gh-146302: make Py_IsInitialized() thread-safe and reflect true init completion

Py_IsInitialized() previously returned 1 before Py_InitializeEx() had
finished, because the runtime flag was set before site.py was imported.
This caused a race in embedders like PyO3 where a second thread could
observe an initialized interpreter before sys.path was fully configured.

Move the initialized=1 store to the end of init_interp_main(), after
site import, lazy imports, tier 2 optimizer, and dict watcher setup.

Access both `initialized` and `core_initialized` through new inline
accessors that use acquire/release atomics, eliminating the C-standard
data race and ensuring correct visibility on weakly-ordered
architectures.

Fix PySys_AddAuditHook() to check core_initialized (not initialized)
when deciding whether a thread state is available, since tstate exists
after core init completes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gpshead

Doc/c-api/interp-lifecycle.rst

@@ -410,6 +410,11 @@ Initializing and finalizing the interpreter
    (zero) if not.  After :c:func:`Py_FinalizeEx` is called, this returns false until
    :c:func:`Py_Initialize` is called again.
 
+   .. versionchanged:: next
+      This function no longer returns true until initialization has fully
+      completed, including import of the :mod:`site` module.  Previously it
+      could return true while :c:func:`Py_Initialize` was still running.
+
 
 .. c:function:: int Py_IsFinalizing()
 

Include/internal/pycore_runtime.h

@@ -56,6 +56,29 @@ _PyRuntimeState_SetFinalizing(_PyRuntimeState *runtime, PyThreadState *tstate) {
     }
 }
 
+// Use acquire/release (not relaxed) because these are publication flags:
+// when a reader sees 1, all prior initialization writes must be visible.
+
+static inline int
+_PyRuntimeState_GetCoreInitialized(_PyRuntimeState *runtime) {
+    return _Py_atomic_load_int_acquire(&runtime->core_initialized);
+}
+
+static inline void
+_PyRuntimeState_SetCoreInitialized(_PyRuntimeState *runtime, int initialized) {
+    _Py_atomic_store_int_release(&runtime->core_initialized, initialized);
+}
+
+static inline int
+_PyRuntimeState_GetInitialized(_PyRuntimeState *runtime) {
+    return _Py_atomic_load_int_acquire(&runtime->initialized);
+}
+
+static inline void
+_PyRuntimeState_SetInitialized(_PyRuntimeState *runtime, int initialized) {
+    _Py_atomic_store_int_release(&runtime->initialized, initialized);
+}
+
 
 #ifdef __cplusplus
 }

Include/internal/pycore_runtime_structs.h

@@ -158,10 +158,18 @@ struct pyruntimestate {
     /* Is Python preinitialized? Set to 1 by Py_PreInitialize() */
     int preinitialized;
 
-    /* Is Python core initialized? Set to 1 by _Py_InitializeCore() */
+    /* Is Python core initialized? Set to 1 by _Py_InitializeCore().
+
+       Use _PyRuntimeState_GetCoreInitialized() and
+       _PyRuntimeState_SetCoreInitialized() to access it,
+       don't access it directly. */
     int core_initialized;
 
-    /* Is Python fully initialized? Set to 1 by Py_Initialize() */
+    /* Is Python fully initialized? Set to 1 by Py_Initialize().
+
+       Use _PyRuntimeState_GetInitialized() and
+       _PyRuntimeState_SetInitialized() to access it,
+       don't access it directly. */
     int initialized;
 
     /* Set by Py_FinalizeEx(). Only reset to NULL if Py_Initialize()

Misc/NEWS.d/next/C_API/2026-03-22-00-00-00.gh-issue-146302.PyIsInit.rst

@@ -0,0 +1,3 @@
+:c:func:`Py_IsInitialized` no longer returns true until initialization has
+fully completed, including import of the :mod:`site` module.  The underlying
+runtime flags now use atomic operations with acquire/release memory ordering.

Python/preconfig.c

@@ -928,7 +928,7 @@ _PyPreConfig_Write(const PyPreConfig *src_config)
         return status;
     }
 
-    if (_PyRuntime.core_initialized) {
+    if (_PyRuntimeState_GetCoreInitialized(&_PyRuntime)) {
         /* bpo-34008: Calling this functions after Py_Initialize() ignores
            the new configuration. */
         return _PyStatus_OK();

Python/pylifecycle.c

@@ -166,13 +166,13 @@ int (*_PyOS_mystrnicmp_hack)(const char *, const char *, Py_ssize_t) = \
 int
 _Py_IsCoreInitialized(void)
 {
-    return _PyRuntime.core_initialized;
+    return _PyRuntimeState_GetCoreInitialized(&_PyRuntime);
 }
 
 int
 Py_IsInitialized(void)
 {
-    return _PyRuntime.initialized;
+    return _PyRuntimeState_GetInitialized(&_PyRuntime);
 }
 
 
@@ -491,7 +491,7 @@ static PyStatus
 pycore_init_runtime(_PyRuntimeState *runtime,
                     const PyConfig *config)
 {
-    if (runtime->initialized) {
+    if (_PyRuntimeState_GetInitialized(runtime)) {
         return _PyStatus_ERR("main interpreter already initialized");
     }
 
@@ -984,7 +984,7 @@ pyinit_config(_PyRuntimeState *runtime,
     }
 
     /* Only when we get here is the runtime core fully initialized */
-    runtime->core_initialized = 1;
+    _PyRuntimeState_SetCoreInitialized(runtime, 1);
     return _PyStatus_OK();
 }
 
