gh-123681: Check NORMALIZE_CENTURY behavior at runtime; require C99 (GH-136022)

A runtime check is needed to support cross-compiling.

Remove the _Py_NORMALIZE_CENTURY macro.
Remove _pydatetime.py's _can_support_c99.

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: encukou

Lib/_pydatetime.py

@@ -213,17 +213,6 @@ def _need_normalize_century():
             _normalize_century = True
     return _normalize_century
 
-_supports_c99 = None
-def _can_support_c99():
-    global _supports_c99
-    if _supports_c99 is None:
-        try:
-            _supports_c99 = (
-                _time.strftime("%F", (1900, 1, 1, 0, 0, 0, 0, 1, 0)) == "1900-01-01")
-        except ValueError:
-            _supports_c99 = False
-    return _supports_c99
-
 # Correctly substitute for %z and %Z escapes in strftime formats.
 def _wrap_strftime(object, format, timetuple):
     # Don't call utcoffset() or tzname() unless actually needed.
@@ -283,7 +272,7 @@ def _wrap_strftime(object, format, timetuple):
                     newformat.append(Zreplace)
                 # Note that datetime(1000, 1, 1).strftime('%G') == '1000' so
                 # year 1000 for %G can go on the fast path.
-                elif ((ch in 'YG' or ch in 'FC' and _can_support_c99()) and
+                elif ((ch in 'YG' or ch in 'FC') and
                         object.year < 1000 and _need_normalize_century()):
                     if ch == 'G':
                         year = int(_time.strftime("%G", timetuple))

Lib/test/datetimetester.py

@@ -1807,7 +1807,7 @@ def test_bool(self):
         self.assertTrue(self.theclass.min)
         self.assertTrue(self.theclass.max)
 
-    def test_strftime_y2k(self):
+    def check_strftime_y2k(self, specifier):
         # Test that years less than 1000 are 0-padded; note that the beginning
         # of an ISO 8601 year may fall in an ISO week of the year before, and
         # therefore needs an offset of -1 when formatting with '%G'.
@@ -1821,22 +1821,28 @@ def test_strftime_y2k(self):
             (1000, 0),
             (1970, 0),
         )
-        specifiers = 'YG'
-        if _time.strftime('%F', (1900, 1, 1, 0, 0, 0, 0, 1, 0)) == '1900-01-01':
-            specifiers += 'FC'
         for year, g_offset in dataset:
-            for specifier in specifiers:
-                with self.subTest(year=year, specifier=specifier):
-                    d = self.theclass(year, 1, 1)
-                    if specifier == 'G':
-                        year += g_offset
-                    if specifier == 'C':
-                        expected = f"{year // 100:02d}"
-                    else:
-                        expected = f"{year:04d}"
-                        if specifier == 'F':
-                            expected += f"-01-01"
-                    self.assertEqual(d.strftime(f"%{specifier}"), expected)
+            with self.subTest(year=year, specifier=specifier):
+                d = self.theclass(year, 1, 1)
+                if specifier == 'G':
+                    year += g_offset
+                if specifier == 'C':
+                    expected = f"{year // 100:02d}"
+                else:
+                    expected = f"{year:04d}"
+                    if specifier == 'F':
+                        expected += f"-01-01"
+                self.assertEqual(d.strftime(f"%{specifier}"), expected)
+
+    def test_strftime_y2k(self):
+        self.check_strftime_y2k('Y')
+        self.check_strftime_y2k('G')
+
+    def test_strftime_y2k_c99(self):
+        # CPython requires C11; specifiers new in C99 must work.
+        # (Other implementations may want to disable this test.)
+        self.check_strftime_y2k('F')
+        self.check_strftime_y2k('C')
 
     def test_replace(self):
         cls = self.theclass

Misc/NEWS.d/next/Build/2025-01-03-13-02-06.gh-issue-123681.gQ67nK.rst

@@ -0,0 +1,3 @@
+Check the ``strftime()`` behavior at runtime instead of at the compile time
+to support cross-compiling.
+Remove the internal macro ``_Py_NORMALIZE_CENTURY``.

