René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Data race in test_ssl.test_sni_callback_race","articleBody":"# Bug report\n\nThere is a sporadic thread sanitizer reported data race in a newly added test\n\n* GH-150018\n\nFor example:\n\n* https://github.com/python/cpython/actions/runs/26217633343/job/77144045276\n\nHere is a summary from Claude with my edits:\n\n-------\n\nOn the free-threaded TSAN build, `test.test_ssl.ContextTests.test_sni_callback_race` sporadically reports a data race inside OpenSSL: one thread reads an ASN.1 string via `ASN1_STRING_cmp` (no lock held) while another writes the same heap block via `ASN1_STRING_set` (holding an internal `CRYPTO_THREAD_lock` rwlock). The test itself does not crash.\n\n### Reproducer\n\n```bash\nCC=clang-20 ./configure -C --disable-gil --with-pydebug --with-thread-sanitizer\nmake -j\n\nfor i in $(seq 1 30); do\n  TSAN_OPTIONS=\"halt_on_error=1\" \\\n    ./python -m test test_ssl -v -m test_sni_callback_race \u003e /tmp/run_$i.log 2\u003e\u00261\n  [ $? -ne 0 ] \u0026\u0026 echo \"FAILED on run $i\" \u0026\u0026 break\ndone\n```\n\nReproduces within ~15 runs on a 22-core machine.\n\n### TSAN report (abridged)\n\n```\nWARNING: ThreadSanitizer: data race\n\n  Read of size 8 by thread T11:\n    #0 memcmp\n    #1 ASN1_STRING_cmp           (libcrypto.so.3)\n    ...\n    #22 thread_run               Modules/_threadmodule.c:388\n\n  Previous write of size 8 by thread T12 (mutexes: write M0):\n    #0 memcpy\n    #1 ASN1_STRING_set           (libcrypto.so.3)\n    ...\n    #22 thread_run               Modules/_threadmodule.c:388\n\n  Location is heap block of size 21 allocated by ASN1_STRING_set\n  Mutex M0 created by CRYPTO_THREAD_lock_new (libcrypto.so.3)\n\nSUMMARY: ThreadSanitizer: data race in memcmp\n```\n\nThe writer holds an OpenSSL-owned rwlock; the reader does not take the same lock. Both call sites enter from two Python worker threads doing concurrent SSL handshakes on the same `SSLContext`.\n\n## Environment\n\n- CPython `main` @ c35b0f2b624 (3.16.0a0, free-threading debug TSAN)\n- Clang 20.1.8\n- OpenSSL 3.0.13 (30 Jan 2024)\n- Linux 6.8.0-101 x86_64\n\ncc @kiri11 @encukou \n\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-150193\n* gh-153269\n* gh-153270\n* gh-153320\n* gh-153349\n* gh-153350\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/colesbury","@type":"Person","name":"colesbury"},"datePublished":"2026-05-21T14:34:05.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/150191/cpython/issues/150191"}