gh-153711: Test availability for dup3() and pipe2() on iOS

Author: encukou

Doc/library/os.rst

@@ -1448,7 +1448,7 @@ 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, macOS >= 27.0, not WASI, not iOS.
+   .. availability:: Unix, macOS >= 27.0, iOS >= 27.0, not WASI.
 
    .. versionadded:: 3.3
 

Misc/NEWS.d/next/Library/2026-08-05-10-13-23.gh-issue-153711.9g9dVz.rst

@@ -0,0 +1,3 @@
+On iOS, add :func:`os.pipe2`, and use the syscalls :manpage:`pipe2 (2)` and
+:manpage:`dup3 (2)` internally, when available at both build-time and
+run-time.

Modules/posixmodule.c

@@ -504,8 +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, *)
-#  define HAVE_PIPE2_RUNTIME __builtin_available(macOS 27.0, *)
+#  define HAVE_DUP3_RUNTIME __builtin_available(macOS 27.0, iOS 27.0, tvOS 27.0, watchOS 27.0, visionOS 27.0, *)
+#  define HAVE_PIPE2_RUNTIME __builtin_available(macOS 27.0, iOS 27.0, tvOS 27.0, watchOS 27.0, visionOS 27.0, *)
 
 #  define HAVE_POSIX_SPAWN_SETSID_RUNTIME __builtin_available(macOS 10.15, *)
 

configure.ac

@@ -5540,7 +5540,7 @@ fi
 # header definition prevents usage - autoconf doesn't use the headers), or
 # raise an error if used at runtime. Force these symbols off.
 if test "$ac_sys_system" != "iOS" ; then
-  AC_CHECK_FUNCS([dup3 getentropy getgroups pipe2 system])
+  AC_CHECK_FUNCS([getentropy getgroups system])
 fi
 
 AC_CHECK_DECL([dirfd],