Thread: Add dblink function to check if a named connection exists

I have locked down access to all dblink_* functions, so that only
certain privileged users have access to them, and instead provide a set
of SRF functions defined as security definer functions, where I connect
to the remote server, fetch some data, disconnect from remote server,
and return the data.
One obvious disadvantage of this approach, is that I need to connect and
disconnect in every function. A possible solution to this, would be
having a function f.ex dblink_exists('connection_name') that returns
true/false depending on whether the  connection already exists. This
way, I could just check if a named connection exists, and establish a
connection if not, and wait until the end of the session to disconnect
all established connections.

I've attached a patch with a suggested implementation of such a function.


--
Tommy Gildseth
Index: dblink.c
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/dblink/dblink.c,v
retrieving revision 1.73
diff -c -c -r1.73 dblink.c
*** dblink.c    4 Apr 2008 17:02:56 -0000    1.73
--- dblink.c    28 May 2008 08:06:23 -0000
***************
*** 192,208 ****
                  freeconn = true; \
              } \
      } while (0)
-
  #define DBLINK_GET_NAMED_CONN \
      do { \
!             char *conname = text_to_cstring(PG_GETARG_TEXT_PP(0)); \
              rconn = getConnectionByName(conname); \
              if(rconn) \
                  conn = rconn->conn; \
-             else \
-                 DBLINK_CONN_NOT_AVAIL; \
      } while (0)

  #define DBLINK_INIT \
      do { \
              if (!pconn) \
--- 192,214 ----
                  freeconn = true; \
              } \
      } while (0)
  #define DBLINK_GET_NAMED_CONN \
      do { \
!             char *conname = NULL; \
!             DBLINK_GET_NAMED_CONN_IF_EXISTS; \
!             if(!rconn) \
!                 DBLINK_CONN_NOT_AVAIL; \
!     } while (0)
!
! #define DBLINK_GET_NAMED_CONN_IF_EXISTS \
!     do { \
!             conname = text_to_cstring(PG_GETARG_TEXT_PP(0)); \
              rconn = getConnectionByName(conname); \
              if(rconn) \
                  conn = rconn->conn; \
      } while (0)

+
  #define DBLINK_INIT \
      do { \
              if (!pconn) \
***************
*** 1056,1061 ****
--- 1062,1090 ----
      PG_RETURN_INT32(PQisBusy(conn));
  }

+
+ /*
+  * Checks if a given named remote connection exists
+  *
+  * Returns 1 if the connection is busy, 0 otherwise
+  * Params:
+  *    text connection_name - name of the connection to check
+  *
+  */
+ PG_FUNCTION_INFO_V1(dblink_exists);
+ Datum
+ dblink_exists(PG_FUNCTION_ARGS)
+ {
+     PGconn       *conn = NULL;
+     remoteConn *rconn = NULL;
+     char *conname = NULL;
+
+     DBLINK_INIT;
+     DBLINK_GET_NAMED_CONN_IF_EXISTS;
+
+     PG_RETURN_BOOL(conn != NULL);
+ }
+
  /*
   * Cancels a running request on a connection
   *
Index: dblink.h
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/dblink/dblink.h,v
retrieving revision 1.20
diff -c -c -r1.20 dblink.h
*** dblink.h    4 Apr 2008 16:57:21 -0000    1.20
--- dblink.h    28 May 2008 08:06:23 -0000
***************
*** 49,54 ****
--- 49,55 ----
  extern Datum dblink_get_result(PG_FUNCTION_ARGS);
  extern Datum dblink_get_connections(PG_FUNCTION_ARGS);
  extern Datum dblink_is_busy(PG_FUNCTION_ARGS);
+ extern Datum dblink_exists(PG_FUNCTION_ARGS);
  extern Datum dblink_cancel_query(PG_FUNCTION_ARGS);
  extern Datum dblink_error_message(PG_FUNCTION_ARGS);
  extern Datum dblink_exec(PG_FUNCTION_ARGS);
Index: dblink.sql.in
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/dblink/dblink.sql.in,v
retrieving revision 1.17
diff -c -c -r1.17 dblink.sql.in
*** dblink.sql.in    5 Apr 2008 02:44:42 -0000    1.17
--- dblink.sql.in    28 May 2008 08:06:23 -0000
***************
*** 178,183 ****
--- 178,188 ----
  AS 'MODULE_PATHNAME', 'dblink_is_busy'
  LANGUAGE C STRICT;

+ CREATE OR REPLACE FUNCTION dblink_exists(text)
+ RETURNS boolean
+ AS 'MODULE_PATHNAME', 'dblink_exists'
+ LANGUAGE C STRICT;
+
  CREATE OR REPLACE FUNCTION dblink_get_result(text)
  RETURNS SETOF record
  AS 'MODULE_PATHNAME', 'dblink_get_result'
Index: expected/dblink.out
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/dblink/expected/dblink.out,v
retrieving revision 1.23
diff -c -c -r1.23 dblink.out
*** expected/dblink.out    6 Apr 2008 16:54:48 -0000    1.23
--- expected/dblink.out    28 May 2008 08:06:23 -0000
***************
*** 731,736 ****
--- 731,748 ----
                0
  (1 row)

