Reduce dependency on ctypes when discovering glibc version. (#6678)

news/6543.bugfix

@@ -0,0 +1 @@
+Prefer ``os.confstr`` to ``ctypes`` when extracting glibc version info.

news/6675.bugfix

@@ -0,0 +1 @@
+Prefer ``os.confstr`` to ``ctypes`` when extracting glibc version info.

src/pip/_internal/utils/glibc.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import
 
-import ctypes
+import os
 import re
 import warnings
 
@@ -13,6 +13,33 @@
 def glibc_version_string():
     # type: () -> Optional[str]
     "Returns glibc version string, or None if not using glibc."
+    return glibc_version_string_confstr() or glibc_version_string_ctypes()
+
+
+def glibc_version_string_confstr():
+    # type: () -> Optional[str]
+    "Primary implementation of glibc_version_string using os.confstr."
+    # os.confstr is quite a bit faster than ctypes.DLL. It's also less likely
+    # to be broken or missing. This strategy is used in the standard library
+    # platform module:
+    # https://github.com/python/cpython/blob/fcf1d003bf4f0100c9d0921ff3d70e1127ca1b71/Lib/platform.py#L175-L183
+    try:
+        # os.confstr("CS_GNU_LIBC_VERSION") returns a string like "glibc 2.17":
+        _, version = os.confstr("CS_GNU_LIBC_VERSION").split()
+    except (AttributeError, OSError, ValueError):
+        # os.confstr() or CS_GNU_LIBC_VERSION not available (or a bad value)...
+        return None
+    return version
+
+
+def glibc_version_string_ctypes():
+    # type: () -> Optional[str]
+    "Fallback implementation of glibc_version_string using ctypes."
+
+    try:
+        import ctypes
+    except ImportError:
+        return None
 
     # ctypes.CDLL(None) internally calls dlopen(NULL), and as the dlopen
     # manpage says, "If filename is NULL, then the returned handle is for the
@@ -56,7 +83,7 @@ def check_glibc_version(version_str, required_major, minimum_minor):
 
 def have_compatible_glibc(required_major, minimum_minor):
     # type: (int, int) -> bool
-    version_str = glibc_version_string()  # type: Optional[str]
+    version_str = glibc_version_string()
     if version_str is None:
         return False
     return check_glibc_version(version_str, required_major, minimum_minor)

tests/unit/test_utils.py

@@ -27,7 +27,10 @@
 )
 from pip._internal.utils.deprecation import PipDeprecationWarning, deprecated
 from pip._internal.utils.encoding import BOMS, auto_decode
-from pip._internal.utils.glibc import check_glibc_version
+from pip._internal.utils.glibc import (
+    check_glibc_version, glibc_version_string, glibc_version_string_confstr,
+    glibc_version_string_ctypes,
+)
 from pip._internal.utils.hashes import Hashes, MissingHashes
 from pip._internal.utils.misc import (
     call_subprocess, egg_link_path, ensure_dir, format_command_args,
@@ -704,6 +707,10 @@ def raising_mkdir(*args, **kwargs):
                     pass
 
 
+def raises(error):
+    raise error
+
+
 class TestGlibc(object):
     def test_manylinux_check_glibc_version(self):
         """
@@ -737,6 +744,35 @@ def test_manylinux_check_glibc_version(self):
                     # Didn't find the warning we were expecting
                     assert False
 
+    def test_glibc_version_string(self, monkeypatch):
+        monkeypatch.setattr(
+            os, "confstr", lambda x: "glibc 2.20", raising=False,
+        )
+        assert glibc_version_string() == "2.20"
+
+    def test_glibc_version_string_confstr(self, monkeypatch):
+        monkeypatch.setattr(
+            os, "confstr", lambda x: "glibc 2.20", raising=False,
+        )
+        assert glibc_version_string_confstr() == "2.20"
+
+    @pytest.mark.parametrize("failure", [
+        lambda x: raises(ValueError),
+        lambda x: raises(OSError),
+        lambda x: "XXX",
+    ])
+    def test_glibc_version_string_confstr_fail(self, monkeypatch, failure):
+        monkeypatch.setattr(os, "confstr", failure, raising=False)
+        assert glibc_version_string_confstr() is None
+
+    def test_glibc_version_string_confstr_missing(self, monkeypatch):
+        monkeypatch.delattr(os, "confstr", raising=False)
+        assert glibc_version_string_confstr() is None
+
+    def test_glibc_version_string_ctypes_missing(self, monkeypatch):
+        monkeypatch.setitem(sys.modules, "ctypes", None)
+        assert glibc_version_string_ctypes() is None
+
 
 @pytest.mark.parametrize('version_info, expected', [
     ((), (0, 0, 0)),