Handle dlopen(NULL) failure in glibc fallback (#12716)

Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

news/12716.bugfix.rst

@@ -0,0 +1 @@
+Avoid dlopen failure for glibc detection in musl builds

src/pip/_internal/utils/glibc.py

@@ -40,7 +40,20 @@ def glibc_version_string_ctypes() -> Optional[str]:
     # manpage says, "If filename is NULL, then the returned handle is for the
     # main program". This way we can let the linker do the work to figure out
     # which libc our process is actually using.
-    process_namespace = ctypes.CDLL(None)
+    #
+    # We must also handle the special case where the executable is not a
+    # dynamically linked executable. This can occur when using musl libc,
+    # for example. In this situation, dlopen() will error, leading to an
+    # OSError. Interestingly, at least in the case of musl, there is no
+    # errno set on the OSError. The single string argument used to construct
+    # OSError comes from libc itself and is therefore not portable to
+    # hard code here. In any case, failure to call dlopen() means we
+    # can't proceed, so we bail on our attempt.
+    try:
+        process_namespace = ctypes.CDLL(None)
+    except OSError:
+        return None
+
     try:
         gnu_get_libc_version = process_namespace.gnu_get_libc_version
     except AttributeError:
@@ -50,7 +63,7 @@ def glibc_version_string_ctypes() -> Optional[str]:
 
     # Call gnu_get_libc_version, which returns a string like "2.5"
     gnu_get_libc_version.restype = ctypes.c_char_p
-    version_str = gnu_get_libc_version()
+    version_str: str = gnu_get_libc_version()
     # py2 / py3 compatibility:
     if not isinstance(version_str, str):
         version_str = version_str.decode("ascii")