fix: detect musl libc when downloading python-build-standalone (#2889)

* fix: detect musl libc when downloading python-build-standalone

`platform.libc_ver()` only ever reports glibc and returns `("", "")` on
musl systems, so `"musl" in platform.libc_ver()` was never true and musl
hosts (e.g. Alpine) always selected the `gnu` python-build-standalone
asset, which won't run.

Detect musl by shelling out to `ldd --version`, which prints "musl libc"
on musl-based systems. This probes the actual system loader rather than
the host Python's linkage, so it stays correct even when cibuildwheel
runs under a relocatable/statically-linked interpreter. Falls back to
glibc when `ldd` is unavailable.

Assisted-by: ClaudeCode:claude-opus-4.8

* test: add type annotations to satisfy mypy and ruff

Assisted-by: ClaudeCode:claude-opus-4.8

cibuildwheel/util/python_build_standalone.py

@@ -5,6 +5,7 @@ import functools
 import hashlib
 import json
 import platform
+import subprocess
 import typing
 
 from filelock import FileLock
@@ -42,6 +43,26 @@ class PythonBuildStandaloneError(Exception):
     """Errors related to python-build-standalone."""
 
 
+def _is_musl_libc() -> bool:
+    """
+    Detect whether the host uses musl libc (e.g. Alpine Linux).
+
+    ``platform.libc_ver()`` only ever reports glibc, so we shell out to ``ldd``,
+    which prints "musl libc" on musl-based systems (and "GNU libc"/"GLIBC" on
+    glibc systems). If ``ldd`` is unavailable, we fall back to assuming glibc.
+    """
+    try:
+        ldd = subprocess.run(
+            ["ldd", "--version"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+    except OSError:
+        return False
+    return "musl" in (ldd.stdout + ldd.stderr).lower()
+
+
 def _get_platform_identifiers() -> tuple[str, str, str | None]:
     """
     Detects the current platform and returns architecture, platform, and libc
@@ -67,7 +88,7 @@ def _get_platform_identifiers() -> tuple[str, str, str | None]:
     # Map OS + Libc
     if system == "Linux":
         platform_identifier = "unknown-linux"
-        libc_identifier = "musl" if "musl" in (platform.libc_ver() or ("", "")) else "gnu"
+        libc_identifier = "musl" if _is_musl_libc() else "gnu"
     elif system == "Darwin":
         platform_identifier = "apple-darwin"
     elif system == "Windows":

unit_test/python_build_standalone_test.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+import platform
+import subprocess
+from typing import Any
+
+import pytest
+
+from cibuildwheel.util import python_build_standalone as pbs
+
+TYPE_CHECKING = False
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+# Real-world `ldd --version` outputs.
+GLIBC_STDOUT = "ldd (Ubuntu GLIBC 2.35-0ubuntu3.1) 2.35\n"
+MUSL_STDERR = "musl libc (x86_64)\nVersion 1.2.4\nDynamic Program Loader\n"
+
+
+def _fake_ldd(
+    stdout: str = "", stderr: str = ""
+) -> Callable[..., subprocess.CompletedProcess[str]]:
+    def run(*args: Any, **kwargs: Any) -> subprocess.CompletedProcess[str]:
+        return subprocess.CompletedProcess(args=args, returncode=0, stdout=stdout, stderr=stderr)
+
+    return run
+
+
+def test_is_musl_libc_detects_musl(monkeypatch: pytest.MonkeyPatch) -> None:
+    # musl's ldd prints to stderr and exits non-zero, but capture both streams
+    monkeypatch.setattr(subprocess, "run", _fake_ldd(stderr=MUSL_STDERR))
+    assert pbs._is_musl_libc() is True
+
+
+def test_is_musl_libc_detects_glibc(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(subprocess, "run", _fake_ldd(stdout=GLIBC_STDOUT))
+    assert pbs._is_musl_libc() is False
+
+
+def test_is_musl_libc_missing_ldd(monkeypatch: pytest.MonkeyPatch) -> None:
+    def run(*args: Any, **kwargs: Any) -> subprocess.CompletedProcess[str]:
+        msg = "ldd"
+        raise FileNotFoundError(msg)
+
+    monkeypatch.setattr(subprocess, "run", run)
+    assert pbs._is_musl_libc() is False
+
+
+@pytest.mark.parametrize(
+    ("musl", "expected_libc"),
+    [(True, "musl"), (False, "gnu")],
+)
+def test_linux_platform_identifiers_libc(
+    monkeypatch: pytest.MonkeyPatch, musl: bool, expected_libc: str
+) -> None:
+    monkeypatch.setattr(platform, "system", lambda: "Linux")
+    monkeypatch.setattr(platform, "machine", lambda: "x86_64")
+    monkeypatch.setattr(pbs, "_is_musl_libc", lambda: musl)
+
+    arch, platform_id, libc = pbs._get_platform_identifiers()
+    assert arch == "x86_64"
+    assert platform_id == "unknown-linux"
+    assert libc == expected_libc