Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 72 bytes)

I ran into the above error message while modifying some custom Drupal code. At first, I thought I just needed to increase the memory limit due to some newly called core functions consuming more than what was allocated.

There's good discussion about different ways to change the PHP memory limit in the article  Fatal error: Allowed memory size of X bytes exhausted (tried to allocate Y bytes)...

On my local stack, one relevant file is /etc/php5/apache2/php.ini which had a limit of 512M that I had set about two years ago. That seemed like a lot already. I upped it to 640M, but got the same error at exactly the same line of code.

So I fired up a debugger to trace through. When execution reached the loop body containing the reported line where execution died (line 4 below), I single-stepped, single-stepped, single-stepped, ... and it didn't leave after the expected number of iterations.

1:  $dirname = pathinfo($current_path, PATHINFO_DIRNAME);
2:  while ($dirname && $dirname != '.') {
3:    $page['#cache']['tags'][] = 'optimizely:' . $dirname . '/*';
4:    $dirname = pathinfo($dirname, PATHINFO_DIRNAME);
5:  }

Line 3 kept adding entries to the $page array until memory was exhausted. It was my own coding mistake in inadvertently creating a runaway infinite loop.
∞

Sources:

Fatal error: Allowed memory size of X bytes exhausted (tried to allocate Y bytes)...
https://www.drupal.org/node/76156