GH-149142: Fix decimal mpd_context_t.status race in free-threading

Lib/test/test_free_threading/test_decimal.py

@@ -0,0 +1,32 @@
+import unittest
+
+from test.support import threading_helper
+
+import decimal
+
+N_THREADS = 8
+ITERATIONS = 100_000
+
+@threading_helper.requires_working_threading()
+class TestDecimal(unittest.TestCase):
+    def test_add_status(self):
+        # prec=4 makes "1.23456" Inexact|Rounded
+        shared_ctx = decimal.Context(prec=4)
+        def worker():
+            for _ in range(ITERATIONS):
+                shared_ctx.create_decimal("1.23456")
+        threading_helper.run_concurrently(worker, N_THREADS)
+
+
+    def test_clear_flags(self):
+        shared_ctx = decimal.Context(prec=4)
+
+        def producer():
+            for _ in range(ITERATIONS):
+                shared_ctx.create_decimal("1.23456")
+
+        def clearer():
+            for _ in range(ITERATIONS):
+                shared_ctx.clear_flags()
+
+        threading_helper.run_concurrently([producer]*N_THREADS + [clearer]*N_THREADS)

Misc/NEWS.d/next/Library/2026-05-30-00-08-49.gh-issue-149142.zWeH0z.rst

@@ -0,0 +1,3 @@
+Fix data races in :meth:`decimal.Context.create_decimal` and
+:meth:`decimal.Context.clear_flags` when updating
+:class:`decimal.Context` status flags in the :term:`free-threaded build`.

Modules/_decimal/_decimal.c

@@ -218,6 +218,9 @@ typedef struct {
 
 typedef struct PyDecContextObject {
     PyObject_HEAD
+#ifdef Py_GIL_DISABLED
+    PyMutex ctx_lock;
+#endif
     mpd_context_t ctx;
     PyObject *traps;
     PyObject *flags;
@@ -248,6 +251,16 @@ typedef struct {
 #define CTX(v) (&_PyDecContextObject_CAST(v)->ctx)
 #define CtxCaps(v) (_PyDecContextObject_CAST(v)->capitals)
 
+#ifdef Py_GIL_DISABLED
+#define CTX_LOCK_INIT(v) _PyDecContextObject_CAST(v)->ctx_lock = (PyMutex){0}
+#define CTX_LOCK(v) PyMutex_Lock(&_PyDecContextObject_CAST(v)->ctx_lock)
+#define CTX_UNLOCK(v) PyMutex_Unlock(&_PyDecContextObject_CAST(v)->ctx_lock)
+#else
+#define CTX_LOCK_INIT(v) ((void)0)
+#define CTX_LOCK(v) ((void)0)
+#define CTX_UNLOCK(v) ((void)0)
+#endif
+
 static inline decimal_state *
 get_module_state_from_ctx(PyObject *v)
 {
@@ -613,7 +626,9 @@ dec_addstatus(PyObject *context, uint32_t status)
     mpd_context_t *ctx = CTX(context);
     decimal_state *state = get_module_state_from_ctx(context);
 
+    CTX_LOCK(context);
     ctx->status |= status;
+    CTX_UNLOCK(context);
     if (status & (ctx->traps|MPD_Malloc_error)) {
         PyObject *ex, *siglist;
 
@@ -1418,7 +1433,9 @@ static PyObject *
 _decimal_Context_clear_flags_impl(PyObject *self)
 /*[clinic end generated code: output=c86719a70177d0b6 input=a06055e2f3e7edb1]*/
 {
+    CTX_LOCK(self);
     CTX(self)->status = 0;
+    CTX_UNLOCK(self);
     Py_RETURN_NONE;
 }
 
@@ -1449,6 +1466,7 @@ context_new(PyTypeObject *type,
     if (self == NULL) {
         return NULL;
     }
+    CTX_LOCK_INIT(self);
 
     self->traps = PyObject_CallObject((PyObject *)state->PyDecSignalDict_Type, NULL);
     if (self->traps == NULL) {
@@ -1566,8 +1584,12 @@ context_repr(PyObject *self)
 #endif
     ctx = CTX(self);
 
+    CTX_LOCK(self);
+    uint32_t ctx_status = ctx->status;
+    CTX_UNLOCK(self);
+
     mem = MPD_MAX_SIGNAL_LIST;
-    n = mpd_lsnprint_signals(flags, mem, ctx->status, dec_signal_string);
+    n = mpd_lsnprint_signals(flags, mem, ctx_status, dec_signal_string);
     if (n < 0 || n >= mem) {
         INTERNAL_ERROR_PTR("context_repr");
     }
@@ -1594,6 +1616,7 @@ init_basic_context(PyObject *v)
     ctx.round = MPD_ROUND_HALF_UP;
 
     *CTX(v) = ctx;
+    CTX_LOCK_INIT(v);
     CtxCaps(v) = 1;
 }
 
@@ -1606,6 +1629,7 @@ init_extended_context(PyObject *v)
     ctx.traps = 0;
 
     *CTX(v) = ctx;
+    CTX_LOCK_INIT(v);
     CtxCaps(v) = 1;
 }
 
@@ -1719,7 +1743,11 @@ _decimal_Context___reduce___impl(PyObject *self, PyTypeObject *cls)
 
     ctx = CTX(self);
 
-    flags = signals_as_list(state, ctx->status);
+    CTX_LOCK(self);
+    uint32_t ctx_status = ctx->status;
+    CTX_UNLOCK(self);
+
+    flags = signals_as_list(state, ctx_status);
     if (flags == NULL) {
         return NULL;
     }
@@ -3495,7 +3523,9 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
             *wcmp = NULL;
         }
         else {
+            CTX_LOCK(context);
             ctx->status |= MPD_Float_operation;
+            CTX_UNLOCK(context);
             *wcmp = PyDec_FromFloatExact(state, w, context);
         }
     }
@@ -3510,7 +3540,9 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
                 *wcmp = NULL;
             }
             else {
+                CTX_LOCK(context);
                 ctx->status |= MPD_Float_operation;
+                CTX_UNLOCK(context);
                 *wcmp = PyDec_FromFloatExact(state, tmp, context);
                 Py_DECREF(tmp);
             }