[WASM Exception]: Stacktrace of thrown exceptions in release build

[ravisumit33]

With javascript exceptions, we used to override __cxa_throw to log the callstack of thrown exception in production (release build). We are moving to wasm exceptions now and are wondering how can we achieve the same with this. I tried exporting __cxa_throw in emitted js and override it as we did before but that is not getting called now with wasm exceptions.

[ravisumit33]

@kripken Is it even possible to get the stacktrace in release build with wasm exceptions ?

[kripken]

Is the exception caught? If not, then an error handler on the page can get a stack trace.

Or, if it is caught, then a page-level error handler won't get it. Then you need to do something when a C++ exception is thrown, I guess, to log the stack at that time. I don't know if there is any kind of hook for that in the wasm exceptions library code - @aheejin ?

[aheejin]

In Wasm exception mode, stack traces are embedded automatically when ASSERTIONS is enabled. ASSERTIONS is enabled by default in -O0, and you can enable it manually by -sASSERTIONS in optimized builds. See https://emscripten.org/docs/porting/exceptions.html#debugging-exceptions.
(You also need -g or -gsource-map to display Wasm function names)

When ASSERTIONS is enabled, uncaught exceptions will automatically display stack traces. You can also catch it in JS and manually print or examine it using the stack property, like:

try {
  call_wasm_function();
} catch (e) {
  // Do things with `e.stack`
}

[ravisumit33]

@aheejin @kripken I guess enabling -sASSERTIONS in optimized builds have a performance cost, right ?

[kripken]

Yes, assertions add overhead. But it might not matter in some cases, so it's worth measuring.

[ravisumit33]

@aheejin @kripken Enabling assertions in production may have some other side effects like increase in error cases which were handled earlier without assertions. Can we have a flag just for enabling stack traces (e.g. EXCEPTION_STACKTRACE=1) without enabling all the assertions ?

[kripken]

That sounds reasonable to me, unless @aheejin there is some technical reason not to do so? I'm not sure offhand how best to manage building of system/lib/libcxxabi/src/cxa_exception.cpp but we can figure that out I hope.

[ravisumit33]

@aheejin What are your views on this ?