postgres_fdw: Add function to list cached connections to foreign servers.

This commit adds function postgres_fdw_get_connections() to return
the foreign server names of all the open connections that postgres_fdw
established from the local session to the foreign servers. This function
also returns whether each connection is valid or not.

This function is useful when checking all the open foreign server connections.
If we found some connection to drop, from the result of function, probably
we can explicitly close them by the function that upcoming commit will add.

This commit bumps the version of postgres_fdw to 1.1 since it adds
new function.

Author: Bharath Rupireddy, tweaked by Fujii Masao
Reviewed-by: Zhijie Hou, Alexey Kondratov, Zhihong Yu, Fujii Masao
Discussion: https://postgr.es/m/2d5cb0b3-a6e8-9bbb-953f-879f47128faa@oss.nttdata.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -13,12 +13,18 @@ DO $d$
             OPTIONS (dbname '$$||current_database()||$$',
                      port '$$||current_setting('port')||$$'
             )$$;
+        EXECUTE $$CREATE SERVER loopback3 FOREIGN DATA WRAPPER postgres_fdw
+            OPTIONS (dbname '$$||current_database()||$$',
+                     port '$$||current_setting('port')||$$'
+            )$$;
+
     END;
 $d$;
 CREATE USER MAPPING FOR public SERVER testserver1
 	OPTIONS (user 'value', password 'value');
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback2;
+CREATE USER MAPPING FOR public SERVER loopback3;
 -- ===================================================================
 -- create objects used through FDW loopback server
 -- ===================================================================
@@ -129,6 +135,11 @@ CREATE FOREIGN TABLE ft6 (
 	c2 int NOT NULL,
 	c3 text
 ) SERVER loopback2 OPTIONS (schema_name 'S 1', table_name 'T 4');
+CREATE FOREIGN TABLE ft7 (
+	c1 int NOT NULL,
+	c2 int NOT NULL,
+	c3 text
+) SERVER loopback3 OPTIONS (schema_name 'S 1', table_name 'T 4');
 -- ===================================================================
 -- tests for validator
 -- ===================================================================
@@ -199,7 +210,8 @@ ALTER FOREIGN TABLE ft2 ALTER COLUMN c1 OPTIONS (column_name 'C 1');
  public | ft4   | loopback  | (schema_name 'S 1', table_name 'T 3') | 
  public | ft5   | loopback  | (schema_name 'S 1', table_name 'T 4') | 
  public | ft6   | loopback2 | (schema_name 'S 1', table_name 'T 4') | 
-(5 rows)
+ public | ft7   | loopback3 | (schema_name 'S 1', table_name 'T 4') | 
+(6 rows)
 
 -- Test that alteration of server options causes reconnection
 -- Remote's errors might be non-English, so hide them to ensure stable results
@@ -9040,6 +9052,13 @@ DROP PROCEDURE terminate_backend_and_wait(text);
 -- ===================================================================
 -- This test case is for closing the connection in pgfdw_xact_callback
 BEGIN;
+-- List all the existing cached connections. Only loopback2 should be output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+ server_name | valid 
+-------------+-------
+ loopback2   | t
+(1 row)
+
 -- Connection xact depth becomes 1 i.e. the connection is in midst of the xact.
 SELECT 1 FROM ft1 LIMIT 1;
  ?column? 
@@ -9047,9 +9066,49 @@ SELECT 1 FROM ft1 LIMIT 1;
         1
 (1 row)
 
+SELECT 1 FROM ft7 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+-- List all the existing cached connections. loopback and loopback3
+-- also should be output as valid connections.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+ server_name | valid 
+-------------+-------
+ loopback    | t
+ loopback2   | t
+ loopback3   | t
+(3 rows)
+
 -- Connection is not closed at the end of the alter statement in
 -- pgfdw_inval_callback. That's because the connection is in midst of this
 -- xact, it is just marked as invalid.
 ALTER SERVER loopback OPTIONS (ADD use_remote_estimate 'off');
+DROP SERVER loopback3 CASCADE;
+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 
+-------------+-------
+ loopback    | f
+ loopback2   | t
+             | f
+(3 rows)
+
 -- The invalid connection gets closed in pgfdw_xact_callback during commit.
 COMMIT;
+-- List all the existing cached connections. loopback and loopback3
+-- should not be output because they should be closed at the end of
+-- the above transaction.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+ server_name | valid 
+-------------+-------
+ loopback2   | t
+(1 row)
+