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Date: Fri, 7 Oct 2022 15:32:19 +0200
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Subject: Preventing stack overflows
From: arnaud.lb@gmail.com (Arnaud Le Blanc)

Hi internals,

I would like to propose a way to detect stack overflows before they 
happen, with the goal of improving debugability.

Stack overflows are caused by excessive stack usage, typically due to 
deep recursions. Recursion in PHP doesn't use recursion internally, but 
some constructs still do, and it is not always possible to avoid without 
dramatically increasing complexity or degrading performances. Some 
examples of these constructs can be found at [1][2][3].

Programs that overflow the stack are terminated with a SIGSEGV, and the 
user is left with no hint as to which code is causing the issue. This 
makes debugging difficult.

Xdebug makes debugging easier by throwing an exception when the function 
call nesting level exceeds a certain limit [4], which is very useful. 
However, this can be overly limiting because this does not discriminate 
between the calls that actually use the stack and those that do not.

Nikita proposed an other solution a while ago [5] that limits in terms 
of VM reentrances, so that only the constructs that actually cause 
internal recursion count towards the limit. One issue is that not all VM 
frames will consume the same amount of stack, and the maximum stack size 
depends on the system, so it's hard to determine a good default value 
for the limit that is not overly limiting.

I would like to propose an alternative [6] that limits in terms of stack 
size. One issue is that it increases the internals complexity a bit, but 
this would make debugging easier without limiting recursion too much 
compared to what the system would allow.

Any thoughts?

[1] https://bugs.php.net/bug.php?id=64196
[2] https://bugs.php.net/bug.php?id=75509
[3] https://github.com/php/php-src/issues/8315
[4] https://xdebug.org/docs/develop#max_nesting_level
[5] https://externals.io/message/108348
[6] https://github.com/php/php-src/pull/9104