called `Option::unwrap()` on a `None` value (Upgrading a weak Arc in layout)

[saschanaz]

Clicking report button does not really 'prefill' the issue and Servo does not let me copy-paste, so I'm just attaching the screenshot.

Repro was able on this page: http://comic.naver.com/webtoon/list.nhn?titleId=670152&weekday=sun

[emilio]

Thanks for reporting!

Full backtrace:

stack backtrace:
   0:     0x55c6af994651 - backtrace::backtrace::libunwind::trace
                        at /home/emilio/projects/moz/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.2.3/src/backtrace/libunwind.rs:54
                         - backtrace::backtrace::trace<closure>
                        at /home/emilio/projects/moz/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.2.3/src/backtrace/mod.rs:70
   1:     0x55c6af995200 - backtrace::capture::{{impl}}::new
                        at /home/emilio/projects/moz/servo/target/debug/build/backtrace-3bc1ff360ebb00cb/out/capture.rs:79
   2:     0x55c6aee1c35b - servo::main::{{closure}}
                        at /home/emilio/projects/moz/servo/components/servo/main.rs:121
   3:     0x55c6b3904485 - std::panicking::rust_panic_with_hook::h2224f33fb7bf2f4c
   4:     0x55c6b3904352 - std::panicking::begin_panic::hcb11a4dc6d779ae5
   5:     0x55c6b3904280 - std::panicking::begin_panic_fmt::h310416c62f3935b3
   6:     0x55c6b3904201 - rust_begin_unwind
   7:     0x55c6b3941eaf - core::panicking::panic_fmt::hc5789f4e80194729
   8:     0x55c6b3941ddb - core::panicking::panic::h1953378f4b37b561
   9:     0x55c6b24414c9 - {{inlined-root}}::unwrap<alloc::arc::Arc<Flow>>
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/libcore/option.rs:21
  10:     0x55c6b2588614 - layout::flow::{{impl}}::generated_containing_block_size
                        at /home/emilio/projects/moz/servo/components/layout/flow.rs:1560
  11:     0x55c6b2540a9a - layout::block::{{impl}}::containing_block_size
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:666
  12:     0x55c6b255d150 - layout::block::{{impl}}::containing_block_inline_size
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:2746
  13:     0x55c6b2553948 - layout::block::ISizeAndMarginsComputer::compute_inline_size_constraint_inputs<layout::block::AbsoluteNonReplaced>
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:2293
  14:     0x55c6b2557997 - layout::block::ISizeAndMarginsComputer::compute_used_inline_size<layout::block::AbsoluteNonReplaced>
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:2419
  15:     0x55c6b2540806 - layout::block::{{impl}}::compute_used_inline_size
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:599
  16:     0x55c6b2542d85 - layout::block::{{impl}}::propagate_and_compute_used_inline_size
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:1303
  17:     0x55c6b25449f4 - layout::block::{{impl}}::compute_inline_sizes
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:1646
  18:     0x55c6b254517c - layout::block::{{impl}}::assign_inline_sizes
                        at /home/emilio/projects/moz/servo/components/layout/block.rs:1736
  19:     0x55c6b25db740 - layout::traversal::{{impl}}::process
                        at /home/emilio/projects/moz/servo/components/layout/traversal.rs:158
  20:     0x55c6b25b5c9b - layout::parallel::ParallelPreorderFlowTraversal::run_parallel_helper<layout::traversal::AssignISizes>
                        at /home/emilio/projects/moz/servo/components/layout/parallel.rs:161
                         - layout::parallel::{{impl}}::run_parallel
                        at /home/emilio/projects/moz/servo/components/layout/parallel.rs:190
  21:     0x55c6b25b621e - layout::parallel::assign_inline_sizes
                        at /home/emilio/projects/moz/servo/components/layout/parallel.rs:209
  22:     0x55c6af804b46 - {{inlined-root}}::start<layout::context::SharedLayoutContext,style::parallel::WorkQueueData>
                        at /home/emilio/projects/moz/servo/components/style/workqueue.rs:184
  23:     0x55c6af8979d9 - _<style..workqueue..WorkQueue<QueueData, WorkData>>::new::_{{closure}}::hd8ec97a472d9ec2a
  24:     0x55c6af8518d0 - {{inlined-root}}::call_once<(),closure>
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/libstd/panic.rs:256
  25:     0x55c6af6a5b83 - {{inlined-root}}::do_call<std::panic::AssertUnwindSafe<closure>,()>
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/libstd/panicking.rs:327
  26:     0x55c6b390c876 - __rust_maybe_catch_panic
  27:     0x55c6af6a54fa - {{inlined-root}}::try<(),std::panic::AssertUnwindSafe<closure>>
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/libstd/panicking.rs:303
  28:     0x55c6af6a3974 - {{inlined-root}}::catch_unwind<std::panic::AssertUnwindSafe<closure>,()>
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/libstd/panic.rs:312
  29:     0x55c6af892237 - std::thread::Builder::spawn::_{{closure}}::h9c31998f7624ebe9
  30:     0x55c6af758bd7 - {{inlined-root}}::call_box<(),closure>
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/liballoc/boxed.rs:587
  31:     0x55c6b3902832 - std::sys::thread::Thread::new::thread_start::he0bf102845911132
  32:     0x7f6979ae6453 - start_thread
  33:     0x7f69796127de - __GI___clone
  34:                0x0 - <unknown>

[emilio]

Seems like this upgrade() call is what is failing. Scary. Probably a consequence of some of the incremental restyle changes (After some more thought, I don't think so)... Sigh.

https://github.com/servo/servo/blob/master/components/layout/flow.rs#L1560

[emilio]

Nope, I confirmed this wasn't due to my style system changes, by using a build from May 26th (2bb109b).

cc @pcwalton

[nox]

This still happens.

[dralley]

This still reproduces