Allow using fast destruction path when ASAN is in use

[danog]

Disabling the fast path when a custom allocator is in use hid the #18833 issue, and made it super hard to debug, this re-enables it if php is compiled with asan.

[danog]

Ping @nielsdos

[nielsdos]

By design, fast shutdown does not explicitly free all request data but defers that to ZendMM. If ZendMM is not used, but you use fast shutdown, then you get real leaks.
Therefore, your patch will report leaks with ASAN in non-debug builds.

[nielsdos]

A compromise could be to make it opt-in when __SANITIZE_ADDRESS__ is set. For example, you could have an environment variable similar to USE_ZEND_ALLOC, e.g. ZEND_FORCE_FAST_SHUTDOWN. Then you check if that environment variable is set along with !EG(full_tables_cleanup) probably.

[dstogov]

fast_shutdown relays on Zend MM that frees the whole request heap at once. It doesn't make sense without it and will cause memory leaks.

[danog]

Makes sense, added a ZEND_FORCE_FAST_SHUTDOWN envvar conditional: leak detection must be disabled when using this option, i.e.

ASAN_OPTIONS=detect_leaks=0 USE_ZEND_ALLOC=0 ZEND_FORCE_FAST_SHUTDOWN=1 php a.php

Detects #18833 without emitting redundant leak issues.

[nielsdos]

As a debugging feature, in combination with ASAN_OPTIONS=detect_leaks=0, I can get behind this.