Fix GH-14361: Deep recursion in zend_cfg.c causes segfault instead of error

[nielsdos]

Building the CFG for JIT or optimizations can run into deep recursion, and depending on the stack limit cause a segfault. This patch uses the stack limit check to fail out of those cases. This prevents a segfault.

We use this check elsewhere in the compiler as well. However, in this case we just make optimizations or JIT fail without aborting the application such that code can still execute.

The attached test case will succeed both without and with this patch, it is mainly intended to test if propagating failure doesn't cause crashes. To reproduce the issue, set a low stack limit using ulimit -s and run the original test case from #14361

[dstogov]

I think it's better to switch to iterative reachability mark algorithm (e.g. using zend_worklist)

[nielsdos]

I think it's better to switch to iterative reachability mark algorithm (e.g. using zend_worklist)

I agree. I changed it.
To make review simpler: first commit changes indentation. Second commit makes it iterative.
Now it still passes the zend_basic_block *b pointer such that the amount of changes are smaller, but I can change that to an index if you prefer that.

[dstogov]

Thanks. This looks almost fine.
Please make the final touch - remove the redundant tail-call optimization.

I don't care about passing zend_basic_block or its index.
Maybe, it would be better to not pass it at all and push the start BB here.

[dstogov]

Iterative algorithm doesn't need tail call optimization any more.
This code may be removed as well as while (!finished) { loop.

[nielsdos]

Thanks, removed the tail call optimization

Maybe, it would be better to not pass it at all and push the start BB here.

We currently don't always start from the entry block so I didn't change it here.

[dstogov]

Looks fine.