Accessing V8 Stack Trace in Signal Handler

[Patric Lemo]

Hello all: 

I am working on a project where I need to print the JS stack trace when a JS process becomes unresponsive. My approach involves setting a periodic alarm using the `alarm()` function, and in the timeout function, I attempt to print the JS stack trace.

I'm experiencing an issue where `v8::StackTrace::CurrentStackTrace` returns a frame count of 0 when invoked within the signal handler. I understand there could be some complications using the V8 API within a signal handler due to its asynchronous signal-unsafe nature.

Here is a simplified version of my current approach:

// Assume isolate and context are correctly setup
void signalHandler(int signal) {
    // Try to get stack trace and print
    v8::HandleScope handleScope(isolate);
    v8::Local<v8::StackTrace> stackTrace = v8::StackTrace::CurrentStackTrace(
        isolate,
        10,
        v8::StackTrace::kDetailed
    );
    int frameCount = stackTrace->GetFrameCount();
    printf("Frame Count: %d\n", frameCount);
    // Iterate and print each frame here...
}

int main() {
    // Setup V8, isolate and context here...
    signal(SIGALRM, signalHandler);
    alarm(10);  // Set alarm
    // Rest of the program...
}

Is my approach flawed? Is there a safer or more effective method to achieve what I'm trying to do?

Thanks

[Andreas Haas]

Hi Partric,

I am not an expert with stack traces, so I don't know if there is a better method, but I think your approach is flawed. v8::StackTrace::CurrentStackTrace() can only work when code execution made a valid transition from JavaScript to C++, in other words, when there is an Exit frame on the stack. With a signal you transition from JavaScript code directly to the signal handling code, without an Exit frame. However, without the Exit frame V8 cannot find the beginning of the JavaScript stack, and can therefore not generate a stack trace.

You could try to request an interrupt with https://source.chromium.org/chromium/chromium/src/+/refs/heads/main:v8/include/v8-isolate.h;l=1115;drc=8d3b19d9432fa06963a1efbd8b85d5d310c3a05a, and then create the stack trace from within the callback. Maybe that works better.

Cheers, Andreas

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[Simon Zünd]

Hi Patric,

one more thing to note is that signal handlers might have a separate stack, depending on whether one was set up with "sigaltstack".

As Andreas mentioned, your best bet is to request an interrupt from the signal handler (or a separate thread) and capture the stack trace in the callback.

To view this discussion on the web visit https://groups.google.com/d/msgid/v8-dev/CAELSTveAc5EhxCBbKvixwHmTy1G9tfwnKPDRW%3D45S%3DPA8qPGYA%40mail.gmail.com.

[Patric Lemo]

Hey Andreas, Simon,

Just wanted to drop a quick note to say thanks. Your advice hit the mark and the interrupt request method worked like a charm. Really appreciate it!

Take care,
Patric Lemo