Modules/_datetimemodule.c

@@ -1760,6 +1760,24 @@ format_utcoffset(char *buf, size_t buflen, const char *sep,
     return 0;
 }
 
+/* Check whether year with century should be normalized for strftime. */
+inline static int
+normalize_century(void)
+{
+    static int cache = -1;
+    if (cache < 0) {
+        char year[5];
+        struct tm date = {
+            .tm_year = -1801,
+            .tm_mon = 0,
+            .tm_mday = 1
+        };
+        cache = (strftime(year, sizeof(year), "%Y", &date) &&
+                 strcmp(year, "0099") != 0);
+    }
+    return cache;
+}
+
 static PyObject *
 make_somezreplacement(PyObject *object, char *sep, PyObject *tzinfoarg)
 {
@@ -1931,10 +1949,9 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             }
             replacement = freplacement;
         }
-#ifdef _Py_NORMALIZE_CENTURY
-        else if (ch == 'Y' || ch == 'G'
-                 || ch == 'F' || ch == 'C'
-        ) {
+        else if (normalize_century()
+                 && (ch == 'Y' || ch == 'G' || ch == 'F' || ch == 'C'))
+        {
             /* 0-pad year with century as necessary */
             PyObject *item = PySequence_GetItem(timetuple, 0);
             if (item == NULL) {
@@ -1985,7 +2002,6 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             }
             continue;
         }
-#endif
         else {
             /* percent followed by something else */
             continue;

Tools/c-analyzer/cpython/ignored.tsv

@@ -228,6 +228,7 @@ Modules/_datetimemodule.c	datetime_isoformat	specs	-
 Modules/_datetimemodule.c	parse_hh_mm_ss_ff	correction	-
 Modules/_datetimemodule.c	time_isoformat	specs	-
 Modules/_datetimemodule.c	-	capi_types	-
+Modules/_datetimemodule.c	normalize_century	cache	-
 Modules/_decimal/_decimal.c	-	cond_map_template	-
 Modules/_decimal/_decimal.c	-	dec_signal_string	-
 Modules/_decimal/_decimal.c	-	dflt_ctx	-

configure

@@ -6800,57 +6800,6 @@ then
   [Define if you have struct stat.st_mtimensec])
 fi
 
-AC_CACHE_CHECK([whether year with century should be normalized for strftime], [ac_cv_normalize_century], [
-AC_RUN_IFELSE([AC_LANG_SOURCE([[
-#include <time.h>
-#include <string.h>
-
-int main(void)
-{
-  char year[5];
-  struct tm date = {
-    .tm_year = -1801,
-    .tm_mon = 0,
-    .tm_mday = 1
-  };
-  if (strftime(year, sizeof(year), "%Y", &date) && !strcmp(year, "0099")) {
-    return 1;
-  }
-  return 0;
-}
-]])],
-[ac_cv_normalize_century=yes],
-[ac_cv_normalize_century=no],
-[ac_cv_normalize_century=yes])])
-if test "$ac_cv_normalize_century" = yes
-then
-  AC_DEFINE([_Py_NORMALIZE_CENTURY], [1],
-  [Define if year with century should be normalized for strftime.])
-fi
-
-AC_CACHE_CHECK([whether C99-compatible strftime specifiers are supported], [ac_cv_strftime_c99_support], [
-AC_RUN_IFELSE([AC_LANG_SOURCE([[
-#include <time.h>
-#include <string.h>
-
-int main(void)
-{
-  char full_date[11];
-  struct tm date = {
-    .tm_year = 0,
-    .tm_mon = 0,
-    .tm_mday = 1
-  };
-  if (strftime(full_date, sizeof(full_date), "%F", &date) && !strcmp(full_date, "1900-01-01")) {
-    return 0;
-  }
-  return 1;
-}
-]])],
-[ac_cv_strftime_c99_support=yes],
-[AC_MSG_ERROR([Python requires C99-compatible strftime specifiers])],
-[ac_cv_strftime_c99_support=])])
-
 dnl check for ncursesw/ncurses and panelw/panel
 dnl NOTE: old curses is not detected.
 dnl have_curses=[no, yes]

configure.ac

@@ -2023,9 +2023,6 @@
 /* HACL* library can compile SIMD256 implementations */
 #undef _Py_HACL_CAN_COMPILE_VEC256
 
-/* Define if year with century should be normalized for strftime. */
-#undef _Py_NORMALIZE_CENTURY
-
 /* Define to force use of thread-safe errno, h_errno, and other functions */
 #undef _REENTRANT