gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms

Add HAVE_DUP3_RUNTIME / HAVE_PIPE2_RUNTIME weak-linking so Xcode 27 SDKs
do not crash on macOS < 27 when the symbols are weak_import NULL.

Author: Vamsi-klu

Doc/library/os.rst

@@ -1448,7 +1448,8 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
    Return a pair of file descriptors ``(r, w)`` usable for reading and writing,
    respectively.
 
-   .. availability:: Unix, not WASI, not macOS, not iOS.
+   .. availability:: Unix, not WASI.  Availability on macOS requires macOS 27.0
+      or later; availability on iOS requires iOS 27.0 or later.
 
    .. versionadded:: 3.3
 

Lib/test/test_os/test_posix.py

@@ -2388,6 +2388,22 @@ def test_pwritev(self):
             self.assertNotHasAttr(os, "pwritev")
             self.assertNotHasAttr(os, "preadv")
 
+    def test_pipe2(self):
+        self._verify_available("HAVE_PIPE2")
+        if self.mac_ver >= (27, 0):
+            self.assertHasAttr(os, "pipe2")
+        else:
+            self.assertNotHasAttr(os, "pipe2")
+
+    def test_dup3(self):
+        self._verify_available("HAVE_DUP3")
+        r, w = os.pipe()
+        self.addCleanup(os.close, r)
+        self.addCleanup(os.close, w)
+        # Must not crash even when dup3 unavailable at runtime.
+        # os.dup2 returns fd2 (here w); do not double-close.
+        os.dup2(r, w, inheritable=False)
+
     def test_stat(self):
         self._verify_available("HAVE_FSTATAT")
         if self.mac_ver >= (10, 10):

Misc/NEWS.d/next/macOS/2026-07-26-07-01-41.gh-issue-153711.ad8g_Q.rst

@@ -0,0 +1,3 @@
+Guard ``os.dup2`` and ``os.pipe``/``os.pipe2`` against calling ``dup3``/``pipe2``
+when the symbols are weak-imported but unavailable at runtime (Xcode 27 SDKs on
+older macOS).

Modules/posixmodule.c

@@ -504,6 +504,8 @@ static const unsigned int _Py_STATX_KNOWN = (STATX_BASIC_STATS | STATX_BTIME
 #  define HAVE_MKFIFOAT_RUNTIME __builtin_available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
 #  define HAVE_MKNODAT_RUNTIME __builtin_available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
 #  define HAVE_PTSNAME_R_RUNTIME __builtin_available(macOS 10.13.4, iOS 11.3, tvOS 11.3, watchOS 4.3, *)
+#  define HAVE_DUP3_RUNTIME __builtin_available(macOS 27.0, iOS 27.0, tvOS 27.0, watchOS 27.0, *)
+#  define HAVE_PIPE2_RUNTIME __builtin_available(macOS 27.0, iOS 27.0, tvOS 27.0, watchOS 27.0, *)
 
 #  define HAVE_POSIX_SPAWN_SETSID_RUNTIME __builtin_available(macOS 10.15, *)
 
@@ -589,6 +591,14 @@ static const unsigned int _Py_STATX_KNOWN = (STATX_BASIC_STATS | STATX_BTIME
 #    define HAVE_PTSNAME_R_RUNTIME (ptsname_r != NULL)
 #  endif
 
+#  ifdef HAVE_DUP3
+#    define HAVE_DUP3_RUNTIME (dup3 != NULL)
+#  endif
+
+#  ifdef HAVE_PIPE2
+#    define HAVE_PIPE2_RUNTIME (pipe2 != NULL)
+#  endif
+
 #endif
 
 #ifdef HAVE_FUTIMESAT
@@ -619,6 +629,8 @@ static const unsigned int _Py_STATX_KNOWN = (STATX_BASIC_STATS | STATX_BTIME
 #  define HAVE_MKFIFOAT_RUNTIME 1
 #  define HAVE_MKNODAT_RUNTIME 1
 #  define HAVE_PTSNAME_R_RUNTIME 1
+#  define HAVE_DUP3_RUNTIME 1
+#  define HAVE_PIPE2_RUNTIME 1
 #endif
 
 
@@ -11910,17 +11922,22 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
 
 #ifdef HAVE_DUP3
     if (!inheritable && dup3_works != 0) {
-        Py_BEGIN_ALLOW_THREADS
-        res = dup3(fd, fd2, O_CLOEXEC);
-        Py_END_ALLOW_THREADS
-        if (res < 0) {
-            if (dup3_works == -1)
-                dup3_works = (errno != ENOSYS);
-            if (dup3_works) {
-                posix_error();
-                return -1;
+        if (HAVE_DUP3_RUNTIME) {
+            Py_BEGIN_ALLOW_THREADS
+            res = dup3(fd, fd2, O_CLOEXEC);
+            Py_END_ALLOW_THREADS
+            if (res < 0) {
+                if (dup3_works == -1)
+                    dup3_works = (errno != ENOSYS);
+                if (dup3_works) {
+                    posix_error();
+                    return -1;
+                }
             }
         }
+        else {
+            dup3_works = 0;
+        }
     }
 
     if (inheritable || dup3_works == 0)
@@ -12779,9 +12796,13 @@ os_pipe_impl(PyObject *module)
 #else
 
 #ifdef HAVE_PIPE2
-    Py_BEGIN_ALLOW_THREADS
-    res = pipe2(fds, O_CLOEXEC);
-    Py_END_ALLOW_THREADS
+    res = -1;
+    errno = ENOSYS;
+    if (HAVE_PIPE2_RUNTIME) {
+        Py_BEGIN_ALLOW_THREADS
+        res = pipe2(fds, O_CLOEXEC);
+        Py_END_ALLOW_THREADS
+    }
 
     if (res != 0 && errno == ENOSYS)
     {
@@ -18811,6 +18832,18 @@ posixmodule_exec(PyObject *m)
     }
 #endif
 
+#if defined(HAVE_PIPE2)
+    if (HAVE_PIPE2_RUNTIME) {} else {
+        PyObject *dct = PyModule_GetDict(m);
+        if (dct == NULL) {
+            return -1;
+        }
+        if (PyDict_PopString(dct, "pipe2", NULL) < 0) {
+            return -1;
+        }
+    }
+#endif
+
 #ifdef HAVE_STATX
     if (statx == NULL) {
         PyObject* dct = PyModule_GetDict(m);