Potential issue with concurrent access to Isolate::pending_deopts

[mkustermann]

From this buildbot log:

FAILED: dartk-vm release_x64 language_2/await_future_test
Expected: Pass
Actual: Crash
--- Command "vm" (took 06.000545s):
DART_CONFIGURATION=ReleaseX64 out/ReleaseX64/dart --hot-reload-test-mode --optimization-counter-threshold=5 --sync-async --ignore-unrecognized-flags --packages=/b/s/w/ir/cache/builder/sdk/.packages /b/s/w/ir/cache/builder/sdk/tests/language_2/await_future_test.dart
exit code:
-6
stdout:
unittest-suite-wait-for-done
stderr:
tag handler failed: 'file:///b/s/w/ir/cache/builder/sdk/tests/language_2/await_future_test.dart' is not a kernel file
...
===== CRASH =====
version=2.1.0-edge.02b2deefd6009a8064ba70c1cd80e2d79dfaeed6 (Fri Aug 31 01:19:24 2018 +0000) on "linux_x64"
si_signo=Segmentation fault(11), si_code=1, si_addr=0x548
Dumping native stack trace for thread 32f3
  [0x000055993cbeb8b0] dart::Isolate::FindPendingDeopt(unsigned long) const
  [0x000055993cbeb8b0] dart::Isolate::FindPendingDeopt(unsigned long) const
  [0x000055993cdb14d4] dart::Thread::VisitObjectPointers(dart::ObjectPointerVisitor*, dart::ValidationPolicy)
  [0x000055993cdb3c54] dart::ThreadRegistry::VisitObjectPointers(dart::ObjectPointerVisitor*, dart::ValidationPolicy)
  [0x000055993cf10dca] dart::Scavenger::IterateRoots(dart::Isolate*, dart::ScavengerVisitor*)
  [0x000055993cf117e9] dart::Scavenger::Scavenge()
  [0x000055993cf11c50] dart::Scavenger::Evacuate()
  [0x000055993cf07754] dart::Heap::EvacuateNewSpace(dart::Thread*, dart::Heap::GCReason)
  [0x000055993cf0623d] dart::Heap::AllocateOld(long, dart::HeapPage::PageType)
  [0x000055993cc1c09c] dart::Object::Allocate(long, long, dart::Heap::Space)
  [0x000055993cc7e767] dart::ICData::NewDescriptor(dart::Zone*, dart::Function const&, dart::String const&, dart::Array const&, long, long, dart::ICData::RebindRule, dart::AbstractType const&)
  [0x000055993cc7ea0f] dart::ICData::New(dart::Function const&, dart::String const&, dart::Array const&, long, long, dart::ICData::RebindRule, dart::AbstractType const&)
  [0x000055993cc7d00e] dart::ICData::NewFrom(dart::ICData const&, long)
  [0x000055993ce627e2] dart::CallSpecializer::TryCreateICData(dart::InstanceCallInstr*)
  [0x000055993ce622f6] dart::CallSpecializer::ApplyClassIds()
  [0x000055993ce6dd10] Unknown symbol
  [0x000055993ce6d655] dart::CompilerPass::Run(dart::CompilerPassState*) const
  [0x000055993ce6d825] dart::CompilerPass::RunPipeline(dart::CompilerPass::PipelineMode, dart::CompilerPassState*)
  [0x000055993cef09c2] dart::CompileParsedFunctionHelper::Compile(dart::CompilationPipeline*)
  [0x000055993cef189a] Unknown symbol
  [0x000055993cef2877] dart::Compiler::CompileOptimizedFunction(dart::Thread*, dart::Function const&, long)
  [0x000055993cef4614] dart::BackgroundCompiler::Run()
  [0x000055993cdb37dd] dart::ThreadPool::Worker::Loop()
  [0x000055993cdb3674] dart::ThreadPool::Worker::Main(unsigned long)
  [0x000055993ccc0b69] Unknown symbol
-- End of DumpStackTrace
--- Re-run this test:
python tools/test.py -m release -n dartk-reload-linux-release-x64 language_2/await_future_test

By looking at the code of Isolate::FindPendingDeopt():

uword Isolate::FindPendingDeopt(uword fp) const {
  for (intptr_t i = 0; i < pending_deopts_->length(); i++) {
    if ((*pending_deopts_)[i].fp() == fp) {
      return (*pending_deopts_)[i].pc();
    }
  }
  FATAL("Missing pending deopt entry");
  return 0;

It seems to not hit the FATAL but rather a SEGV which might indicate there is some concurrent access of the background compiler to Isolate::pending_deopts_

/cc @mraleph