Fix musl detection in Python 3.12 and below

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 <[email protected]>
+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 <[email protected]>
+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()