If a heap type does not have Py_TPFLAGS_HAVE_GC or doesn't not implement tp_traverse, the garbage collector may fail to collect the type and most of its instances at Python exit. The problem is worse when sub-interpreters are involved: if a sub-interpreter exits and does not clear all of its memory, the other interpreters will continue running with these leaks.

See issue gh-87138 for the rationale.

I wrote an article on the lock type which had this bug and caused a memory leak: https://vstinner.github.io/subinterpreter-leaks.html

Linked PRs

• [draft] gh-116946: Emit RuntimeError if a heap type does not implement traverse #116947
• gh-116946: fully implement GC protocol for zlib objects #138290
• [3.14] gh-116946: fully implement GC protocol for zlib objects (GH-138290) #138327
• [3.13] gh-116946: fully implement GC protocol for zlib objects (GH-138290) #138328
• gh-116946: fully implement GC protocol for _tkinter.Tk{app,tt}Object #138331
• gh-116946: fully implement GC protocol for _curses_panel.panel #138333
• [3.14] gh-116946: fully implement GC protocol for _curses_panel.panel (GH-138333) #138427
• [3.13] gh-116946: fully implement GC protocol for _curses_panel.panel (GH-138333) #138428
• gh-116946: fully implement GC protocol for _functools._lru_list_elem #138334
• gh-116946: add Py_TPFLAGS_IMMUTABLETYPE to select.[e]poll #138340
• gh-116946: add Py_TPFLAGS_IMMUTABLETYPE to _random.Random #138341
• gh-116946: add Py_TPFLAGS_IMMUTABLETYPE to several internal types #138582
• Revert "gh-116946: fully implement GC protocol for _tkinter.Tk{app,tt}Object (#138331)" #138807
• gh-116946: Remove gc-support from some immutable types #139073
• gh-116946: partial revert gh-139073 for thread handle type #139474
• GH-116946: Eliminate the need for the GC in the _thread.lock and _thread.RLock #141268
• GH-116946: revert eliminate the need for the GC in the _thread.lock and _thread.RLock  #141448
• gh-116946: Implement the GC protocol for _tkinter tkapp and tktimertoken #152310
• gh-116946: remove unnecessary gc from _decimal types #156034

The following PRs were directly closed because the types are immutable and empty:

• gh-116946: fully implement GC protocol for _thread._localdummy #138329
• gh-116946: fully implement GC protocol for _tokenize.TokenizerIter #138330
• gh-116946: fully implement GC protocol for _ssl.Certificate #138332

The following PRs were wrongly merged so we reverted them:

• gh-116946: fully implement GC protocol for bz2 objects #138266
• [3.14] gh-116946: fully implement GC protocol for bz2 objects (GH-138266) #138320
• [3.13] gh-116946: fully implement GC protocol for bz2 objects (GH-138266) #138322
• gh-116946: fully implement GC protocol for lzma objects #138288
• [3.14] gh-116946: fully implement GC protocol for lzma objects (GH-138288) #138321
• [3.13] gh-116946: fully implement GC protocol for lzma objects (GH-138288) #138323
• gh-116946: fully implement GC protocol for _hashlib objects #138289
• [3.14] gh-116946: fully implement GC protocol for _hashlib objects (GH-138289) #138324
• [3.13] gh-116946: fully implement GC protocol for _hashlib objects (GH-138289) #138326

PRs for reverting the above commits:

• gh-116946: Revert GC protocol for immutable empty heap types (GH-138266, GH-138288, GH-138289) #138338
• [3.13] gh-116946: Revert GC protocol for immutable empty heap types (GH-138322, GH-138323, GH-138326) #138337