[issue41422] C Unpickler memory leak via memo

[kale-smoothie]

kale-smoothie <suraj-patel-95 at outlook.com> added the comment:

The leak demonstrated in the attachment is, to my understanding, caused by memoizing the closure returned from the `find_class` method that's used to intercept global references. The cycle is then: Unpickler, memo table, closure, Unpickler (via cell reference to `self`).

My proposed patch visits every entry in the memo table.

Pre-patch run of valgrind on leak_pickler.py:

==20339== HEAP SUMMARY:
==20339==     in use at exit: 190,189,238 bytes in 2,406,919 blocks
==20339==   total heap usage: 3,150,288 allocs, 743,369 frees, 233,766,596 bytes allocated
==20339== 
==20339== LEAK SUMMARY:
==20339==    definitely lost: 0 bytes in 0 blocks
==20339==    indirectly lost: 0 bytes in 0 blocks
==20339==      possibly lost: 190,176,150 bytes in 2,406,835 blocks
==20339==    still reachable: 13,088 bytes in 84 blocks
==20339==         suppressed: 0 bytes in 0 blocks
==20339== Rerun with --leak-check=full to see details of leaked memory

Post-patch run of valgrind on leak_pickler.py:

==20880== HEAP SUMMARY:
==20880==     in use at exit: 667,277 bytes in 6,725 blocks
==20880==   total heap usage: 2,853,739 allocs, 2,847,014 frees, 216,473,216 bytes allocated
==20880== 
==20880== LEAK SUMMARY:
==20880==    definitely lost: 0 bytes in 0 blocks
==20880==    indirectly lost: 0 bytes in 0 blocks
==20880==      possibly lost: 654,624 bytes in 6,646 blocks
==20880==    still reachable: 12,653 bytes in 79 blocks
==20880==         suppressed: 0 bytes in 0 blocks
==20880== Rerun with --leak-check=full to see details of leaked memory

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41422>
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