postgres_fdw: Add functions to discard cached connections.

This commit introduces two new functions postgres_fdw_disconnect()
and postgres_fdw_disconnect_all(). The former function discards
the cached connections to the specified foreign server. The latter discards
all the cached connections. If the connection is used in the current
transaction, it's not closed and a warning message is emitted.

For example, these functions are useful when users want to explicitly
close the foreign server connections that are no longer necessary and
then to prevent them from eating up the foreign servers connections
capacity.

Author: Bharath Rupireddy, tweaked a bit by Fujii Masao
Reviewed-by: Alexey Kondratov, Zhijie Hou, Zhihong Yu, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACVvrp5=AVp2PupEm+nAC8S4buqR3fJMmaCoc7ftT0aD2A@mail.gmail.com

1 files changed, 94 insertions, 4 deletions

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 28b82f5f9dc..9473ab07623 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -19,7 +19,10 @@ DO $d$
             OPTIONS (dbname '$$||current_database()||$$',
                      port '$$||current_setting('port')||$$'
             )$$;
-
+        EXECUTE $$CREATE SERVER loopback4 FOREIGN DATA WRAPPER postgres_fdw
+            OPTIONS (dbname '$$||current_database()||$$',
+                     port '$$||current_setting('port')||$$'
+            )$$;
     END;
 $d$;
 
@@ -28,6 +31,7 @@ CREATE USER MAPPING FOR public SERVER testserver1
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback2;
 CREATE USER MAPPING FOR public SERVER loopback3;
+CREATE USER MAPPING FOR public SERVER loopback4;
 
 -- ===================================================================
 -- create objects used through FDW loopback server
@@ -154,6 +158,12 @@ CREATE FOREIGN TABLE ft7 (
 	c3 text
 ) SERVER loopback3 OPTIONS (schema_name 'S 1', table_name 'T 4');
 
+CREATE FOREIGN TABLE ft8 (
+	c1 int NOT NULL,
+	c2 int NOT NULL,
+	c3 text
+) SERVER loopback4 OPTIONS (schema_name 'S 1', table_name 'T 4');
+
 -- ===================================================================
 -- tests for validator
 -- ===================================================================
@@ -2710,9 +2720,9 @@ COMMIT;
 -- Clean up
 DROP PROCEDURE terminate_backend_and_wait(text);
 
--- ===================================================================
--- test connection invalidation cases
--- ===================================================================
+-- =============================================================================
+-- test connection invalidation cases and postgres_fdw_get_connections function
+-- =============================================================================
 -- This test case is for closing the connection in pgfdw_xact_callback
 BEGIN;
 -- List all the existing cached connections. Only loopback2 should be output.
@@ -2739,6 +2749,86 @@ COMMIT;
 -- the above transaction.
 SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
 
+-- =======================================================================
+-- test postgres_fdw_disconnect and postgres_fdw_disconnect_all functions
+-- =======================================================================
+-- Return true as all cached connections are closed.
+SELECT postgres_fdw_disconnect_all();
+-- Ensure to cache loopback connection.
+SELECT 1 FROM ft1 LIMIT 1;
+BEGIN;
+-- Ensure to cache loopback2 connection.
+SELECT 1 FROM ft6 LIMIT 1;
+-- List all the existing cached connections. loopback and loopback2 should be
+-- output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+-- Issue a warning and return false as loopback2 connection is still in use and
+-- can not be closed.
+SELECT postgres_fdw_disconnect('loopback2');
+-- Close loopback connection, return true and issue a warning as loopback2
+-- connection is still in use and can not be closed.
+SELECT postgres_fdw_disconnect_all();
+-- List all the existing cached connections. loopback2 should be output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+-- Ensure to cache loopback connection.
+SELECT 1 FROM ft1 LIMIT 1;
+-- Ensure to cache loopback4 connection.
+SELECT 1 FROM ft8 LIMIT 1;
+-- List all the existing cached connections. loopback, loopback2, loopback4
+-- should be output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+DROP SERVER loopback4 CASCADE;
+-- Return false as connections are still in use, warnings are issued.
+SELECT postgres_fdw_disconnect_all();
+COMMIT;
+-- Close loopback2 connection and return true.
+SELECT postgres_fdw_disconnect('loopback2');
+-- List all the existing cached connections. loopback should be output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+-- Return false as loopback2 connectin is closed already.
+SELECT postgres_fdw_disconnect('loopback2');
+-- Return an error as there is no foreign server with given name.
+SELECT postgres_fdw_disconnect('unknownserver');
+-- Close loopback connection and return true.
+SELECT postgres_fdw_disconnect_all();
+-- List all the existing cached connections. No connection exists, so NULL
+-- should be output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+
+-- =============================================================================
+-- test case for having multiple cached connections for a foreign server
+-- =============================================================================
+CREATE ROLE multi_conn_user1 SUPERUSER;
+CREATE ROLE multi_conn_user2 SUPERUSER;
+CREATE USER MAPPING FOR multi_conn_user1 SERVER loopback;
+CREATE USER MAPPING FOR multi_conn_user2 SERVER loopback;
+
+-- Will cache loopback connection with user mapping for multi_conn_user1
+SET ROLE multi_conn_user1;
+SELECT 1 FROM ft1 LIMIT 1;
+RESET ROLE;
+
+-- Will cache loopback connection with user mapping for multi_conn_user2
+SET ROLE multi_conn_user2;
+SELECT 1 FROM ft1 LIMIT 1;
+RESET ROLE;
+
+-- Should output two connections for loopback server
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+
+-- Close loopback connections and return true.
+SELECT postgres_fdw_disconnect('loopback');
+
+-- List all the existing cached connections. No connection exists, so NULL
+-- should be output.
+SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
+
+-- Clean up
+DROP USER MAPPING FOR multi_conn_user1 SERVER loopback;
+DROP USER MAPPING FOR multi_conn_user2 SERVER loopback;
+DROP ROLE multi_conn_user1;
+DROP ROLE multi_conn_user2;
+
 -- ===================================================================
 -- batch insert
 -- ===================================================================