Testcase found while fuzzing mozilla-central rev b5b42ed4d6a0 (built with --enable-debug --enable-fuzzing).

Testcase can be reproduced using the following commands:

$ pip install fuzzfetch grizzly-framework
$ python -m fuzzfetch --build b5b42ed4d6a0 --debug --fuzzing -n mc-debug
$ python -m grizzly.replay --xvfb ./mc-debug/firefox ./testcase.zip

Assertion failure: get() (dereferencing a UniquePtr containing nullptr with ->), at /builds/worker/workspace/obj-build/dist/include/mozilla/UniquePtr.h:284

    #0 0x7f449d320058 in operator-> /builds/worker/workspace/obj-build/dist/include/mozilla/UniquePtr.h:284:5
    #1 0x7f449d320058 in mozilla::DecodedStream::SetPreservesPitch(bool) /builds/worker/checkouts/gecko/dom/media/mediasink/DecodedStream.cpp:705:7
    #2 0x7f449cfc7456 in mozilla::MediaDecoderStateMachine::PreservesPitchChanged() /builds/worker/checkouts/gecko/dom/media/MediaDecoderStateMachine.cpp:3703:15
    #3 0x7f449d0d881c in mozilla::WatchManager<mozilla::MediaDecoderStateMachine>::PerCallbackWatcher::Notify()::'lambda'()::operator()() const /builds/worker/workspace/obj-build/dist/include/mozilla/StateWatching.h:251:38
    #4 0x7f449d0d86e4 in mozilla::detail::RunnableFunction<mozilla::WatchManager<mozilla::MediaDecoderStateMachine>::PerCallbackWatcher::Notify()::'lambda'()>::Run() /builds/worker/workspace/obj-build/dist/include/nsThreadUtils.h:534:5
    #5 0x7f4499948145 in mozilla::SimpleTaskQueue::DrainTasks() /builds/worker/workspace/obj-build/dist/include/mozilla/TaskDispatcher.h:42:10
    #6 0x7f4499948203 in DrainDirectTasks /builds/worker/checkouts/gecko/xpcom/threads/TaskQueue.cpp:270:16
    #7 0x7f4499948203 in non-virtual thunk to mozilla::TaskQueue::DrainDirectTasks() /builds/worker/checkouts/gecko/xpcom/threads/TaskQueue.cpp
    #8 0x7f449994080b in MaybeDrainDirectTasks /builds/worker/workspace/obj-build/dist/include/mozilla/TaskDispatcher.h:240:41
    #9 0x7f449994080b in mozilla::AutoTaskDispatcher::TaskGroupRunnable::Run() /builds/worker/workspace/obj-build/dist/include/mozilla/TaskDispatcher.h:224:7
    #10 0x7f4499947566 in mozilla::TaskQueue::Runner::Run() /builds/worker/checkouts/gecko/xpcom/threads/TaskQueue.cpp:208:20
    #11 0x7f449995fb4b in nsThreadPool::Run() /builds/worker/checkouts/gecko/xpcom/threads/nsThreadPool.cpp:303:14
    #12 0x7f44999567b8 in nsThread::ProcessNextEvent(bool, bool*) /builds/worker/checkouts/gecko/xpcom/threads/nsThread.cpp:1153:16
    #13 0x7f449995d3aa in NS_ProcessNextEvent(nsIThread*, bool) /builds/worker/checkouts/gecko/xpcom/threads/nsThreadUtils.cpp:548:10
    #14 0x7f449a27983b in mozilla::ipc::MessagePumpForNonMainThreads::Run(base::MessagePump::Delegate*) /builds/worker/checkouts/gecko/ipc/glue/MessagePump.cpp:300:20
    #15 0x7f449a1e26c7 in MessageLoop::RunInternal() /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:335:10
    #16 0x7f449a1e25e2 in RunHandler /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:328:3
    #17 0x7f449a1e25e2 in MessageLoop::Run() /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:310:3
    #18 0x7f4499952f4f in nsThread::ThreadFunc(void*) /builds/worker/checkouts/gecko/xpcom/threads/nsThread.cpp:395:10
    #19 0x7f44b073aa57 in _pt_root /builds/worker/checkouts/gecko/nsprpub/pr/src/pthreads/ptthread.c:201:5
    #20 0x7f44b0cac608 in start_thread /build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477:8

Same root cause as bug 1708536, and I forgot to address this one in bug1708536.

Depends on D114015

https://hg.mozilla.org/mozilla-central/rev/5727a6f52c63

Bugmon Analysis
Verified bug as fixed on rev mozilla-central 20210503214210-c97286566c45.
Removing bugmon keyword as no further action possible. Please review the bug and re-add the keyword for further analysis.

:alwu, since this bug contains a bisection range, could you fill (if possible) the regressed_by field?
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Sorry, bug in the bot.