fix: fix decimal.Context.status race in free-threading

Author: LindaSummer

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 +1,2 @@
+Fix a data race in :class:`decimal.Context` status flag updates in the
+:term:`free-threaded build`.

Modules/_decimal/_decimal.c

@@ -218,6 +218,7 @@ typedef struct {
 
 typedef struct PyDecContextObject {
     PyObject_HEAD
+    PyMutex ctx_lock;
     mpd_context_t ctx;
     PyObject *traps;
     PyObject *flags;
@@ -227,6 +228,7 @@ typedef struct PyDecContextObject {
 } PyDecContextObject;
 
 #define _PyDecContextObject_CAST(op)    ((PyDecContextObject *)(op))
+#define _PyDecContextObject_LOCKED_CAST(op)    ((PyDecContextObject *)(op))
 
 typedef struct {
     PyObject_HEAD
@@ -246,6 +248,7 @@ typedef struct {
 #define SdFlagAddr(v) (_PyDecSignalDictObject_CAST(v)->flags)
 #define SdFlags(v) (*_PyDecSignalDictObject_CAST(v)->flags)
 #define CTX(v) (&_PyDecContextObject_CAST(v)->ctx)
+#define CTX_LOCK(v) (&_PyDecContextObject_CAST(v)->ctx_lock)
 #define CtxCaps(v) (_PyDecContextObject_CAST(v)->capitals)
 
 static inline decimal_state *
@@ -611,9 +614,12 @@ static int
 dec_addstatus(PyObject *context, uint32_t status)
 {
     mpd_context_t *ctx = CTX(context);
+    PyMutex* ctx_lock = CTX_LOCK(context);
     decimal_state *state = get_module_state_from_ctx(context);
 
+    PyMutex_Lock(ctx_lock);
     ctx->status |= status;
+    PyMutex_Unlock(ctx_lock);
     if (status & (ctx->traps|MPD_Malloc_error)) {
         PyObject *ex, *siglist;
 
@@ -1418,7 +1424,10 @@ static PyObject *
 _decimal_Context_clear_flags_impl(PyObject *self)
 /*[clinic end generated code: output=c86719a70177d0b6 input=a06055e2f3e7edb1]*/
 {
+    PyMutex* ctx_lock = CTX_LOCK(self);
+    PyMutex_Lock(ctx_lock);
     CTX(self)->status = 0;
+    PyMutex_Unlock(ctx_lock);
     Py_RETURN_NONE;
 }
 
@@ -1437,6 +1446,7 @@ context_new(PyTypeObject *type,
 {
     PyDecContextObject *self = NULL;
     mpd_context_t *ctx;
+    PyMutex* ctx_lock;
 
     decimal_state *state = get_module_state_by_def(type);
     if (type == state->PyDecContext_Type) {
@@ -1449,6 +1459,7 @@ context_new(PyTypeObject *type,
     if (self == NULL) {
         return NULL;
     }
+    self->ctx_lock = (PyMutex){0};
 
     self->traps = PyObject_CallObject((PyObject *)state->PyDecSignalDict_Type, NULL);
     if (self->traps == NULL) {
@@ -1471,8 +1482,12 @@ context_new(PyTypeObject *type,
         *ctx = dflt_ctx;
     }
 
+    ctx_lock = CTX_LOCK(self);
+
     SdFlagAddr(self->traps) = &ctx->traps;
+    PyMutex_Lock(ctx_lock);
     SdFlagAddr(self->flags) = &ctx->status;
+    PyMutex_Unlock(ctx_lock);
 
     CtxCaps(self) = 1;
     self->tstate = NULL;
@@ -1556,6 +1571,7 @@ static PyObject *
 context_repr(PyObject *self)
 {
     mpd_context_t *ctx;
+    PyMutex* ctx_lock;
     char flags[MPD_MAX_SIGNAL_LIST];
     char traps[MPD_MAX_SIGNAL_LIST];
     int n, mem;
@@ -1566,8 +1582,13 @@ context_repr(PyObject *self)
 #endif
     ctx = CTX(self);
 
