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
+ * adding back any hardcoded global connection settings (at present, there
- * fallback_application_name of 'citus').
+ * 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/expected/mx_application_name.out

@@ -0,0 +1,14 @@
+CREATE SCHEMA mx_app_name;
+CREATE TABLE output (line text);
+-- a hack to run command with another user
+\COPY output FROM PROGRAM 'PGAPPNAME=mx_app_name psql -c "CREATE TABLE dist_1(a int)" -h localhost -p 57636 -U postgres -d regression'
+\COPY output FROM PROGRAM 'PGAPPNAME=mx_app_name psql -c "SELECT create_distributed_table(''dist_1'', ''a'');" -h localhost -p 57636 -U postgres -d regression'
+-- ensure the command executed fine
+SELECT count(*) FROM pg_dist_partition WHERE logicalrelid = 'dist_1'::regclass;
+ count
+---------------------------------------------------------------------
+     1
+(1 row)
+
+\COPY output FROM PROGRAM 'PGAPPNAME=mx_app_name psql -c "DROP TABLE dist_1;" -h localhost -p 57636 -U postgres -d regression'
+DROP SCHEMA mx_app_name CASCADE;

src/test/regress/multi_mx_schedule

@@ -59,6 +59,7 @@ test: locally_execute_intermediate_results
 test: multi_mx_alter_distributed_table
 test: update_colocation_mx
 test: resync_metadata_with_sequences
+test: mx_application_name
 
 # should be executed sequentially because it modifies metadata
 test: local_shard_execution_dropped_column

src/test/regress/sql/mx_application_name.sql

@@ -0,0 +1,14 @@
+CREATE SCHEMA mx_app_name;
+
+CREATE TABLE output (line text);
+
+-- a hack to run command with another user
+\COPY output FROM PROGRAM 'PGAPPNAME=mx_app_name psql -c "CREATE TABLE dist_1(a int)" -h localhost -p 57636 -U postgres -d regression'
+\COPY output FROM PROGRAM 'PGAPPNAME=mx_app_name psql -c "SELECT create_distributed_table(''dist_1'', ''a'');" -h localhost -p 57636 -U postgres -d regression'
+
+-- ensure the command executed fine
+SELECT count(*) FROM pg_dist_partition WHERE logicalrelid = 'dist_1'::regclass;
+
+\COPY output FROM PROGRAM 'PGAPPNAME=mx_app_name psql -c "DROP TABLE dist_1;" -h localhost -p 57636 -U postgres -d regression'
+
+DROP SCHEMA mx_app_name CASCADE;