Reduce dependency on ctypes when discovering glibc version.

src/pip/_internal/utils/glibc.py

@@ -10,17 +10,32 @@
     from typing import Optional, Tuple
 
 
-def glibc_version_string():
+def glibc_version_string_os():
     # type: () -> Optional[str]
-    "Returns glibc version string, or None if not using glibc."
+    """Returns glibc version string, or None if not using glibc.
 
-    try:  # https://github.com/pypa/pip/issues/6675#issue-463147612
-        glibc_version = os.confstr("CS_GNU_LIBC_VERSION").split()
+    This should be paired with glibc_version_string_ctypes as a fallback:
+    glibc_ver = glibc_version_string_os() or glibc_version_string_ctypes()
+    """
+
+    # 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):
-        pass  # os.confstr() or CS_GNU_LIBC_VERSION not available...
-    else:
-        if len(glibc_version) == 2:
-            return glibc_version[-1]
+        # 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 ctypes implementation of glibc_version_string_os."
 
     try:
         import ctypes
@@ -69,7 +84,7 @@ 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_os() or glibc_version_string_ctypes()
     if version_str is None:
         return False
     return check_glibc_version(version_str, required_major, minimum_minor)
@@ -99,7 +114,7 @@ def libc_ver():
     Returns a tuple of strings (lib, version) which default to empty strings
     in case the lookup fails.
     """
-    glibc_version = glibc_version_string()
+    glibc_version = glibc_version_string_os() or glibc_version_string_ctypes()
     if glibc_version is None:
         return ("", "")
     else:

tests/unit/test_utils.py

@@ -26,7 +26,9 @@
 )
 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_ctypes, glibc_version_string_os,
+)
 from pip._internal.utils.hashes import Hashes, MissingHashes
 from pip._internal.utils.misc import (
     call_subprocess, egg_link_path, ensure_dir, format_command_args,
@@ -701,6 +703,30 @@ def test_manylinux_check_glibc_version(self):
                     # Didn't find the warning we were expecting
                     assert False
 
+    def test_glibc_version_string_os_fail(self, monkeypatch):
+
+        if hasattr(os, "confstr"):
+
+            def raises(error):
+                raise error()
+
+            monkeypatch.setattr(os, "confstr", lambda x: raises(ValueError))
+            assert glibc_version_string_os() is None
+
+            monkeypatch.setattr(os, "confstr", lambda x: raises(OSError))
+            assert glibc_version_string_os() is None
+
+            monkeypatch.setattr(os, "confstr", lambda x: "XXX")
+            assert glibc_version_string_os() is None
+
+            monkeypatch.delattr(os, "confstr")
+
+        assert glibc_version_string_os() is None
+
+    def test_glibc_version_string_ctypes_fail(self, monkeypatch):
+        monkeypatch.setitem(sys.modules, "ctypes", None)
+        assert glibc_version_string_ctypes() is None
+
 
 @pytest.mark.parametrize('version_info, expected', [
     (None, None),