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gh-116738: make entering of contextvars.Context thread safe #143074

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gh-116738: make entering of contextvars.Context thread safe #143074

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68f8ec6
make entering of context thread safe
kumaraditya303 Dec 21, 2025
7fb7931
📜🤖 Added by blurb_it.
blurb-it[bot] Dec 22, 2025
cc959da
code review: make exiting thread safe
kumaraditya303 Dec 22, 2025
6a3e53c
Merge branch 'context' of https://github.com/kumaraditya303/cpython i…
kumaraditya303 Dec 22, 2025
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-12-22-16-22-02.gh-issue-116738.caQuq_.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix thread safety of :func:`contextvars.Context.run`.
15 changes: 10 additions & 5 deletions Python/context.c
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Expand Up @@ -195,16 +195,20 @@ _PyContext_Enter(PyThreadState *ts, PyObject *octx)
{
ENSURE_Context(octx, -1)
PyContext *ctx = (PyContext *)octx;
#ifdef Py_GIL_DISABLED
int already_entered = _Py_atomic_exchange_int(&ctx->ctx_entered, 1);
#else
int already_entered = ctx->ctx_entered;
ctx->ctx_entered = 1;
#endif

if (ctx->ctx_entered) {
if (already_entered) {
_PyErr_Format(ts, PyExc_RuntimeError,
"cannot enter context: %R is already entered", ctx);
return -1;
}

ctx->ctx_prev = (PyContext *)ts->context; /* borrow */
ctx->ctx_entered = 1;

ts->context = Py_NewRef(ctx);
context_switched(ts);
return 0;
Expand All @@ -225,8 +229,9 @@ _PyContext_Exit(PyThreadState *ts, PyObject *octx)
{
ENSURE_Context(octx, -1)
PyContext *ctx = (PyContext *)octx;
int already_entered = FT_ATOMIC_LOAD_INT_RELAXED(ctx->ctx_entered);

if (!ctx->ctx_entered) {
if (!already_entered) {
PyErr_Format(PyExc_RuntimeError,
"cannot exit context: %R has not been entered", ctx);
return -1;
Expand All @@ -243,7 +248,7 @@ _PyContext_Exit(PyThreadState *ts, PyObject *octx)
Py_SETREF(ts->context, (PyObject *)ctx->ctx_prev);

ctx->ctx_prev = NULL;
ctx->ctx_entered = 0;
FT_ATOMIC_STORE_INT(ctx->ctx_entered, 0);
context_switched(ts);
return 0;
}
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