postgres_fdw: Extend postgres_fdw_get_connections to return user name.

This commit adds a "user_name" output column to
the postgres_fdw_get_connections function, returning the name
of the local user mapped to the foreign server for each connection.
If a public mapping is used, it returns "public."

This helps identify postgres_fdw connections more easily,
such as determining which connections are invalid, closed,
or used within the current transaction.

No extension version bump is needed, as commit c297a47c5f
already handled it for v18~.

Author: Hayato Kuroda
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/b492a935-6c7e-8c08-e485-3c1d64d7d10f@oss.nttdata.com

contrib/postgres_fdw/connection.c

@@ -1997,8 +1997,8 @@ pgfdw_finish_abort_cleanup(List *pending_entries, List *cancel_requested,
 
 /* Number of output arguments (columns) for various API versions */
 #define POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_1	2
-#define POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_2	4
-#define POSTGRES_FDW_GET_CONNECTIONS_COLS	4	/* maximum of above */
+#define POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_2	5
+#define POSTGRES_FDW_GET_CONNECTIONS_COLS	5	/* maximum of above */
 
 /*
  * Internal function used by postgres_fdw_get_connections variants.
@@ -2014,10 +2014,13 @@ pgfdw_finish_abort_cleanup(List *pending_entries, List *cancel_requested,
  *
  * For API version 1.2 and later, this function takes an input parameter
  * to check a connection status and returns the following
- * additional values along with the two values from version 1.1:
+ * additional values along with the three values from version 1.1:
  *
+ * - user_name - the local user name of the active connection. In case the
+ *   user mapping is dropped but the connection is still active, then the
+ *   user name will be NULL in the output.
  * - used_in_xact - true if the connection is used in the current transaction.
- * - closed: true if the connection is closed.
+ * - closed - true if the connection is closed.
  *
  * No records are returned when there are no cached connections at all.
  */
@@ -2056,6 +2059,7 @@ postgres_fdw_get_connections_internal(FunctionCallInfo fcinfo,
 		ForeignServer *server;
 		Datum		values[POSTGRES_FDW_GET_CONNECTIONS_COLS] = {0};
 		bool		nulls[POSTGRES_FDW_GET_CONNECTIONS_COLS] = {0};
+		int			i = 0;
 
 		/* We only look for open remote connections */
 		if (!entry->conn)
@@ -2100,28 +2104,61 @@ postgres_fdw_get_connections_internal(FunctionCallInfo fcinfo,
 			Assert(entry->conn && entry->xact_depth > 0 && entry->invalidated);
 
 			/* Show null, if no server name was found */
-			nulls[0] = true;
+			nulls[i++] = true;
 		}
 		else
-			values[0] = CStringGetTextDatum(server->servername);
+			values[i++] = CStringGetTextDatum(server->servername);
 
-		values[1] = BoolGetDatum(!entry->invalidated);
+		if (api_version >= PGFDW_V1_2)
+		{
+			HeapTuple	tp;
+
+			/* Use the system cache to obtain the user mapping */
+			tp = SearchSysCache1(USERMAPPINGOID, ObjectIdGetDatum(entry->key));
+
+			/*
+			 * Just like in the foreign server case, user mappings can also be
+			 * dropped in the current explicit transaction. Therefore, the
+			 * similar check as in the server case is required.
+			 */
+			if (!HeapTupleIsValid(tp))
+			{
+				/*
+				 * If we reach here, this entry must have been invalidated in
+				 * pgfdw_inval_callback, same as in the server case.
+				 */
+				Assert(entry->conn && entry->xact_depth > 0 &&
+					   entry->invalidated);
+
+				nulls[i++] = true;
+			}
+			else
+			{
+				Oid			userid;
+
+				userid = ((Form_pg_user_mapping) GETSTRUCT(tp))->umuser;
+				values[i++] = CStringGetTextDatum(MappingUserName(userid));
+				ReleaseSysCache(tp);
+			}
+		}
+
+		values[i++] = BoolGetDatum(!entry->invalidated);
 
 		if (api_version >= PGFDW_V1_2)
 		{
 			bool		check_conn = PG_GETARG_BOOL(0);
 
 			/* Is this connection used in the current transaction? */
-			values[2] = BoolGetDatum(entry->xact_depth > 0);
+			values[i++] = BoolGetDatum(entry->xact_depth > 0);
 
