Fix musl detection in Python 3.12 and below (#827)

CPython 3.10 and below don't even attempt to fix the target triple for
musl. CPython 3.11 and 3.12 do, but in our setup it doesn't work right
because of an autoconf bug. CPython 3.13 rewrites the target triple
detection logic and works fine.

Fixes #724.

Author: geofft

cpython-unix/build-cpython-host.sh

@@ -37,17 +37,12 @@ pushd "Python-${PYTHON_VERSION}"
 # Clang 13 actually prints something with --print-multiarch, confusing CPython's
 # configure. This is reported as https://bugs.python.org/issue45405. We nerf the
 # check since we know what we're doing.
-if [ "${CC}" = "clang" ]; then
+if [[ "${CC}" = "clang" || "${CC}" = "musl-clang" ]]; then
   if [ -n "${PYTHON_MEETS_MINIMUM_VERSION_3_13}" ]; then
     patch -p1 -i ${ROOT}/patch-disable-multiarch-13.patch
   else
     patch -p1 -i ${ROOT}/patch-disable-multiarch.patch
   fi
-elif [ "${CC}" = "musl-clang" ]; then
-  # Similarly, this is a problem for musl Clang on Python 3.13+
-  if [ -n "${PYTHON_MEETS_MINIMUM_VERSION_3_13}" ]; then
-    patch -p1 -i ${ROOT}/patch-disable-multiarch-13.patch
-  fi
 fi
 
 autoconf

cpython-unix/build-cpython.sh

@@ -115,6 +115,14 @@ if [ -n "${CROSS_COMPILING}" ]; then
     fi
 fi
 
+# CPython <=3.10 doesn't properly detect musl. CPython <=3.12 tries, but fails
+# in our environment because of an autoconf bug. CPython >=3.13 is fine.
+if [ -n "${PYTHON_MEETS_MAXIMUM_VERSION_3_10}" ]; then
+    patch -p1 -i ${ROOT}/patch-cpython-configure-target-triple-musl-3.10.patch
+elif [ -n "${PYTHON_MEETS_MAXIMUM_VERSION_3_12}" ]; then
+    patch -p1 -i ${ROOT}/patch-cpython-configure-target-triple-musl-3.12.patch
+fi
+
 # Clang 13 actually prints something with --print-multiarch, confusing CPython's
 # configure. This is reported as https://bugs.python.org/issue45405. We nerf the
 # check since we know what we're doing.
@@ -1133,11 +1141,10 @@ mkdir -p "${LIB_DYNLOAD}"
 touch "${LIB_DYNLOAD}/.empty"
 
 # Symlink libpython so we don't have 2 copies.
+# TODO(geofft): Surely we can get PYTHON_ARCH out of the build?
 case "${TARGET_TRIPLE}" in
 aarch64-unknown-linux-*)
-    # In Python 3.13+, the musl target is identified in cross compiles and the output directory
-    # is named accordingly.
-    if [[ "${CC}" = "musl-clang" && -n "${PYTHON_MEETS_MINIMUM_VERSION_3_13}" ]]; then
+    if [[ "${CC}" = "musl-clang" ]]; then
         PYTHON_ARCH="aarch64-linux-musl"
     else
         PYTHON_ARCH="aarch64-linux-gnu"
@@ -1176,9 +1183,7 @@ s390x-unknown-linux-gnu)
     PYTHON_ARCH="s390x-linux-gnu"
     ;;
 x86_64-unknown-linux-*)
-    # In Python 3.13+, the musl target is identified in cross compiles and the output directory
-    # is named accordingly.
-    if [[ "${CC}" = "musl-clang" && -n "${PYTHON_MEETS_MINIMUM_VERSION_3_13}" ]]; then
+    if [[ "${CC}" = "musl-clang" ]]; then
         PYTHON_ARCH="x86_64-linux-musl"
     else
         PYTHON_ARCH="x86_64-linux-gnu"

