iframe-append-to-child-document.html panics while trying to deal with a framed content changed notification

[jdm]

It's disabled claiming that #14411 is the cause of it failing. I tried reenabling it and running it, and got this output:

 0:04.88 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) Full command: /Users/jdm/src/servo/target/debug/servo -w --hard-fail -u Servo/wptrunner -Z replace-surrogates http://web-platform.test:8000/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html --user-stylesheet /Users/jdm/src/servo/resources/ahem.css
(pid:71028) "called `Option::unwrap()` on a `None` value (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(1) }, at /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/src/libcore/option.rs:323)"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "stack backtrace:"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   0:        0x113c342f1 - backtrace::backtrace::trace::hfa16fa86cb3d9083"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   1:        0x113c343b8 - backtrace::capture::Backtrace::new::hc8466b6723d46b7f"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   2:        0x10fabf08b - servo::main::{{closure}}::hed9c19bd2cb5f61a"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   3:        0x11694ea04 - std::panicking::rust_panic_with_hook::h33761bada49f3713"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   4:        0x11694e894 - std::panicking::begin_panic::h37a3cca06423c62e"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   5:        0x11694e802 - std::panicking::begin_panic_fmt::h4c0c43306d52c68f"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   6:        0x11694e767 - rust_begin_unwind"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   7:        0x116977f90 - core::panicking::panic_fmt::hbd633f652cd3a047"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   8:        0x116977e94 - core::panicking::panic::h723a4d81a96970ca"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "   9:        0x11098e1ae - <core::option::Option<T>>::unwrap::h45423c7bf1d14c70"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  10:        0x11104043c - script::script_thread::ScriptThread::handle_framed_content_changed::hac7737301cb49934"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  11:        0x111036002 - script::script_thread::ScriptThread::handle_msg_from_constellation::ha25d10a68b4c228e"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  12:        0x111033514 - script::script_thread::ScriptThread::handle_msgs::{{closure}}::h6ce7f4d07d6a2912"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  13:        0x1110340e1 - script::script_thread::ScriptThread::profile_event::h5037a6d26e784851"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  14:        0x111031c5a - script::script_thread::ScriptThread::handle_msgs::hd49e656fd4612832"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  15:        0x1110301a5 - script::script_thread::ScriptThread::start::h73f2d5ef35ee5e25"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  16:        0x11102c647 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::{{closure}}::{{closure}}::hd98dca41bd5e51fd"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  17:        0x11072e296 - profile_traits::mem::ProfilerChan::run_with_memory_reporting::h40004b401012d19e"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  18:        0x11102cb58 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::{{closure}}::hb716a2d7dbf26580"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  19:        0x110b48d49 - <std::thread::local::LocalKey<T>>::with::h68a4795350991987"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  20:        0x11102d017 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::ha7213a5460711713"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  21:        0x110ebc66a - <std::panic::AssertUnwindSafe<F> as core::ops::FnOnce<()>>::call_once::hb85bb569e3930eca"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  22:        0x110adadb9 - std::panicking::try::do_call::hdce947d793edb536"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  23:        0x11694f91a - __rust_maybe_catch_panic"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  24:        0x110a22a33 - std::panicking::try::h313c30e98c400556"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  25:        0x1109f80b4 - std::panic::catch_unwind::hc4b3ba6d690c8822"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  26:        0x110a01acb - std::thread::Builder::spawn::{{closure}}::hb776ef25827276dd"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  27:        0x110cd13a2 - <F as alloc::boxed::FnBox<A>>::call_box::h86607921970b3111"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  28:        0x11694d984 - std::sys::imp::thread::Thread::new::thread_start::he1b44499d44cc4a4"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  29:     0x7fff8cebe898 - _pthread_body"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "  30:     0x7fff8cebe729 - _pthread_start"
 0:05.00 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "ERROR:servo: called `Option::unwrap()` on a `None` value"
 0:05.10 PROCESS_OUTPUT: Thread-TestrunnerManager-1 (pid:71028) "Pipeline failed in hard-fail mode.  Crashing!"

@asajeffrey This might be up your alley.

[asajeffrey]

Re-ran the test. Good news, it no longer panics! Bad news, it FAILs instead :/