Handle dlopen(NULL) failure in glibc fallback (#13)

(cherry picked from commit 601bcf8)

Manually removed type annotation added in that commit to keep Python 2.7
support for Pex vendored Pip.

---------

Co-authored-by: Charlie Marsh <crmarsh416@gmail.com>
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

@@ -49,7 +49,20 @@ def glibc_version_string_ctypes():
     # 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: