René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"The REPL hangs under low memory on main","articleBody":"# Bug report\n\n### Bug description:\n\nIts kind of the same issue like https://github.com/python/cpython/issues/134163\nso cc @ZeroIntensity @picnixz \n\nbut the root cause is very different \n\n\n```python\nimport _testcapi\n_testcapi.set_nomemory(100) # or 0 or 1000\n```\nit will print the error and hang forever\n\n```py\n[hyi@rocky cpython]$ ./python \nPython 3.15.0a0 (heads/main:8e0bf4fe5d7, Oct 14 2025, 14:01:02) [GCC 11.5.0 20240719 (Red Hat 11.5.0-5)] on linux\nType \"help\", \"copyright\", \"credits\" or \"license\" for more information.\n\u003e\u003e\u003e import _testcapi\n\u003e\u003e\u003e _testcapi.set_nomemory(0)\nobject address  : 0xffff9c12dea0\nobject refcount : 3\nobject type     : 0x927c48\nobject type name: MemoryError\nobject repr     : \nlost sys.stderr\n# hang here\n```\n\nthe root cause is easy but the fix maybe hard and need to discuss so only issue\nand digging here no patch\n\nsome dig:\n\n- it only happens in main 3.15 and recent commits\n- it happends on all system\n- both old and default REPL hava it\n- its hard to reproduce without repl\n\nthe root cause can check bt\n\n```bt\n➜  ~  lldb -p 23325\n(lldb) process attach --pid 23325\nwarning: python.exe was compiled with optimization - stepping may behave oddly; variables may not be available.\nProcess 23325 stopped\n* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP\n    frame #0: 0x0000000100bac6c4 python.exe`BaseException_clear(op=0x00000001011c1e40) at exceptions.c:135:5 [opt]\n   132 \tBaseException_clear(PyObject *op)\n   133 \t{\n   134 \t   PyBaseExceptionObject *self = PyBaseExceptionObject_CAST(op);\n-\u003e 135 \t   Py_CLEAR(self-\u003edict);\n   136 \t   Py_CLEAR(self-\u003eargs);\n   137 \t   Py_CLEAR(self-\u003enotes);\n   138 \t   Py_CLEAR(self-\u003etraceback);\nTarget 0: (python.exe) stopped.\nExecutable binary set to \"/Users/hyi/prs/cpython/python.exe\".\nArchitecture set to: arm64-apple-macosx-.\n(lldb) bt\n* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP\n  * frame #0: 0x0000000100bac6c4 python.exe`BaseException_clear(op=0x00000001011c1e40) at exceptions.c:135:5 [opt]\n    frame #1: 0x0000000100bac58c python.exe`MemoryError_dealloc(op=0x00000001011c1e40) at exceptions.c:4099:11 [opt]\n    frame #2: 0x0000000100bf8fac python.exe`_Py_Dealloc(op=0x00000001011c1e40) at object.c:3200:5 [opt]\n    frame #3: 0x0000000100d0c9ac python.exe`format_unraisable_v [inlined] Py_DECREF(op=0x00000001011c1e40) at refcount.h:418:9 [opt]\n    frame #4: 0x0000000100d0c9a4 python.exe`format_unraisable_v [inlined] Py_XDECREF(op=0x00000001011c1e40) at refcount.h:511:9 [opt]\n    frame #5: 0x0000000100d0c9a4 python.exe`format_unraisable_v(format=\u003cunavailable\u003e, va=\"8X\\xa2\\U00000001\\U00000001\", obj=0x0000000000000000) at errors.c:1746:5 [opt]\n    frame #6: 0x0000000100d0c428 python.exe`PyErr_FormatUnraisable(format=\u003cunavailable\u003e) at errors.c:1769:5 [opt]\n    frame #7: 0x0000000100d8017c python.exe`atexit_callfuncs(state=\u003cunavailable\u003e) at atexitmodule.c:113:9 [opt] [artificial]\n    frame #8: 0x0000000100d80070 python.exe`_PyAtExit_Call(interp=\u003cunavailable\u003e) at atexitmodule.c:151:5 [opt] [artificial]\n    frame #9: 0x0000000100d471b0 python.exe`make_pre_finalization_calls(tstate=0x0000000100ff8bc0, subinterpreters=1) at pylifecycle.c:2108:9 [opt]\n    frame #10: 0x0000000100d468dc python.exe`_Py_Finalize(runtime=\u003cunavailable\u003e) at pylifecycle.c:2167:5 [opt]\n    frame #11: 0x0000000100d46828 python.exe`Py_FinalizeEx at pylifecycle.c:2378:12 [opt] [artificial]\n    frame #12: 0x0000000100d76854 python.exe`Py_RunMain at main.c:774:9 [opt]\n    frame #13: 0x0000000100d76fec python.exe`pymain_main(args=0x000000016f2b6140) at main.c:802:12 [opt]\n    frame #14: 0x0000000100d7708c python.exe`Py_BytesMain(argc=\u003cunavailable\u003e, argv=\u003cunavailable\u003e) at main.c:826:12 [opt]\n    frame #15: 0x000000019a2e5d54 dyld`start + 7184\n(lldb) \n```\n\n- with the perf can check the fucntion\n```bt\n[hyi@rocky ~]$ perf annotate -l --stdio --symbol PyErr_FormatUnraisable\n\nSorted summary for file /home/hyi/cpython/python\n----------------------------------------------\n\n   55.56 errors.c:1765\n   29.63 errors.c:1771\n   14.81 errors.c:1768\n Percent |      Source code \u0026 Disassembly of python for task-clock:upppH (27 samples, percent: local period)\n------------------------------------------------------------------------------------------------------------\n         :\n         :\n         :\n         : 3      Disassembly of section .text:\n         :\n         : 5      0000000000614ac0 \u003cPyErr_FormatUnraisable\u003e:\n         : 6      _PyErr_Clear(tstate); /* Just in case */\n         : 7      }\n         :\n         : 9      void\n         : 10     PyErr_FormatUnraisable(const char *format, ...)\n         : 11     {\n    0.00 :   614ac0: stp     x29, x30, [sp, #-272]!\n         : 13     va_list va;\n         :\n         : 15     va_start(va, format);\n    0.00 :   614ac4: mov     w9, #0xffffffc8                 // #-56\n    0.00 :   614ac8: mov     w8, #0xffffff80                 // #-128\n         : 18     {\n    0.00 :   614acc: mov     x29, sp\n         : 20     va_start(va, format);\n    0.00 :   614ad0: add     x10, sp, #0xd0\n    0.00 :   614ad4: add     x11, sp, #0x110\n```\n\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-140103\n* gh-140161\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/yihong0618","@type":"Person","name":"yihong0618"},"datePublished":"2025-10-14T07:25:05.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":9},"url":"https://github.com/140080/cpython/issues/140080"}