Remove the reliance on socket.getfqdn()

Author: rayluo

msal/managed_identity.py

@@ -6,7 +6,6 @@
 import json
 import logging
 import os
-import socket
 import sys
 import time
 from urllib.parse import urlparse  # Python 3+
@@ -146,7 +145,10 @@ class ManagedIdentityClient(object):
         (like what a ``PublicClientApplication`` does),
         not a token with application permissions for an app.
     """
-    __instance, _tenant = None, "managed_identity"  # Placeholders
+    __instance = "localhost"  # We used to get this value from socket.getfqdn()
+        # but it is unreliable because getfqdn() either hangs or returns empty value
+        # on some misconfigured machines
+    _tenant = "managed_identity"
     _TOKEN_SOURCE = "token_source"
     _TOKEN_SOURCE_IDP = "identity_provider"
     _TOKEN_SOURCE_CACHE = "cache"
@@ -252,11 +254,6 @@ def __init__(
         self._token_cache = token_cache or TokenCache()
         self._client_capabilities = client_capabilities
 
-    def _get_instance(self):
-        if self.__instance is None:
-            self.__instance = socket.getfqdn()  # Moved from class definition to here
-        return self.__instance
-
     def acquire_token_for_client(
         self,
         *,
@@ -302,7 +299,7 @@ def acquire_token_for_client(
                 target=[resource],
                 query=dict(
                     client_id=client_id_in_cache,
-                    environment=self._get_instance(),
+                    environment=self.__instance,
                     realm=self._tenant,
                     home_account_id=None,
                 ),
@@ -344,7 +341,7 @@ def acquire_token_for_client(
                     client_id=client_id_in_cache,
                     scope=[resource],
                     token_endpoint="https://{}/{}".format(
-                        self._get_instance(), self._tenant),
+                        self.__instance, self._tenant),
                     response=result,
                     params={},
                     data={},

tests/test_mi.py

@@ -190,8 +190,12 @@ def test_happy_path_of_vm(self):
             headers={'Metadata': 'true'},
             )
 
-    @patch("msal.managed_identity.socket.getfqdn", new=lambda: "MixedCaseHostName")
-    def test_happy_path_of_windows_vm(self):
+    @patch.object(ManagedIdentityClient, "_ManagedIdentityClient__instance", "MixedCaseHostName")
+    def test_happy_path_of_theoretical_mixed_case_hostname(self):
+        """Historically, we used to get the host name from socket.getfqdn(),
+        which could return a mixed-case host name on Windows.
+        Although we no longer use getfqdn(), we still keep this test case to ensure we tolerate it.
+        """
         self.test_happy_path_of_vm()
 
     @patch.dict(os.environ, {"AZURE_POD_IDENTITY_AUTHORITY_HOST": "http://localhost:1234//"})