cpython-unix/patch-cpython-configure-target-triple-musl-3.10.patch

@@ -0,0 +1,31 @@
+From d7a70fe9ffa0e4e173bae545cdcba62dc7a2a1f1 Mon Sep 17 00:00:00 2001
+From: Geoffrey Thomas <geofft@ldpreload.com>
+Date: Sat, 25 Oct 2025 18:49:45 -0400
+Subject: [PATCH 1/1] LOCAL: configure.ac: Fix musl detection
+Forwarded: not-needed
+
+Newer versions of Python rework this code significantly, so this does
+not need to be upstreamed.
+---
+ configure.ac | 5 +++++
+ 1 file changed, 5 insertions(+)
+
+diff --git a/configure.ac b/configure.ac
+index ac3be3850a9..9c7c02ea6de 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -866,6 +866,11 @@ EOF
+ 
+ if $CPP $CPPFLAGS conftest.c >conftest.out 2>/dev/null; then
+   PLATFORM_TRIPLET=`grep -v '^#' conftest.out | grep -v '^ *$' | tr -d ' 	'`
++  case "$CC" in
++  musl-*)
++    PLATFORM_TRIPLET=`echo "$PLATFORM_TRIPLET" | sed 's/linux-gnu/linux-musl/'`
++    ;;
++  esac
+   AC_MSG_RESULT([$PLATFORM_TRIPLET])
+ else
+   AC_MSG_RESULT([none])
+-- 
+2.39.5 (Apple Git-154)
+

cpython-unix/patch-cpython-configure-target-triple-musl-3.12.patch

@@ -0,0 +1,36 @@
+From 851c49bafb1fe11c02eafe4850e952cafec548b7 Mon Sep 17 00:00:00 2001
+From: Geoffrey Thomas <geofft@ldpreload.com>
+Date: Sat, 25 Oct 2025 18:49:45 -0400
+Subject: [PATCH 1/1] LOCAL: configure.ac: Fix musl detection
+Forwarded: not-needed
+
+We set HOST_CC to regular (glibc) clang and CC to musl-clang.
+AC_CANONICAL_HOST sets build_os based on the output of config.guess,
+which looks at HOST_CC in preference to CC, and therefore misreports the
+target triple as gnu. Directly checking CC works for us.
+
+Newer versions of Python rework this code significantly to not rely on
+build_os and do musl detection internally, so this does not need to be
+upstreamed.
+---
+ configure.ac | 4 ++--
+ 1 file changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/configure.ac b/configure.ac
+index 1a02d19f1b2..ee743c11aa5 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -1108,8 +1108,8 @@ EOF
+ 
+ if $CPP $CPPFLAGS conftest.c >conftest.out 2>/dev/null; then
+   PLATFORM_TRIPLET=`grep -v '^#' conftest.out | grep -v '^ *$' | tr -d ' 	'`
+-  case "$build_os" in
+-  linux-musl*)
++  case "$CC" in
++  musl-*)
+     PLATFORM_TRIPLET=`echo "$PLATFORM_TRIPLET" | sed 's/linux-gnu/linux-musl/'`
+     ;;
+   esac
+-- 
+2.39.5 (Apple Git-154)
+

src/verify_distribution.py

@@ -2,6 +2,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
+import importlib.machinery
 import os
 import struct
 import sys
@@ -255,6 +256,17 @@ def test_hash_algorithm(self):
             msg=f"{sys.hash_info.algorithm=!r} is not siphash",
         )
 
+    def test_libc_identity(self):
+        def assertLibc(value):
+            for libc in ("-gnu", "-musl"):
+                if os.environ["TARGET_TRIPLE"].endswith(libc):
+                    self.assertIn(libc, value)
+                else:
+                    self.assertNotIn(libc, value)
+
+        assertLibc(sys.implementation._multiarch)
+        assertLibc(importlib.machinery.EXTENSION_SUFFIXES[0])
+
 
 if __name__ == "__main__":
     unittest.main()