Fix musl detection in Python 3.12 and below

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 doens't work right
because of an autoconf bug. CPython 3.13 rewrites the target triple
detection logic and works fine.

Meanwhile, the previous patch to this part of configure.ac is no longer
relevant because clang stopped implementing --print-multiarch in LLVM 16
(llvm/llvm-project@e05edb19), so we can drop those patches.

Fixes #724.

Author: geofft

cpython-unix/build-cpython.sh

@@ -115,15 +115,14 @@ if [ -n "${CROSS_COMPILING}" ]; then
     fi
 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.
-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 ${ROOT}/patch-disable-multiarch.patch
-    fi
+# CPython <=3.10 doesn't properly detect musl. CPython <=3.12 tries, but fails in our environment
+# because of an autoconf bug. CPython >=3.13 is fine.
+# Backport the musl detection code from CPython 3.13 so it loads
+# extension modules with the right suffix.
+if [ -n "${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
 
 # Python 3.11 supports using a provided Python to use during bootstrapping

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)
+

cpython-unix/patch-disable-multiarch-13.patch

@@ -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()