I have Cap’nProto RPC (in C++, on macOS) working well on the main thread, but I
want to move it to a background thread for various reasons. However, my process
is now raising SIGUSR1 signals, which the debugger breaks at, preventing the
code from working.
I’m following the directions in the docs: in my background thread’s main
function I call kj::setupAsyncIo(), then I call kj::getCurrentThreadExecutor()
to get the Executor object. My main thread uses that Executor reference to pass
lambdas that will run on the background thread and do the RPC stuff.
My immediate problem is that as soon as the main thread schedules a call with
the Executor, LLDB (in Xcode) takes control, letting me know that the
background thread has raised a SIGUSR1 signal. The backtrace is:
* thread #5, stop reason = signal SIGUSR1
* frame #0: 0x00000001ab672eb0 libsystem_kernel.dylib`poll + 8
frame #1: 0x0000000100bb0c78 `kj::UnixEventPort::PollContext::run(int) + 40
frame #2: 0x0000000100bb09ec `kj::UnixEventPort::wait() + 352
frame #3: 0x0000000100ba4710 `kj::EventLoop::wait() + 40
frame #4: 0x0000000100ba4c90
`kj::_::waitImpl(kj::Own<kj::_::PromiseNode>&&, kj::_::ExceptionOrValue&,
kj::WaitScope&) + 436
frame #5: 0x0000000100ba52fc `kj::_::NeverDone::wait(kj::WaitScope&) const
+ 76
…
If I continue from this, the code seems to be deadlocked: nothing happens
until I kill the process.
If I run without LLDB, the process gets past this point; the Executor seems to
behave correctly and my lambda starts running on the background thread.
However, working without a debugger is unacceptable; there’s no way I’m going
to be able to troubleshoot the rest of my code without LLDB.
Is there a workaround to this? Like, a way to get LLDB to ignore the signal, or
to get Cap’nP to use something other than a signal? I’m guessing I haven’t run
into this before because I’ve only been using a single thread and for whatever
reason Cap’nProto only uses this signal for inter-thread messaging … ?
—Jens
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