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gh-111956: Add thread-safe one-time initialization. (gh-111960)
1 parent b6dcbed commit 1669481

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Lines changed: 1061 additions & 955 deletions

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‎Include/internal/pycore_ast_state.h‎

Lines changed: 5 additions & 1 deletion
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‎Include/internal/pycore_lock.h‎

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Original file line numberDiff line numberDiff line change
@@ -46,6 +46,7 @@ typedef struct _PyMutex PyMutex;
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#define _Py_UNLOCKED 0
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#define _Py_LOCKED 1
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#define _Py_HAS_PARKED 2
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#define _Py_ONCE_INITIALIZED 4
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// (private) slow path for locking the mutex
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PyAPI_FUNC(void) _PyMutex_LockSlow(PyMutex *m);
@@ -166,6 +167,35 @@ _PyRawMutex_Unlock(_PyRawMutex *m)
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_PyRawMutex_UnlockSlow(m);
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}
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// A data structure that can be used to run initialization code once in a
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// thread-safe manner. The C++11 equivalent is std::call_once.
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typedef struct {
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uint8_t v;
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} _PyOnceFlag;
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// Type signature for one-time initialization functions. The function should
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// return 0 on success and -1 on failure.
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typedef int _Py_once_fn_t(void *arg);
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// (private) slow path for one time initialization
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PyAPI_FUNC(int)
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_PyOnceFlag_CallOnceSlow(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg);
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// Calls `fn` once using `flag`. The `arg` is passed to the call to `fn`.
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//
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// Returns 0 on success and -1 on failure.
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//
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// If `fn` returns 0 (success), then subsequent calls immediately return 0.
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// If `fn` returns -1 (failure), then subsequent calls will retry the call.
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static inline int
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_PyOnceFlag_CallOnce(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg)
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{
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if (_Py_atomic_load_uint8(&flag->v) == _Py_ONCE_INITIALIZED) {
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return 0;
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}
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return _PyOnceFlag_CallOnceSlow(flag, fn, arg);
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}
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#ifdef __cplusplus
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}
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#endif

‎Include/internal/pycore_modsupport.h‎

Lines changed: 9 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,8 @@
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#ifndef Py_INTERNAL_MODSUPPORT_H
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#define Py_INTERNAL_MODSUPPORT_H
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#include "pycore_lock.h" // _PyOnceFlag
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36
#ifdef __cplusplus
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extern "C" {
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#endif
@@ -65,15 +68,16 @@ PyAPI_FUNC(void) _PyArg_BadArgument(
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// --- _PyArg_Parser API ---------------------------------------------------
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typedef struct _PyArg_Parser {
68-
int initialized;
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const char *format;
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const char * const *keywords;
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const char *fname;
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const char *custom_msg;
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int pos; /* number of positional-only arguments */
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int min; /* minimal number of arguments */
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int max; /* maximal number of positional arguments */
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PyObject *kwtuple; /* tuple of keyword parameter names */
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_PyOnceFlag once; /* atomic one-time initialization flag */
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int is_kwtuple_owned; /* does this parser own the kwtuple object? */
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int pos; /* number of positional-only arguments */
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int min; /* minimal number of arguments */
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int max; /* maximal number of positional arguments */
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PyObject *kwtuple; /* tuple of keyword parameter names */
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struct _PyArg_Parser *next;
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} _PyArg_Parser;
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‎Include/internal/pycore_runtime.h‎

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -27,7 +27,6 @@ extern "C" {
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#include "pycore_unicodeobject.h" // struct _Py_unicode_runtime_state
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struct _getargs_runtime_state {
30-
PyThread_type_lock mutex;
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struct _PyArg_Parser *static_parsers;
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};
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‎Misc/NEWS.d/next/C API/2023-11-10-10-24-28.gh-issue-111956.ImE6Cx.rst‎

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
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Add internal-only one-time initialization API: ``_PyOnceFlag`` and
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``_PyOnceFlag_CallOnce``.

‎Modules/_testinternalcapi/test_lock.c‎

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Original file line numberDiff line numberDiff line change
@@ -341,13 +341,45 @@ test_lock_benchmark(PyObject *module, PyObject *obj)
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Py_RETURN_NONE;
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}
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static int
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init_maybe_fail(void *arg)
346+
{
347+
int *counter = (int *)arg;
348+
(*counter)++;
349+
if (*counter < 5) {
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// failure
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return -1;
352+
}
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assert(*counter == 5);
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return 0;
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}
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static PyObject *
358+
test_lock_once(PyObject *self, PyObject *obj)
359+
{
360+
_PyOnceFlag once = {0};
361+
int counter = 0;
362+
for (int i = 0; i < 10; i++) {
363+
int res = _PyOnceFlag_CallOnce(&once, init_maybe_fail, &counter);
364+
if (i < 4) {
365+
assert(res == -1);
366+
}
367+
else {
368+
assert(res == 0);
369+
assert(counter == 5);
370+
}
371+
}
372+
Py_RETURN_NONE;
373+
}
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344375
static PyMethodDef test_methods[] = {
345376
{"test_lock_basic", test_lock_basic, METH_NOARGS},
346377
{"test_lock_two_threads", test_lock_two_threads, METH_NOARGS},
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{"test_lock_counter", test_lock_counter, METH_NOARGS},
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{"test_lock_counter_slow", test_lock_counter_slow, METH_NOARGS},
349380
_TESTINTERNALCAPI_BENCHMARK_LOCKS_METHODDEF
350381
{"test_lock_benchmark", test_lock_benchmark, METH_NOARGS},
382+
{"test_lock_once", test_lock_once, METH_NOARGS},
351383
{NULL, NULL} /* sentinel */
352384
};
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