 			/*
 			 * If a connection status check is requested and supported, return
 			 * whether the connection is closed. Otherwise, return NULL.
 			 */
 			if (check_conn && pgfdw_conn_checkable())
-				values[3] = BoolGetDatum(pgfdw_conn_check(entry->conn) != 0);
+				values[i++] = BoolGetDatum(pgfdw_conn_check(entry->conn) != 0);
 			else
-				nulls[3] = true;
+				nulls[i++] = true;
 		}
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10472,13 +10472,15 @@ NOTICE:  drop cascades to 2 other objects
 DETAIL:  drop cascades to user mapping for public on server loopback3
 drop cascades to foreign table ft7
 -- List all the existing cached connections. loopback and loopback3
--- should be output as invalid connections. Also the server name for
--- loopback3 should be NULL because the server was dropped.
-SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
- server_name | valid | used_in_xact | closed 
--------------+-------+--------------+--------
- loopback    | f     | t            | 
-             | f     | t            | 
+-- should be output as invalid connections. Also the server name and user name
+-- for loopback3 should be NULL because both server and user mapping were
+-- dropped.
+SELECT server_name, user_name = CURRENT_USER as "user_name = CURRENT_USER", valid, used_in_xact, closed
+FROM postgres_fdw_get_connections() ORDER BY 1;
+ server_name | user_name = CURRENT_USER | valid | used_in_xact | closed 
+-------------+--------------------------+-------+--------------+--------
+ loopback    | t                        | f     | t            | 
+             |                          | f     | t            | 
 (2 rows)
 
 -- The invalid connections get closed in pgfdw_xact_callback during commit.

contrib/postgres_fdw/postgres_fdw--1.1--1.2.sql

@@ -11,7 +11,8 @@ DROP FUNCTION postgres_fdw_get_connections ();
 
 CREATE FUNCTION postgres_fdw_get_connections (
     IN check_conn boolean DEFAULT false, OUT server_name text,
-    OUT valid boolean, OUT used_in_xact boolean, OUT closed boolean)
+    OUT user_name text, OUT valid boolean, OUT used_in_xact boolean,
+    OUT closed boolean)
 RETURNS SETOF record
 AS 'MODULE_PATHNAME', 'postgres_fdw_get_connections_1_2'
 LANGUAGE C STRICT PARALLEL RESTRICTED;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3382,9 +3382,11 @@ SELECT server_name FROM postgres_fdw_get_connections() ORDER BY 1;
 ALTER SERVER loopback OPTIONS (ADD use_remote_estimate 'off');
 DROP SERVER loopback3 CASCADE;
 -- List all the existing cached connections. loopback and loopback3
--- should be output as invalid connections. Also the server name for
--- loopback3 should be NULL because the server was dropped.
-SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+-- should be output as invalid connections. Also the server name and user name
+-- for loopback3 should be NULL because both server and user mapping were
+-- dropped.
+SELECT server_name, user_name = CURRENT_USER as "user_name = CURRENT_USER", valid, used_in_xact, closed
+FROM postgres_fdw_get_connections() ORDER BY 1;
 -- The invalid connections get closed in pgfdw_xact_callback during commit.
 COMMIT;
 -- All cached connections were closed while committing above xact, so no

doc/src/sgml/postgres-fdw.sgml

@@ -779,7 +779,8 @@ OPTIONS (ADD password_required 'false');
    <varlistentry>
     <term><function>postgres_fdw_get_connections(
       IN check_conn boolean DEFAULT false, OUT server_name text,
-      OUT valid boolean, OUT used_in_xact boolean, OUT closed boolean)
+      OUT user_name text, OUT valid boolean, OUT used_in_xact boolean,
+      OUT closed boolean)
       returns setof record</function></term>
     <listitem>
      <para>
@@ -806,10 +807,12 @@ OPTIONS (ADD password_required 'false');
      <para>
       Example usage of the function:
 <screen>
- server_name | valid | used_in_xact | closed
--------------+-------+--------------+--------
- loopback1   | t     | t            |
- loopback2   | f     | t            |
+postgres=# SELECT * FROM postgres_fdw_get_connections(true);
+ server_name | user_name | valid | used_in_xact | closed
+-------------+-----------+-------+--------------+--------
+ loopback1   | postgres  | t     | t            | f
+ loopback2   | public    | t     | t            | f
+ loopback3   |           | f     | t            | f
 </screen>
       The output columns are described in
       <xref linkend="postgres-fdw-get-connections-columns"/>.
@@ -836,6 +839,16 @@ OPTIONS (ADD password_required 'false');
          invalid), this will be <literal>NULL</literal>.
         </entry>
        </row>
+       <row>
+        <entry><structfield>user_name</structfield></entry>
+        <entry><type>text</type></entry>
+        <entry>
+         Name of the local user mapped to the foreign server of this
+         connection, or <literal>public</literal> if a public mapping is used.
+         If the user mapping is dropped but the connection remains open
+         (i.e., marked as invalid), this will be <literal>NULL</literal>.
+        </entry>
+       </row>
        <row>
         <entry><structfield>valid</structfield></entry>
         <entry><type>boolean</type></entry>