OdbBackend: acquire the GIL in the callbacks, fix write_cb's type argument - #1496
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The callbacks libgit2 calls for an OdbBackend implemented in Python called into Python without holding the GIL. That works when libgit2 is entered from the C extension, which keeps the GIL, but not from cffi, which releases it around every C call: Index.add on a repository with such a backend crashes the interpreter (access violation on Windows). Acquire the GIL with PyGILState_Ensure, like the filter callbacks do. Assisted-by: Claude Fable 5.1
The type was passed to PyObject_CallMethod with the "n" format, which reads a Py_ssize_t, but git_object_t is an int. On 64-bit Windows write_cb received garbage in the upper 32 bits, e.g. 2753074036739 (0x28100000003) instead of 3 for a blob. Assisted-by: Claude Fable 5.1
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With an
OdbBackendimplemented in Python,Index.add(path)crashes the interpreter (access violation on Windows, Python 3.13, pygit2 1.20.1), single-threaded.Cause: the odb backend callbacks in
src/odb_backend.ccall into Python without holding the GIL. That works when libgit2 is entered from the C extension, which keeps the GIL, but cffi releases it around every C call, sogit_index_add_bypathreaches the callbacks without it.Fix:
PyGILState_Ensure/PyGILState_Releasein each callback, the same pattern as insrc/filter.c. It is re-entrant, so it is fine when the caller already holds the GIL.Second commit:
write_cbreceived itstypeargument through thenformat (Py_ssize_t) thoughgit_object_tis an int; on 64-bit Windows it got e.g. 2753074036739 instead of 3 for a blob. It is now passed asi.Tests:
test_index_add(run in a subprocess, since the failure is a crash) andtest_write_cb. Both fail before and pass after, on Windows x64 with MSVC, CPython 3.13 and libgit2 1.9.7; the full suite passes with the first commit.Not done here:
src/refdb_backend.chas the same missing GIL. Possibly related: #1004.This PR (analysis, patch, tests and this description) was prepared by Claude Fable 5.1, working for @cboos.
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