Fix NULL dereference in zend_signal_handler_unblock

[Appla]

This PR attempts to Fixes #21252
Alternative fixes

[devnexen]

php 8.3 is EOL, it s 8.4 minimum now

[devnexen]

I have more confidence in the 2nd fix personally. cc @bwoebi

[iluuu1994]

zend_signal_handler_unblock() is only called when there are pending signals. This is handled by ZEND_SIGNAL_UNBLOCK_INTERRUPTIONS():

php-src/Zend/zend_signal.h

Line 85 in 8c42cf4

# define ZEND_SIGNAL_UNBLOCK_INTERRUPTIONS() if (((SIGG(depth)--) == SIGG(blocked))) { zend_signal_handler_unblock(); }

(SIGG(depth)--) == SIGG(blocked) is true only when both SIGG(depth) and SIGG(blocked) are 1. SIGG(blocked) is set to 1 by zend_signal_handler_defer() when a signal is delayed.

zend_signal_handler_unblock() will SIG_BLOCK all incoming signals, which should prevent further signals from triggering, or being added to the queue. I don't see how this can cause a race condition.

Looking a bit at APCu, php_apc_finally looks suspicious, because it does not reset EG(bailout) during the execution of the finally block. This means that if a bailout is triggered, the finally block will be executed again. The analogous zend_catch in Zend/zend.h does have this code. This might or might not have something to do with it. I don't know if you have an easy way to test.

Edit: I created a PR here, as the code looks wrong regardless: krakjoe/apcu#611. I suppose you can just wait to see if this fixes your issue, once merged and released.

[iluuu1994]

See #21252 (comment).