gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms by Vamsi-klu · Pull Request #154718 · python/cpython · GitHub

Summary

Fix gh-153711 (deferred-blocker): when CPython is built with an Xcode 27 SDK, dup3 / pipe2 are detected at configure time but only exist at runtime on macOS / iOS 27+. On older OSes they bind as weak_import NULL and the first os.pipe() / non-inheritable os.dup2() jumps to address 0.

What the issue is

Xcode 27 SDKs annotate dup3/pipe2 with __API_AVAILABLE(macos(27.0), iOS(27.0), …).
AC_CHECK_FUNCS synthesizes its own char dup3(); prototype, ignores availability, links against libSystem.tbd, and defines HAVE_DUP3 / HAVE_PIPE2.
Modules/posixmodule.c then calls them unconditionally inside os_dup2_impl / os_pipe_impl / os_pipe2_impl.
With MACOSX_DEPLOYMENT_TARGET < 27, the symbols are weak; on macOS 26 they are NULL → EXC_BAD_ACCESS.
os.pipe() is on the hot path of subprocess, multiprocessing, and asyncio — effectively a total-loss crash for python.org-style builds.

iOS was already excluded from the configure check (gh-150443 / #150444). macOS was not. The issue body already points at the gh-97897 (mkfifoat/mknodat) weak-linking pattern.

Why I solved it that way

Runtime __builtin_available guards, not configure hacks — configure cannot see availability attributes, and we still want the symbols on macOS 27+.
Reuse the existing three-arm HAVE_*_RUNTIME machinery in posixmodule.c (modern / Xcode≤8 / non-Apple) documented in Mac/README.rst. Omitting any arm breaks the build.
Sole-expression if (HAVE_X_RUNTIME) — Apple's compiler rejects if (!HAVE_X_RUNTIME).
Latch dup3_works = 0 when unavailable so the existing dup2+FD_CLOEXEC fallback runs this call and every later call without re-probing.
Seed res = -1; errno = ENOSYS before the pipe2 probe so the existing ENOSYS → pipe() fallback still runs when the symbol is missing (uninitialized res/errno would be a footgun).
Pop os.pipe2 from the module dict when unavailable (same idiom as pwritev/preadv) because os_pipe2_impl has no fallback and would crash on direct call. Existing tests already hasattr(os, "pipe2").
Omit visionOS from __builtin_available — no other macro in the block names it; pre-Xcode-15 clang rejects the spelling.
Honest verification limit: this environment is Linux. Code mirrors Prevent segfaults by adding macOS weaklinking for mkfifoat and mknodat system calls added in macOS 13 Ventura #97897; an OS-mac core dev with Xcode 27 should confirm on macOS 26.

How I did it

Area	Change
Modules/posixmodule.c	HAVE_DUP3_RUNTIME / HAVE_PIPE2_RUNTIME in all 3 arms; guard dup3/pipe2 call sites; pop pipe2 at exec
Lib/test/test_os/test_posix.py	TestPosixWeaklinking.test_pipe2 / test_dup3
Doc/library/os.rst	os.pipe2 availability notes macOS/iOS ≥ 27
Misc/NEWS.d/next/macOS/	blurb for gh-153711

Impact

Unblocks building with Xcode 27 SDKs while deploying to macOS < 27.
On Linux/non-Apple, macros are 1 — zero codegen change (constant-folded).
On macOS ≥ 27, behavior unchanged (fast path still used).
Deferred-blocker across 3.13–3.16 — backports appropriate after OS-mac review.

Testing plan

Linux debug build + ./python -m test test_os.test_posix — PASS (weaklinking class skipped on non-macOS, as designed).
Smoke: os.pipe() / os.dup2(..., inheritable=False) on Linux — OK.
Needed from reviewer / buildbot: macOS 26 host, Xcode 27 SDK, deployment target < 27:
- Before: crash on os.pipe() / os.pipe2 / non-inheritable dup2.
- After: os.pipe() works via fallback; hasattr(os, "pipe2") is False; TestPosixWeaklinking assertions hold.
macOS 27+ : pipe2 present, weaklinking tests assert attrs exist.
CI: macOS jobs (will not fully exercise the Xcode 27 × older OS matrix).

Everything else

Does not undo iOS configure exclusion from gh-150443: Exclude explicit dup3 and pipe2 checks on iOS builds. #150444.
Pattern credit: Ned Deily's Prevent segfaults by adding macOS weaklinking for mkfifoat and mknodat system calls added in macOS 13 Ventura #97897 / commit for mkfifoat/mknodat.
Backports: 3.15, 3.14, 3.13 after merge.
Reviewers: OS-mac / Apple platform SMEs.

