Properly handle exceptions raised while handling server auth messages (…

…#862)

When server sends us an authentication request message and we fail to
process it, we must terminate the connection and propagate the exception
immediately.  Currently asyncpg will just timeout waiting for
`ReadyForQuery` from the server, which will never arrive.

Fixes: #861

asyncpg/protocol/coreproto.pyx

@@ -5,7 +5,7 @@
 # the Apache 2.0 License: http://www.apache.org/licenses/LICENSE-2.0
 
 
-from hashlib import md5 as hashlib_md5  # for MD5 authentication
+import hashlib
 
 
 include "scram.pyx"
@@ -150,15 +150,28 @@ cdef class CoreProtocol:
     cdef _process__auth(self, char mtype):
         if mtype == b'R':
             # Authentication...
-            self._parse_msg_authentication()
-            if self.result_type != RESULT_OK:
+            try:
+                self._parse_msg_authentication()
+            except Exception as ex:
+                # Exception in authentication parsing code
+                # is usually either malformed authentication data
+                # or missing support for cryptographic primitives
+                # in the hashlib module.
+                self.result_type = RESULT_FAILED
+                self.result = apg_exc.InternalClientError(
+                    f"unexpected error while performing authentication: {ex}")
+                self.result.__cause__ = ex
                 self.con_status = CONNECTION_BAD
                 self._push_result()
+            else:
+                if self.result_type != RESULT_OK:
+                    self.con_status = CONNECTION_BAD
+                    self._push_result()
 
-            elif self.auth_msg is not None:
-                # Server wants us to send auth data, so do that.
-                self._write(self.auth_msg)
-                self.auth_msg = None
+                elif self.auth_msg is not None:
+                    # Server wants us to send auth data, so do that.
+                    self._write(self.auth_msg)
+                    self.auth_msg = None
 
         elif mtype == b'K':
             # BackendKeyData
@@ -634,7 +647,7 @@ cdef class CoreProtocol:
 
         # 'md5' + md5(md5(password + username) + salt))
         userpass = ((self.password or '') + (self.user or '')).encode('ascii')
-        hash = hashlib_md5(hashlib_md5(userpass).hexdigest().\
+        hash = hashlib.md5(hashlib.md5(userpass).hexdigest().\
                 encode('ascii') + salt).hexdigest().encode('ascii')
 
         msg.write_bytestring(b'md5' + hash)

tests/test_connect.py

@@ -359,8 +359,13 @@ async def test_auth_password_scram_sha_256(self):
         await self.con.execute(alter_password)
         await self.con.execute("SET password_encryption = 'md5';")
 
-    async def test_auth_unsupported(self):
-        pass
+    @unittest.mock.patch('hashlib.md5', side_effect=ValueError("no md5"))
+    async def test_auth_md5_unsupported(self, _):
+        with self.assertRaisesRegex(
+            exceptions.InternalClientError,
+            ".*no md5.*",
+        ):
+            await self.connect(user='md5_user', password='correctpassword')
 
 
 class TestConnectParams(tb.TestCase):