Description:
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I'm trying to migrate from using op_array->reserved to ZEND_OP_ARRAY_EXTENSION
in a PHP extension. This is recommended in the UPGRADE.INTERNALS, and I anyway
already ran into problems with op_array reserved pointers in PHP 7.3. I see
various crashes when using ZEND_OP_ARRAY_EXTENSION and I could nail down two
major reasons for those crashes.

1. The run time cache that ZEND_OP_ARRAY_EXTENSION accesses is not always 
   initialized, and ZEND_OP_ARRAY_EXTENSION does no safety checks around that.

   I can mitigate this by calling zend_fetch_function for all functions that I
   need to access (zend_fetch_function ensures that the run time cache is
   initialized), however there's no similar way to ensure an initialized run
   time cache for class methods or callables.

   To transition from op_array->reserved to ZEND_OP_ARRAY_EXTENSION, I would
   need a way to ensure that the run time cache on the op_array is initialized.
   This could be an API function that I can call. There exists a function
   init_func_run_time_cache in Zend, but it's static and not part of the public
   API.

2. In one cases (namely in zend_get_call_trampoline_func), the run time cache
   pointer is set to a dummy value of 0x2. Accessing this op_array with 
   ZEND_OP_ARRAY_EXTENSION causes a crash.

   I'd expect ZEND_OP_ARRAY_EXTENSION to check for this special condition and
   handle it accordingly.

The one critical necessary for me (and I think for many others) to transition
from op_array->reserved to ZEND_OP_ARRAY_EXTENSION is a way to ensure an 
initialized run time cache on an op_array, like a function 
init_func_run_time_cache that I can call. Enhanced safety checks in
ZEND_OP_ARRAY_EXTENSION would be a nice-to-have.