Requested reviewers (subject-matter experts)

Could the following SMEs take a look when convenient (I cannot formally request reviews from this fork account):

ned-deily — OS-mac / weak-linking (Prevent segfaults by adding macOS weaklinking for mkfifoat and mknodat system calls added in macOS 13 Ventura #97897 pattern)
ronaldoussoren — macOS
freakboy3742 — Apple/iOS (adjacent iOS can't be built on macOS 26 / Xcode 26 build host #150443)

Thank you!

…latforms 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.

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Keep pipe2 documented as unavailable on iOS

The new iOS availability claim is inaccurate even on iOS 27: configure.ac:5526-5532 unconditionally skips the pipe2 feature check whenever ac_sys_system is iOS, so HAVE_PIPE2 remains undefined and the module never exports os.pipe2. Either retain not iOS here or change the iOS configure policy to enable the function where supported.

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Guard the remaining direct pipe2 call at compile time

When building with an Xcode 27 SDK for a macOS deployment target below 27, removing pipe2 from the module dictionary happens too late to make the build succeed: os_pipe2_impl still calls the availability-annotated pipe2 unguarded at Modules/posixmodule.c:12861, while configure.ac:2797-2802 promotes Clang's unguarded-availability diagnostic to an error. The call needs a lexical availability guard or the same targeted diagnostic suppression used for the runtime-removed preadv/pwritev methods.

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The availability extension only accepts comma-separated known platforms (optionally with 'not ' / '>= version'). Free-form prose after 'not WASI.' was parsed as an unknown platform and failed Docs EPUB/doctest with warnings-as-errors.

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Correct the iOS availability claim

The documentation promises os.pipe2 on iOS 27+, but configure.ac deliberately skips the AC_CHECK_FUNCS invocation containing pipe2 whenever ac_sys_system is iOS (lines 5526–5532). Consequently HAVE_PIPE2 is never defined and the method is not compiled or exported on any iOS version; either enable the configure check for supported iOS versions or continue documenting this as unavailable on iOS.

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Documentation build overview

📚 cpython-previews | 🛠️ Build #33762150 | 📁 Comparing 548b366 against main (f5f5059)

🔍 Preview build

2 files changed
± library/os.html
± whatsnew/changelog.html

Closing this PR. Thank you for the review attention.

Based on pythonGH-154718 Co-Authored-By: Vamsi-klu <nrvamsi13@gmail.com>

pythongh-153711 : Guard dup3/pipe2 with __builtin_available on Apple p…

e5fbe3b

…latforms 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.

bedevere-app Bot added the awaiting review label Jul 26, 2026

bedevere-app Bot mentioned this pull request Jul 26, 2026

macOS: Test availability for dup3() and pipe2() to prevent crashes when building against Xcode 27 #153711

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Vamsi-klu marked this pull request as draft July 26, 2026 08:01

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Vamsi-klu closed this Jul 26, 2026

encukou added a commit to encukou/cpython that referenced this pull request Aug 4, 2026

Add runtime guards for dup3 & pipe2

d6daa55

Based on pythonGH-154718 Co-Authored-By: Vamsi-klu <nrvamsi13@gmail.com>

encukou added a commit to encukou/cpython that referenced this pull request Aug 4, 2026

Add runtime guards for dup3 & pipe2

03077d5

Based on pythonGH-154718 Co-Authored-By: Vamsi-klu <nrvamsi13@gmail.com>

encukou mentioned this pull request Aug 4, 2026

gh-153711: Add runtime guards for dup3 & pipe2 #155174

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gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms by Vamsi-klu · Pull Request #154718 · python/cpython · GitHub

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gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms - #154718

gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms#154718
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-                 .. 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.

               #endif
+              #if defined(HAVE_PIPE2)
+                  if (HAVE_PIPE2_RUNTIME) {} else {

                  respectively.
-                 .. availability:: Unix, not WASI, not macOS, not iOS.
+                 .. availability:: Unix, not WASI, macOS >= 27.0, iOS >= 27.0.

gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms by Vamsi-klu · Pull Request #154718 · python/cpython · GitHub

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gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms - #154718

gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms#154718Vamsi-klu wants to merge 2 commits intopython:mainpython/cpython:mainfrom Vamsi-klu:fix-gh153711-macos-dup3-pipe2-weaklinkVamsi-klu/cpython:fix-gh153711-macos-dup3-pipe2-weaklinkCopy head branch name to clipboard

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Summary

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gh-153711: Guard dup3/pipe2 with __builtin_available on Apple platforms#154718
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