Hello,

Apologies if me dropping in and drafting this issue seems weird. I am a big fan of how tracing can help with managing an application in production and would like to help out with adding first-class tracing hooks to PHP.

In the past I've used the OpenCensus extension to export tracing data to a managed service. This extension leaves a lot to be desired, but one thing it got right in my opinion - ability to declare instrumented function at runtime.

For example, one can declare a traced function and add attributes to it in PHP:

opencensus_trace_function('curl_exec', fn ($resource) => ['attributes' => ['url' => curl_getinfo($resource, CURLINFO_EFFECTIVE_URL)]]);

This is very powerful, in my opinion. If this functionality was available in PHP core, many open source frameworks and libraries could provide automatic instrumentation of their code. Think Symfony automatically instrumenting cache calls or Doctrine - DB calls.

I've researched php-src to find an appropriate place to call code which would instrument selected functions - both internal and user-land.

Since I am very new to php-src and have no C coding experience, so far I haven't gotten anywhere.

I've looked into zend_execute_ex and zend_execute_internal and tried to find where they are called. I've also stumbled upon ZEND_VM_HOT_HANDLER(130, ZEND_DO_UCALL, ANY, ANY, SPEC(RETVAL)).

Could you point me in the right direction: where would it make sense to call code that instruments a function? Or is the only way to override function pointers? Is declaring instrumented function runtime even a viable feature?

Thanks for your time and everything you do for the PHP community!

split up engine improvements in two parts

• additional functionality besides zend_execute_ex for instrumenting ALL function calls

  • before/after userland function call chain
  • extensions should allocate own stack, use array of frames and increment/decrement
  • support for zend vm stack? complicated

• instrument specific function calls

  • internal: keep overwriting handlers like we do now, already the most optimal solution
  • userland:
    - add hook into engine when "function is emitted" from parsing that allows set flag on zend_function. This flag could be checked to prevent JIT-ing the function.
    • on method call with instrumentation flag set: iterate over list of extension handlers (or use chain?)
    • again, could we let extensions manage their stack (simpler from engine pov)
    • downside: your hook gets called even when only other extension registers on it.

int should_instrument(zend_function *function) {
	funciton->flags |= ZEND_INSTURMENTED;
}

Own Stack vs VM Stack:

• problem with allocating space on VM stack, its actually not so easy.