:     0x55555d50baa0 - Interpret
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:2873
  43:     0x55555d4fe3e2 - _ZN2js9RunScriptEP9JSContextRNS_8RunStateE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:399
  44:     0x55555d4ff7aa - _ZN2js13ExecuteKernelEP9JSContextN2JS6HandleIP8JSScriptEER8JSObjectRKNS2_5ValueENS_16AbstractFramePtrEPS9_
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:679
  45:     0x55555d4ffa6a - _ZN2js7ExecuteEP9JSContextN2JS6HandleIP8JSScriptEER8JSObjectPNS2_5ValueE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/vm/Interpreter.cpp:711
  46:     0x55555d278e2e - Evaluate
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsapi.cpp:4406
  47:     0x55555d27916a - Evaluate
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsapi.cpp:4443
  48:     0x55555d2796ec - _ZN2JS8EvaluateEP9JSContextRKNS_22ReadOnlyCompileOptionsEPKDsmNS_13MutableHandleINS_5ValueEEE
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/mozjs_sys-0.50.1/mozjs/js/src/jsapi.cpp:4501
  49:     0x55555810c4dc - mozjs::rust::wrappers::Evaluate2::h0248c6dddcc4cb6e
                        at components/script/lib.rs:1
  50:     0x55555863c944 - script::dom::globalscope::GlobalScope::evaluate_script_on_global_with_result::{{closure}}::he8c9f29caae6f2bd
                        at components/script/dom/globalscope.rs:438
  51:     0x555557ab2bf0 - profile_traits::time::profile::h84ced1b49a125322
                        at /shared/dev/rust/servo/components/profile_traits/time.rs:134
  52:     0x555557930e51 - script::dom::globalscope::GlobalScope::evaluate_script_on_global_with_result::h91d3f3e5e118005d
                        at components/script/dom/globalscope.rs:422
  53:     0x555558624347 - script::dom::htmlscriptelement::HTMLScriptElement::run_a_classic_script::h0b6141388480355c
                        at components/script/dom/htmlscriptelement.rs:569
  54:     0x555558623ce6 - script::dom::htmlscriptelement::HTMLScriptElement::execute::h121b886601b6d84c
                        at components/script/dom/htmlscriptelement.rs:539
  55:     0x555558621b50 - script::dom::htmlscriptelement::HTMLScriptElement::prepare::hc23ce7a231e15cec
                        at components/script/dom/htmlscriptelement.rs:452
  56:     0x555557f0587d - script::dom::servoparser::ServoParser::tokenize::h5833b5c4c0a507c0
                        at components/script/dom/servoparser/mod.rs:473
  57:     0x5555573d9537 - script::dom::servoparser::ServoParser::do_parse_sync::h9dc77b745682a3ea
                        at components/script/dom/servoparser/mod.rs:428
  58:     0x555557f0556f - script::dom::servoparser::ServoParser::parse_sync::{{closure}}::he01b21b2379ffb52
                        at components/script/dom/servoparser/mod.rs:414
  59:     0x555557ab3abc - profile_traits::time::profile::hb114593d5ee765e3
                        at /shared/dev/rust/servo/components/profile_traits/time.rs:134
  60:     0x5555573d929e - script::dom::servoparser::ServoParser::parse_sync::hf7f9305c0c471455
                        at components/script/dom/servoparser/mod.rs:411
  61:     0x5555573d97ed - script::dom::servoparser::ServoParser::parse_bytes_chunk::h64a847441162a5cb
                        at components/script/dom/servoparser/mod.rs:453
  62:     0x5555573dc1f3 - <script::dom::servoparser::ParserContext as net_traits::FetchResponseListener>::process_response_chunk::h214e19640f94ec26
                        at components/script/dom/servoparser/mod.rs:720
  63:     0x55555772251f - script::script_thread::ScriptThread::handle_fetch_chunk::h042ae8fc159a7939
                        at components/script/script_thread.rs:2518
  64:     0x55555770c0d3 - script::script_thread::ScriptThread::handle_msg_from_constellation::ha15ae6c7652fbbf7
                        at components/script/script_thread.rs:1268
  65:     0x55555794a60c - script::script_thread::ScriptThread::handle_msgs::{{closure}}::h6319263d718905a4
                        at components/script/script_thread.rs:1073
  66:     0x55555794aaf5 - script::script_thread::ScriptThread::profile_event::h24ad32198d7857e4
                        at components/script/script_thread.rs:1246
  67:     0x555557709174 - script::script_thread::ScriptThread::handle_msgs::h7f813572bec83627
                        at components/script/script_thread.rs:1067
  68:     0x555557706a33 - script::script_thread::ScriptThread::start::hfffc65ead139c565
                        at components/script/script_thread.rs:899
  69:     0x555557947dbc - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::{{closure}}::hb7eb5271aea54155
                        at components/script/script_thread.rs:580
  70:     0x5555583eba54 - profile_traits::mem::ProfilerChan::run_with_memory_reporting::he9923d74841dc066
                        at /shared/dev/rust/servo/components/profile_traits/mem.rs:63
  71:     0x555557948383 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::h8e07abd934bb999e
                        at components/script/script_thread.rs:579
  72:     0x5555590fd0d4 - std::sys_common::backtrace::__rust_begin_short_backtrace::h2d78b2f1df325efe
                        at /checkout/src/libstd/sys_common/backtrace.rs:133
  73:     0x555558c26f1b - std::thread::Builder::spawn::{{closure}}::{{closure}}::h60029a105377620a
                        at /checkout/src/libstd/thread/mod.rs:406
  74:     0x55555772ae94 - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::h071cb41be9fa08a0
                        at /checkout/src/libstd/panic.rs:293
  75:     0x5555571acab4 - std::panicking::try::do_call::h0634da9ad6ac21c1
                        at /checkout/src/libstd/panicking.rs:479
  76:     0x55555dc0ad6e - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:102
  77:     0x5555570a838c - std::panicking::try::h21b3c835feaab04b
                        at /checkout/src/libstd/panicking.rs:458
  78:     0x55555779098c - std::panic::catch_unwind::h79f92692c3fe610e
                        at /checkout/src/libstd/panic.rs:358
  79:     0x555558c26bf8 - std::thread::Builder::spawn::{{closure}}::he57f2c95280fc725
                        at /checkout/src/libstd/thread/mod.rs:405
  80:     0x555558c27255 - <F as alloc::boxed::FnBox<A>>::call_box::h5fc075c8048227e2
                        at /checkout/src/liballoc/boxed.rs:788
  81:     0x55555dbf5d7b - <alloc::boxed::Box<alloc::boxed::FnBox<A, Output$u3d$R$GT$$u20$$u2b$$u20$$u27$a$GT$$u20$as$u20$core..ops..function..FnOnce$LT$A$GT$$GT$::call_once::h13f1b0bf377a9f5b
                        at /checkout/src/liballoc/boxed.rs:798
                         - std::sys_common::thread::start_thread::hb5b6e0447b9691dc
                        at libstd/sys_common/thread.rs:24
                         - std::sys::unix::thread::Thread::new::thread_start::hcaff0bef1a01c624
                        at libstd/sys/unix/thread.rs:90
  82:     0x7ffff764ef29 - start_thread
  83:     0x7ffff67f1ede - __clone
  84:                0x0 - <unknown>
ERROR 2018-08-19T17:25:22Z: servo: DomRefCell<T> already mutably borrowed: BorrowError
Pipeline failed in hard-fail mode.  Crashing!
Servo exited with return value 1

[jdm]

That looks like the problem that #21028 set out to solve.

[mateon1]

git bisect confirms that the panic changed in that PR (40ea4c4)