+    ctx_lock = CTX_LOCK(self);
+    PyMutex_Lock(ctx_lock);
+    uint32_t ctx_status = ctx->status;
+    PyMutex_Unlock(ctx_lock);
+
     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 +1615,7 @@ init_basic_context(PyObject *v)
     ctx.round = MPD_ROUND_HALF_UP;
 
     *CTX(v) = ctx;
+    *CTX_LOCK(v) = (PyMutex){0};
     CtxCaps(v) = 1;
 }
 
@@ -1606,6 +1628,7 @@ init_extended_context(PyObject *v)
     ctx.traps = 0;
 
     *CTX(v) = ctx;
+    *CTX_LOCK(v) = (PyMutex){0};
     CtxCaps(v) = 1;
 }
 
@@ -1715,11 +1738,16 @@ _decimal_Context___reduce___impl(PyObject *self, PyTypeObject *cls)
     PyObject *traps;
     PyObject *ret;
     mpd_context_t *ctx;
+    PyMutex* ctx_lock;
     decimal_state *state = PyType_GetModuleState(cls);
 
     ctx = CTX(self);
+    ctx_lock = CTX_LOCK(self);
 
-    flags = signals_as_list(state, ctx->status);
+    PyMutex_Lock(ctx_lock);
+    uint32_t ctx_status = ctx->status;
+    PyMutex_Unlock(ctx_lock);
+    flags = signals_as_list(state, ctx_status);
     if (flags == NULL) {
         return NULL;
     }
@@ -1917,11 +1945,17 @@ PyDec_SetCurrentContext(PyObject *self, PyObject *v)
 static PyObject *
 init_current_context(decimal_state *state)
 {
+    mpd_context_t* ctx;
+    PyMutex* ctx_lock;
     PyObject *tl_context = context_copy(state, state->default_context_template);
     if (tl_context == NULL) {
         return NULL;
     }
-    CTX(tl_context)->status = 0;
+    ctx = CTX(tl_context);
+    ctx_lock = CTX_LOCK(tl_context);
+    PyMutex_Lock(ctx_lock);
+    ctx->status = 0;
+    PyMutex_Unlock(ctx_lock);
 
     PyObject *tok = PyContextVar_Set(state->current_context_var, tl_context);
     if (tok == NULL) {
@@ -1982,7 +2016,11 @@ PyDec_SetCurrentContext(PyObject *self, PyObject *v)
         if (v == NULL) {
             return NULL;
         }
-        CTX(v)->status = 0;
+        mpd_context_t* ctx = CTX(v);
+        PyMutex* ctx_lock = CTX_LOCK(v);
+        PyMutex_Lock(ctx_lock);
+        ctx->status = 0;
+        PyMutex_Unlock(ctx_lock);
     }
     else {
         Py_INCREF(v);
@@ -3479,6 +3517,7 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
                int op, PyObject *context)
 {
     mpd_context_t *ctx = CTX(context);
+    PyMutex* ctx_lock = CTX_LOCK(context);
 
     *vcmp = v;
 
@@ -3495,7 +3534,9 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
             *wcmp = NULL;
         }
         else {
+            PyMutex_Lock(ctx_lock);
             ctx->status |= MPD_Float_operation;
+            PyMutex_Unlock(ctx_lock);
             *wcmp = PyDec_FromFloatExact(state, w, context);
         }
     }
@@ -3510,7 +3551,9 @@ convert_op_cmp(PyObject **vcmp, PyObject **wcmp, PyObject *v, PyObject *w,
                 *wcmp = NULL;
             }
             else {
+                PyMutex_Lock(ctx_lock);
                 ctx->status |= MPD_Float_operation;
+                PyMutex_Unlock(ctx_lock);
                 *wcmp = PyDec_FromFloatExact(state, tmp, context);
                 Py_DECREF(tmp);
             }