Intermittent Mac script thread panic on shutdown: Option::unwrap in ScriptMemoryFailsafe::drop or Page::window

[mbrubeck]

mac-rel-css and mac-rel-wpt are failing very very frequently with shutdown panics that happen in different tests each time:

• http://build.servo.org/builders/mac-rel-css/builds/815/steps/test/logs/stdio
• http://build.servo.org/builders/mac-rel-css/builds/816/steps/test/logs/stdio
• http://build.servo.org/builders/mac-rel-css/builds/817/steps/test/logs/stdio
• http://build.servo.org/builders/mac-rel-css/builds/818/steps/test/logs/stdio
• http://build.servo.org/builders/mac-rel-wpt/builds/859/steps/test_1/logs/stdio

The backtraces come in two flavors, but I suspect they are the same underlying bug (maybe even the same code, inlined two different ways):

  ▶ CRASH [expected PASS] /_mozilla/css/block_image.html
  │ 
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ thread 'ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:325
  │ stack backtrace:
  │    1:        0x10c2a3958 - std::sys::backtrace::tracing::imp::write::h9fb600083204ae7f
  │    2:        0x10c2a9c35 - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hca543c34f11229ac
  │    3:        0x10c2a984e - std::panicking::default_hook::hc2c969e7453d080c
  │    4:        0x10b9fcb62 - util::panicking::initiate_panic_hook::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::h0bad41b0b38a855d
  │    5:        0x10c291128 - std::sys_common::unwind::begin_unwind_inner::h30e12d15ce2b2e25
  │    6:        0x10c292a5e - std::sys_common::unwind::begin_unwind_fmt::hb2de8a9968d38523
  │    7:        0x10c2a2c27 - rust_begin_unwind
  │    8:        0x10c2d19f0 - core::panicking::panic_fmt::h257ceb0aa351d801
  │    9:        0x10c2d1cec - core::panicking::panic::h4bb1497076d04ab9
  │   10:        0x10b4dda90 - _<script_thread..ScriptMemoryFailsafe<'a> as core..ops..Drop>::drop::h78647ee7cb9fccc9
  │   11:        0x10b4e0739 - std::sys_common::unwind::try::try_fn::hacee48040437e386
  │   12:        0x10c2a2bbb - __rust_try
  │   13:        0x10c2a2b43 - std::sys_common::unwind::inner_try::h47a4d9cd4a369dcd
  │   14:        0x10b4e0a4a - _<F as std..boxed..FnBox<A>>::call_box::h824e8f60f71d7d45
  │   15:        0x10c2a8cb8 - std::sys::thread::Thread::new::thread_start::h6f266e069bf4ec2b
  │   16:     0x7fff8ad7b059 - _pthread_body
  │   17:     0x7fff8ad7afd6 - _pthread_start
  └ thread panicked while panicking. aborting.

  ▶ CRASH [expected FAIL] /css21_dev/html4/reference/floats-placement-vertical-004-ref2.htm
  │ 
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ thread 'ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:325
  │ stack backtrace:
  │    1:        0x10eac0d18 - std::sys::backtrace::tracing::imp::write::h9fb600083204ae7f
  │    2:        0x10eac6ff5 - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hca543c34f11229ac
  │    3:        0x10eac6c0e - std::panicking::default_hook::hc2c969e7453d080c
  │    4:        0x10e21c242 - util::panicking::initiate_panic_hook::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::heda6930908f06acc
  │    5:        0x10eaae4e8 - std::sys_common::unwind::begin_unwind_inner::h30e12d15ce2b2e25
  │    6:        0x10eaafe1e - std::sys_common::unwind::begin_unwind_fmt::hb2de8a9968d38523
  │    7:        0x10eabffe7 - rust_begin_unwind
  │    8:        0x10eaeedb0 - core::panicking::panic_fmt::h257ceb0aa351d801
  │    9:        0x10eaef0ac - core::panicking::panic::h4bb1497076d04ab9
  │   10:        0x10dbecb70 - script::page::Page::window::h010a05681584eec2
  │   11:        0x10da8ffcc - std::sys_common::unwind::try::try_fn::hd12b2d76c81a0e30
  │   12:        0x10eabff7b - __rust_try
  │   13:        0x10eabff03 - std::sys_common::unwind::inner_try::h47a4d9cd4a369dcd
  │   14:        0x10da902ca - _<F as std..boxed..FnBox<A>>::call_box::he24f36d07ed1c7fd
  │   15:        0x10eac6078 - std::sys::thread::Thread::new::thread_start::h6f266e069bf4ec2b
  │   16:     0x7fff86bcf059 - _pthread_body
  │   17:     0x7fff86bcefd6 - _pthread_start
  └ thread panicked while panicking. aborting.

[cbrewster]

Not sure what is causing these, but I do know what causes the unwrap panic. We set the Frame on Page to None inside fn shut_down_layout in the script thread. When ScriptMemoryFailsafe::drop is called and it hasn't been neutered, it tries to get the window from the Page which causes a panic as we set the Frame to None.

[jdm]

After #11044 this looks like:

  ▶ CRASH [expected PASS] /_mozilla/css/border_collapse_simple_a.html
  │ 
  │ Shutting dow</span><span class="stdout">n the Constellation after generating an output file or exit flag specified
  │ thread &#39;ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }&#39; panicked at &#39;index out of bounds: the len is 0 but the index is 0&#39;, ../src/libcollections/vec.rs:1167
  │ stack backtrace:
  │    1:        0x10317db28 - std::sys::backtrace::tracing::imp::write::h9fb600083204ae7f
  │    2:        0x103183fb5 - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hca543c34f11229ac
  │    3:        0x103183bce - std::panicking::default_hook::hc2c969e7453d080c
  │    4:        0x102912c72 - util::panicking::initiate_panic_hook::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::ha2bd86c312dc8d7a
  │    5:        0x10316b782 - std::panicking::rust_panic_with_hook::hfe203e3083c2b544
  │    6:        0x103184576 - std::panicking::begin_panic::h4889569716505182
  │    7:        0x10316d078 - std::panicking::begin_panic_fmt::h484cd47786497f03
  │    8:        0x1031841cf - rust_begin_unwind
  │    9:        0x1031ab9c0 - core::panicking::panic_fmt::h257ceb0aa351d801
  │   10:        0x1031abb40 - core::panicking::panic_bounds_check::h3956fdcea61ff421
  │   11:        0x101d00e1e - script::dom::browsingcontext::BrowsingContext::active_document::h226f24d08ec44427
  │   12:        0x101d05afa - script::dom::browsingcontext::BrowsingContext::active_window::h2ba128d21d9d19cf
  │   13:        0x10223a17c - _&lt;script_thread..ScriptMemoryFailsafe&lt;&#39;a&gt; as core..ops..Drop&gt;::drop::hc9b03780a30d666b
  │   14:        0x10223cdc7 - std::panicking::try::call::h8d69dc69dac5d183
  │   15:        0x10318773b - __rust_try
  │   16:        0x1031876d5 - __rust_maybe_catch_panic
  │   17:        0x10223d5b4 - _&lt;F as std..boxed..FnBox&lt;A&gt;&gt;::call_box::h2e5eb7eb1a952230
  │   18:        0x103182fd8 - std::sys::thread::Thread::new::thread_start::h6f266e069bf4ec2b
  │   19:     0x7fff8c3c2059 - _pthread_body
  │   20:     0x7fff8c3c1fd6 - _pthread_start
  └ thread panicked while panicking. aborting.