Use a fixed application_name while connecting to remote nodes

Citus heavily relies on application_name, see
`IsCitusInitiatedRemoteBackend()`.

But if the user set the application name, such as export PGAPPNAME=test_name,
Citus uses that name while connecting to the remote node.

With this commit, we ensure that Citus always connects with
the "citus" user name to the remote nodes.

src/backend/distributed/connection/connection_configuration.c

@@ -72,8 +72,8 @@ InitConnParams()
 /*
  * ResetConnParams frees all strings in the keywords and parameters arrays,
  * sets their elements to null, and resets the ConnParamsSize to zero before
- * adding back any hardcoded global connection settings (at present, only the
- * fallback_application_name of 'citus').
+ * adding back any hardcoded global connection settings (at present, there
+ * are no).
  */
 void
 ResetConnParams()
@@ -89,8 +89,6 @@ ResetConnParams()
 	ConnParams.size = 0;
 
 	InvalidateConnParamsHashEntries();
-
-	AddConnParam("fallback_application_name", CITUS_APPLICATION_NAME);
 }
 
 
@@ -253,14 +251,16 @@ GetConnParams(ConnectionHashKey *key, char ***keywords, char ***values,
 		"port",
 		"dbname",
 		"user",
-		"client_encoding"
+		"client_encoding",
+		"application_name"
 	};
 	const char *runtimeValues[] = {
 		key->hostname,
 		nodePortString,
 		key->database,
 		key->user,
-		GetDatabaseEncodingName()
+		GetDatabaseEncodingName(),
+		CITUS_APPLICATION_NAME
 	};
 
 	/*

src/test/regress/citus_tests/config.py

@@ -293,6 +293,7 @@ class CitusUnusualQuerySettingsConfig(CitusMXBaseClusterConfig):
             "citus.recover_2pc_interval": "1s",
             "citus.remote_task_check_interval": "1ms",
             "citus.values_materialization_threshold": "0",
+            "application_name": "non_citus_app_name"
         }
 
 
@@ -317,7 +318,6 @@ class CitusSingleShardClusterConfig(CitusMXBaseClusterConfig):
         super().__init__(arguments)
         self.new_settings = {"citus.shard_count": 1}
 
-
 class CitusNonMxClusterConfig(CitusMXBaseClusterConfig):
     def __init__(self, arguments):
         super().__init__(arguments)