@@ -1217,7 +1217,7 @@ init_interp_main(PyThreadState *tstate)
          * or pure Python code in the standard library won't work.
          */
         if (is_main_interp) {
-            interp->runtime->initialized = 1;
+            _PyRuntimeState_SetInitialized(interp->runtime, 1);
         }
         return _PyStatus_OK();
     }
@@ -1329,8 +1329,6 @@ init_interp_main(PyThreadState *tstate)
             Py_XDECREF(warnings_module);
         }
         Py_XDECREF(warnoptions);
-
-        interp->runtime->initialized = 1;
     }
 
     if (config->site_import) {
@@ -1426,6 +1424,10 @@ init_interp_main(PyThreadState *tstate)
 
     assert(!_PyErr_Occurred(tstate));
 
+    if (is_main_interp) {
+        _PyRuntimeState_SetInitialized(interp->runtime, 1);
+    }
+
     return _PyStatus_OK();
 }
 
@@ -1445,11 +1447,11 @@ static PyStatus
 pyinit_main(PyThreadState *tstate)
 {
     PyInterpreterState *interp = tstate->interp;
-    if (!interp->runtime->core_initialized) {
+    if (!_PyRuntimeState_GetCoreInitialized(interp->runtime)) {
         return _PyStatus_ERR("runtime core not initialized");
     }
 
-    if (interp->runtime->initialized) {
+    if (_PyRuntimeState_GetInitialized(interp->runtime)) {
         return pyinit_main_reconfigure(tstate);
     }
 
@@ -1505,7 +1507,7 @@ Py_InitializeEx(int install_sigs)
     }
     _PyRuntimeState *runtime = &_PyRuntime;
 
-    if (runtime->initialized) {
+    if (_PyRuntimeState_GetInitialized(runtime)) {
         /* bpo-33932: Calling Py_Initialize() twice does nothing. */
         return;
     }
@@ -2210,7 +2212,7 @@ _Py_Finalize(_PyRuntimeState *runtime)
     int status = 0;
 
     /* Bail out early if already finalized (or never initialized). */
-    if (!runtime->initialized) {
+    if (!_PyRuntimeState_GetInitialized(runtime)) {
         return status;
     }
 
@@ -2245,8 +2247,8 @@ _Py_Finalize(_PyRuntimeState *runtime)
        when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
     _PyInterpreterState_SetFinalizing(tstate->interp, tstate);
     _PyRuntimeState_SetFinalizing(runtime, tstate);
-    runtime->initialized = 0;
-    runtime->core_initialized = 0;
+    _PyRuntimeState_SetInitialized(runtime, 0);
+    _PyRuntimeState_SetCoreInitialized(runtime, 0);
 
     // XXX Call something like _PyImport_Disable() here?
 
@@ -2472,7 +2474,7 @@ new_interpreter(PyThreadState **tstate_p,
     }
     _PyRuntimeState *runtime = &_PyRuntime;
 
-    if (!runtime->initialized) {
+    if (!_PyRuntimeState_GetInitialized(runtime)) {
         return _PyStatus_ERR("Py_Initialize must be called first");
     }
 
@@ -3312,10 +3314,10 @@ fatal_error_dump_runtime(int fd, _PyRuntimeState *runtime)
         _Py_DumpHexadecimal(fd, (uintptr_t)finalizing, sizeof(finalizing) * 2);
         PUTS(fd, ")");
     }
-    else if (runtime->initialized) {
+    else if (_PyRuntimeState_GetInitialized(runtime)) {
         PUTS(fd, "initialized");
     }
-    else if (runtime->core_initialized) {
+    else if (_PyRuntimeState_GetCoreInitialized(runtime)) {
         PUTS(fd, "core initialized");
     }
     else if (runtime->preinitialized) {

Python/pystate.c

@@ -330,8 +330,8 @@ init_runtime(_PyRuntimeState *runtime,
 {
     assert(!runtime->preinitializing);
     assert(!runtime->preinitialized);
-    assert(!runtime->core_initialized);
-    assert(!runtime->initialized);
+    assert(!_PyRuntimeState_GetCoreInitialized(runtime));
+    assert(!_PyRuntimeState_GetInitialized(runtime));
     assert(!runtime->_initialized);
 
     runtime->open_code_hook = open_code_hook;

Python/sysmodule.c

@@ -33,6 +33,7 @@ Data members:
 #include "pycore_pymath.h"        // _PY_SHORT_FLOAT_REPR
 #include "pycore_pymem.h"         // _PyMem_DefaultRawFree()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_runtime.h"       // _PyRuntimeState_GetCoreInitialized()
 #include "pycore_pystats.h"       // _Py_PrintSpecializationStats()
 #include "pycore_structseq.h"     // _PyStructSequence_InitBuiltinWithFlags()
 #include "pycore_sysmodule.h"     // export _PySys_GetSizeOf()
@@ -471,7 +472,7 @@ PySys_AddAuditHook(Py_AuditHookFunction hook, void *userData)
        PySys_AddAuditHook() can be called before Python is initialized. */
     _PyRuntimeState *runtime = &_PyRuntime;
     PyThreadState *tstate;
-    if (runtime->initialized) {
+    if (_PyRuntimeState_GetCoreInitialized(runtime)) {
         tstate = _PyThreadState_GET();
     }
     else {