Capture device leaves browser with locks held

[hills]

It looks like I have the basis of a lockup when capture devices are used in Firefox, and this is the smallest reproducable test case I am able to create from the browser.

On Linux with apulse, this is often (but not always) leaving some kind of lock held:

• the Firefox UI can do basic operations but not a lot else.
• Other apps continue to be able to record the same "dsnoop" device.
• Firefox has do be killed with a soft SIGINT to restore most of its functionality; "pkill firefox-bin".

Other platforms seem immune; Mac and Windows.

With experimentation it looks like it's an interaction between opening both playback and capture devices. With capture only I wasn't able to show the result. It could be races on the fast opening/closing when error conditions occur.

The firefox process is left with a thread in this state:

Thread 46 (Thread 0x7fc3c17fe700 (LWP 14599)):
#0  0x00007fc3e364036f in pthread_cond_wait@@GLIBC_2.3.2 () at /lib64/libpthread.so.0
#1  0x00007fc3b6dc8ea7 in pa_threaded_mainloop_wait () at /usr/lib64/apulse/libpulse.so.0
#2  0x00007fc3d3b1b066 in _$LT$cubeb_pulse..backend..stream..PulseStream$u20$as$u20$cubeb_backend..traits..StreamOps$GT$::stop::h0b3214cf8c1e02c3 () at /scratch/mark/tmp/ff/firefox/libxul.so
#3  0x00007fc3d3b14bda in cubeb_backend::capi::capi_stream_stop::h148e9408bfd40feb () at /scratch/mark/tmp/ff/firefox/libxul.so
#4  0x00007fc3d3c4ba1a in cubeb_core::stream::StreamRef::stop::ha9e7c2fa562d26f8 () at /scratch/mark/tmp/ff/firefox/libxul.so
#5  0x00007fc3d3bc76c5 in audioipc_server::server::CubebServer::process_msg::h089c041680ef1cdb () at /scratch/mark/tmp/ff/firefox/libxul.so
#6  0x00007fc3d5859663 in _$LT$audioipc_server..server..CubebServer$u20$as$u20$audioipc..rpc..server..Server$GT$::process::he0be6d27a49be39a () at /scratch/mark/tmp/ff/firefox/libxul.so
#7  0x00007fc3d5855e5b in _$LT$futures..future..map_err..MapErr$LT$A$C$F$GT$$u20$as$u20$futures..future..Future$GT$::poll::h563291d28ca3169b () at /scratch/mark/tmp/ff/firefox/libxul.so
#8  0x00007fc3d5853067 in tokio::runtime::current_thread::runtime::Runtime::block_on::hd21a8602f581c25d () at /scratch/mark/tmp/ff/firefox/libxul.so
#9  0x00007fc3d3bbbac4 in std::sys_common::backtrace::__rust_begin_short_backtrace::hfab3e639f9165888 () at /scratch/mark/tmp/ff/firefox/libxul.so
#10 0x00007fc3d3bbd5c7 in core::ops::function::FnOnce::call_once$u7b$$u7b$vtable.shim$u7d$$u7d$::h6ace096c9215fb5d () at /scratch/mark/tmp/ff/firefox/libxul.so
#11 0x00007fc3d5a7793a in std::sys::unix::thread::Thread::new::thread_start::h5ad4ddffe24373a8 () at /scratch/mark/tmp/ff/firefox/libxul.so
#12 0x00007fc3e363a684 in start_thread () at /lib64/libpthread.so.0
#13 0x00007fc3e28d0eed in clone () at /lib64/libc.so.6

I note that pulseaudio is deferring work to its own worker threads. They are in this state, so I can reasonably see why nothing is moving forward:

Thread 83 (Thread 0x7fc3b44ff700 (LWP 14756)):
#0  0x00007fc3e28c52fd in poll () at /lib64/libc.so.6
#1  0x00007fc3b6dc8c1a in poll_func () at /usr/lib64/apulse/libpulse.so.0
#2  0x00007fc3b6dc4f1e in pa_mainloop_poll () at /usr/lib64/apulse/libpulse.so.0
#3  0x00007fc3b6dc4c31 in pa_mainloop_iterate () at /usr/lib64/apulse/libpulse.so.0
#4  0x00007fc3b6dc52a5 in pa_mainloop_run () at /usr/lib64/apulse/libpulse.so.0
#5  0x00007fc3b6dc8bc2 in mainloop_thread () at /usr/lib64/apulse/libpulse.so.0
#6  0x00007fc3e363a684 in start_thread () at /lib64/libpthread.so.0
#7  0x00007fc3e28d0eed in clone () at /lib64/libc.so.6

Thread 76 (Thread 0x7fc3b6abf700 (LWP 14743)):
#0  0x00007fc3e28c52fd in poll () at /lib64/libc.so.6
#1  0x00007fc3b6dc8c1a in poll_func () at /usr/lib64/apulse/libpulse.so.0
#2  0x00007fc3b6dc4f1e in pa_mainloop_poll () at /usr/lib64/apulse/libpulse.so.0
#3  0x00007fc3b6dc4c31 in pa_mainloop_iterate () at /usr/lib64/apulse/libpulse.so.0
#4  0x00007fc3b6dc52a5 in pa_mainloop_run () at /usr/lib64/apulse/libpulse.so.0
#5  0x00007fc3b6dc8bc2 in mainloop_thread () at /usr/lib64/apulse/libpulse.so.0
#6  0x00007fc3e363a684 in start_thread () at /lib64/libpthread.so.0
#7  0x00007fc3e28d0eed in clone () at /lib64/libc.so.6