PHP: rfc:debug_backtrace_depth

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====== PHP RFC: debug_backtrace_depth(int $limit=0): int ======
  * Version: 0.1
  * Date: 2021-03-13
  * Author: Tyson Andre, tandre@php.net
  * Status: Withdrawn
  * First Published at: https://wiki.php.net/rfc/debug_backtrace_depth
  * Implementation: https://github.com/php/php-src/pull/6653

===== Introduction =====

Inspecting the current stack trace depth is occasionally useful for manually debugging, checking for potential infinite recursion, or for checking if code would hit stack frame limits in extensions such as Xdebug (''xdebug.max_nesting_limit''). It is currently possible to compute the depth through ''count(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, $limit=0))'', but this is verbose and inefficient compared to returning the depth directly.

===== Proposal =====

Add a new function ''debug_backtrace_depth(int $limit=0): int'' that will compute the depth directly rather than build an array with the frame data.

This can be polyfilled with
''min($limit, count(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS)))''.
or less accurately as ''count(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, $limit))''

  * For ''debug_backtrace_depth'', the limit is on the returned depth. If that depth is exceeded, ''$limit'' is returned instead)
  * For ''debug_backtrace'', the preexisting behavior is that the ''$limit'' is on the number of frames scanned, not the size of the returned array, and this may differ. Changing this to be a limit on the returned depth may be an improvement, but is out of scope for this RFC.

When ''$limit <= 0'', there is no limit.

Internally, PHP's stack frames are represented like a singly linked list (i.e. to count them, you need to iterate through the frames), and this RFC does not change that representation. The amount of time needed to compute the depth is proportional to the depth, which is why this RFC supports the optional ''$limit''. Counting the frames is still much faster than constructing an array with the ''file'', ''line'', ''class'', etc. with ''debug_backtrace()''.

==== Example use cases ====

While it is possible to do these things already with ''count(debug_backtrace(...))'', having a dedicated function to do this makes it more concise, efficient, and readable.

=== Tracking down potentially infinite recursion ===

<code php>
function fibonacci(int $n): int|float {
    if (debug_backtrace_depth(1000) >= 1000) {  // temporary debugging code
        throw new RuntimeException("Possibly infinite recursion at n=$n - was a base case forgotten?");
    }
    if ($n === 1) { return 1; }

    return fibonacci($n - 2) + fibonacci($n - 1);
}
echo fibonacci(10);
/*
Warning: Uncaught RuntimeException: Possibly infinite recursion at n=-1988 - was a base case forgotten? in php shell code:3                                          
Stack trace:
#0 php shell code(7): fibonacci(-1988)
#1 php shell code(7): fibonacci(-1986)
...
*/
</code>

=== Checking for compatibility with extensions with stack trace limits ===

For example, in Xdebug https://xdebug.org/docs/all_settings#max_nesting_level has a default value of 256 - if you are developing an application or library which cannot override max_nesting_level you may want to ensure your library does not use deep recursion.

<code php>
// e.g. a logging function
function my_common_function() {
    if (is_running_as_unit_test() && debug_backtrace_depth(250) >= 250) {
        throw new TestFrameworkError("Refactor this code, xdebug.max_nesting_level may be reached in other environments");
    }
    // body of my_common_function()
}
</code>

=== Debugging ===

This makes it more convenient to efficiently visualize how deep recursion is when adding temporary debugging statements, or where recursive calls are made relative to other calls.

<code php>
<?php
function fibonacci_debug(int $n) {
    if ($n <= 1) {
        $result = 1;
    } else {
        $result = fibonacci_debug($n - 2) + fibonacci_debug($n - 1);
    }
    printf("%sfib(%d) = %d\n", str_repeat('|', debug_backtrace_depth() - 1), $n, $result);
    return $result;
}

fibonacci_debug(4);
/*
||fib(0) = 1
||fib(1) = 1
|fib(2) = 2
||fib(1) = 1
|||fib(0) = 1
|||fib(1) = 1
||fib(2) = 2
|fib(3) = 3
fib(4) = 5
 */
</code>

===== RFC Impact =====
==== To SAPIs ====
None, other than creating the new global function ''debug_backtrace_depth''

===== Open Issues =====
Make sure there are no open issues when the vote starts!

===== Unaffected PHP Functionality =====
''debug_backtrace'' and ''debug_print_backtrace'' are unchanged.

===== Future Scope =====
The behavior of ''debug_backtrace'' and ''debug_print_backtrace'' could be changed to make $limit be consistent with the documentation in a separate RFC. https://www.php.net/manual/en/function.debug-backtrace.php#refsect1-function.debug-backtrace-parameters.

===== Proposed PHP Version =====
8.1

===== Discussion =====

==== Limited to a specific use case ====

https://externals.io/message/113502#113821

<blockquote>
There hasn't been much discussion on this one, probably because the
functionality and use cases are so specific. I'm not really convinced by
your examples.

For the purpose of printing debug logs, using an engine-provided depth is
non-ideal, because any intermediate helper functions will count towards the
depth. Tracking your own depth will result in a more meaningful/predictable
output.

For tracking down infinite recursion, isn't this what the xdebug recursion
limit is for? You'll directly get your recursive stack trace, which should
make it obvious where the infinite recursion occurs. Using
debug_backtrace_depth() for this purpose means that you actually already
need to know where you are infinitely recursing in order to place the guard.

If you want to find places where your code would hit the xdebug recursion
limit, then why not run it under xdebug and find out, rather than trying to
simulate the same behavior manually? That again requires that you actually
place relevant guards in the first place, which makes this another chicken
and egg problem.
</blockquote>


===== Proposed Voting Choices =====

Yes/No, requiring a 2/3 majority
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