bpo-35389: platform.libc_ver() uses os.confstr() (GH-10891)

platform.libc_ver() now uses os.confstr('CS_GNU_LIBC_VERSION') if
available and the *executable* parameter is not set. The default
value of the libc_ver() *executable* parameter becomes None.

Quick benchmark on Fedora 29:

python3 -m perf command ./python -S -c 'import platform; platform.libc_ver()'
94.9 ms +- 4.3 ms -> 33.2 ms +- 1.4 ms: 2.86x faster (-65%)

1 files changed, 14 insertions, 1 deletions

diff --git a/Lib/platform.py b/Lib/platform.py
index 74346c4cab..f089a463ef 100755
--- a/Lib/platform.py
+++ b/Lib/platform.py
@@ -169,7 +169,7 @@ _libc_search = re.compile(b'(__libc_init)'
                           b'|'
                           br'(libc(_\w+)?\.so(?:\.(\d[0-9.]*))?)', re.ASCII)
 
-def libc_ver(executable=sys.executable, lib='', version='', chunksize=16384):
+def libc_ver(executable=None, lib='', version='', chunksize=16384):
 
     """ Tries to determine the libc version that the file executable
         (which defaults to the Python interpreter) is linked against.
@@ -184,6 +184,19 @@ def libc_ver(executable=sys.executable, lib='', version='', chunksize=16384):
         The file is read and scanned in chunks of chunksize bytes.
 
     """
+    if executable is None:
+        try:
+            ver = os.confstr('CS_GNU_LIBC_VERSION')
+            # parse 'glibc 2.28' as ('glibc', '2.28')
+            parts = ver.split(maxsplit=1)
+            if len(parts) == 2:
+                return tuple(parts)
+        except (AttributeError, ValueError, OSError):
+            # os.confstr() or CS_GNU_LIBC_VERSION value not available
+            pass
+
+        executable = sys.executable
+
     V = _comparable_version
     if hasattr(os.path, 'realpath'):
         # Python 2.2 introduced os.path.realpath(); it is used