Servo panics when loading a broken script URL with an onerror handler that rewrites the document

[mateon1]

<script async src=x onerror="document.write(0)"></script>

VMware, Inc.
llvmpipe (LLVM 7.0, 128 bits)
3.3 (Core Profile) Mesa 18.3.2
unknown completed load Script("file:///shared/dev/fuzz/domato/x") (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at components/script/document_loader.rs:171)
stack backtrace:
   0:     0x56362ac05756 - backtrace::backtrace::libunwind::trace::h8af3b710f1a5e12e
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace::h2a1223234ae0346f
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/mod.rs:42
   1:     0x56362abffd63 - backtrace::capture::Backtrace::new_unresolved::h2dd57ce7b77a7435
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/capture.rs:88
   2:     0x56362abffcbd - backtrace::capture::Backtrace::new::h8c6b161078e28d12
                        at /home/mateon/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/capture.rs:63
   3:     0x563623bba2b1 - servo::main::{{closure}}::h5fe9a894213f10e8
                        at ports/servo/non_android_main.rs:110
   4:     0x56362ccf32c8 - rust_panic_with_hook
                        at src/libstd/panicking.rs:482
   5:     0x56362ccf2d61 - continue_panic_fmt
                        at src/libstd/panicking.rs:385
   6:     0x56362ccf2cae - begin_panic_fmt
                        at src/libstd/panicking.rs:340
   7:     0x563626e0263f - script::document_loader::DocumentLoader::finish_load::{{closure}}::ha215191539b44bcd
                        at components/script/document_loader.rs:171
   8:     0x5636260cc611 - <core::option::Option<T>>::unwrap_or_else::hc633e9599da3a3fc
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libcore/option.rs:386
   9:     0x56362521615f - script::document_loader::DocumentLoader::finish_load::hf15042593c09a6a9
                        at components/script/document_loader.rs:171
  10:     0x563626629802 - script::dom::document::Document::finish_load::h2cac29fd478adc61
                        at components/script/dom/document.rs:1727
  11:     0x5636257ca5af - <script::dom::htmlscriptelement::ScriptContext as net_traits::FetchResponseListener>::process_response_eof::hd746e7f11044a6ea
                        at components/script/dom/htmlscriptelement.rs:246
  12:     0x56362686081a - <net_traits::FetchResponseMsg as net_traits::Action<T>>::process::h5fba9fa4b20edf5a
                        at /shared/dev/rust/servo/components/net_traits/lib.rs:271
  13:     0x563626bd7ae9 - <script::network_listener::ListenerTask<A, Listener> as script::task::TaskOnce>::run_once::h9e3a1d21f3429766
                        at components/script/network_listener.rs:105
  14:     0x5636257e33a3 - <script::task::CancellableTask<T> as script::task::TaskOnce>::run_once::hc2a5b42eb1198235
                        at components/script/task.rs:122
  15:     0x5636257d82b8 - <T as script::task::TaskBox>::run_box::h97b906a285339a15
                        at components/script/task.rs:64
  16:     0x5636256334b6 - script::script_thread::ScriptThread::handle_msg_from_script::h15d002ec5aef5d9d
                        at components/script/script_thread.rs:1745
  17:     0x5636252064d9 - script::script_thread::ScriptThread::handle_msgs::{{closure}}::h9f46a3c0a7bb7f95
                        at components/script/script_thread.rs:1348
  18:     0x5636252095be - script::script_thread::ScriptThread::profile_event::he7baa525ee50e51f
                        at components/script/script_thread.rs:1583
  19:     0x56362562e7ae - script::script_thread::ScriptThread::handle_msgs::hd718a0404a74804d
                        at components/script/script_thread.rs:1341
  20:     0x56362562b8b7 - script::script_thread::ScriptThread::start::h9d2b2f4b40ef17fd
                        at components/script/script_thread.rs:1178
  21:     0x563625202cb1 - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::{{closure}}::h3d2e50a51a59531c
                        at components/script/script_thread.rs:731
  22:     0x563626928138 - profile_traits::mem::ProfilerChan::run_with_memory_reporting::h9a6fb4b77584b283
                        at /shared/dev/rust/servo/components/profile_traits/mem.rs:88
  23:     0x56362520344d - <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}::hec71b1d6b339dd35
                        at components/script/script_thread.rs:729
  24:     0x5636274860f4 - std::sys_common::backtrace::__rust_begin_short_backtrace::h70010f71bf1d628c
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/sys_common/backtrace.rs:136
  25:     0x5636263dd3a3 - std::thread::Builder::spawn_unchecked::{{closure}}::{{closure}}::hea3251b085d70f10
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/thread/mod.rs:469
  26:     0x5636268a4e73 - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::hbd8938e17c48a216
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panic.rs:309
  27:     0x563626b538e9 - std::panicking::try::do_call::hd59da0280ee94513
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panicking.rs:297
  28:     0x56362ccfde79 - __rust_maybe_catch_panic
                        at src/libpanic_unwind/lib.rs:87
  29:     0x563626a2593f - std::panicking::try::h96247a5f4e51c63e
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panicking.rs:276
  30:     0x5636268f5b75 - std::panic::catch_unwind::hc4559b7cad69ef45
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panic.rs:388
  31:     0x5636263daf2f - std::thread::Builder::spawn_unchecked::{{closure}}::h66879e6d1707cb6d
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/thread/mod.rs:468
  32:     0x5636263de098 - <F as alloc::boxed::FnBox<A>>::call_box::h87e072e6468861a9
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/liballoc/boxed.rs:749
  33:     0x56362ccfd1bd - call_once<(),()>
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/liballoc/boxed.rs:759
                         - start_thread
                        at src/libstd/sys_common/thread.rs:14
                         - thread_start
                        at src/libstd/sys/unix/thread.rs:80
  34:     0x7f8e0f51afa2 - start_thread
  35:     0x7f8e0f42f7ee - __clone
  36:                0x0 - <unknown>
[2019-03-31T17:41:41Z ERROR servo] unknown completed load Script("file:///shared/dev/fuzz/domato/x")
Pipeline failed in hard-fail mode.  Crashing!

[jdm]

I think we should change this panic into a warn! instead.

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[jdm]

This change will also avoid panics like #18928, but we'll keep that one open because there is still useful work to be done to avoid unnecessary loads.

[krk]

@highfive: assign me

[highfive]

Hey @krk! Thanks for your interest in working on this issue. It's now assigned to you!