+ SELECT dblink_exists('dtest1');
+  dblink_exists
+ ---------------
+  t
+ (1 row)
+
+ SELECT dblink_exists('doesnotexist');
+  dblink_exists
+ ---------------
+  f
+ (1 row)
+
  SELECT dblink_disconnect('dtest1');
   dblink_disconnect
  -------------------
Index: sql/dblink.sql
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/dblink/sql/dblink.sql,v
retrieving revision 1.20
diff -c -c -r1.20 dblink.sql
*** sql/dblink.sql    6 Apr 2008 16:54:48 -0000    1.20
--- sql/dblink.sql    28 May 2008 08:06:23 -0000
***************
*** 351,356 ****
--- 351,359 ----

  SELECT dblink_is_busy('dtest1');

+ SELECT dblink_exists('dtest1');
+ SELECT dblink_exists('doesnotexist');
+
  SELECT dblink_disconnect('dtest1');
  SELECT dblink_disconnect('dtest2');
  SELECT dblink_disconnect('dtest3');

Tommy Gildseth <tommy.gildseth@usit.uio.no> writes:
> One obvious disadvantage of this approach, is that I need to connect and 
> disconnect in every function. A possible solution to this, would be 
> having a function f.ex dblink_exists('connection_name') that returns 
> true/false depending on whether the  connection already exists.

Can't you do this already?
SELECT 'myconn' = ANY (dblink_get_connections());

A dedicated function might be a tad faster, but it probably isn't going
to matter compared to the overhead of sending a remote query.
        regards, tom lane

Tom Lane wrote:
> Tommy Gildseth <tommy.gildseth@usit.uio.no> writes:
>> One obvious disadvantage of this approach, is that I need to connect and 
>> disconnect in every function. A possible solution to this, would be 
>> having a function f.ex dblink_exists('connection_name') that returns 
>> true/false depending on whether the  connection already exists.
> 
> Can't you do this already?
> 
>     SELECT 'myconn' = ANY (dblink_get_connections());
> 
> A dedicated function might be a tad faster, but it probably isn't going
> to matter compared to the overhead of sending a remote query.

I agree. The above is about as simple as  SELECT dblink_exists('dtest1');
and probably not measurably slower. If you still think a dedicated 
function is needed, please send the output of some performance testing 
to justify it.

If you really want the notational simplicity, you could use an SQL 
function to wrap it:

CREATE OR REPLACE FUNCTION dblink_exists(text)
RETURNS bool AS $$  SELECT $1 = ANY (dblink_get_connections())
$$ LANGUAGE sql;

contrib_regression=# SELECT dblink_exists('dtest1'); dblink_exists
--------------- f
(1 row)

I guess it might be worthwhile adding the SQL function definition to 
dblink.sql.in as an enhancement in 8.4.

Joe

Joe Conway wrote:

> 
> If you really want the notational simplicity, you could use an SQL 
> function to wrap it:
> 
> CREATE OR REPLACE FUNCTION dblink_exists(text)
> RETURNS bool AS $$
>   SELECT $1 = ANY (dblink_get_connections())
> $$ LANGUAGE sql;


Thanks, that seems like a reasonable way to solve this.


-- 
Tommy Gildseth

Joe Conway wrote:
> Tom Lane wrote:
>> Tommy Gildseth <tommy.gildseth@usit.uio.no> writes:
>>> One obvious disadvantage of this approach, is that I need to connect 
>>> and disconnect in every function. A possible solution to this, would 
>>> be having a function f.ex dblink_exists('connection_name') that 
>>> returns true/false depending on whether the  connection already exists.
>>
>> Can't you do this already?
>>
>>     SELECT 'myconn' = ANY (dblink_get_connections());
>>
>> A dedicated function might be a tad faster, but it probably isn't going
>> to matter compared to the overhead of sending a remote query.
> 
> I agree. The above is about as simple as
>   SELECT dblink_exists('dtest1');
> and probably not measurably slower. If you still think a dedicated 
> function is needed, please send the output of some performance testing 
> to justify it.
> 
> If you really want the notational simplicity, you could use an SQL 
> function to wrap it:
> 
> CREATE OR REPLACE FUNCTION dblink_exists(text)
> RETURNS bool AS $$
>   SELECT $1 = ANY (dblink_get_connections())
> $$ LANGUAGE sql;



dblink_get_connections() returns null if there are no connections 
though, so the above will fail if you haven't already established a 
connection, unless you also check for null, and not just false.

I guess you could rewrite the above function to something like:

CREATE OR REPLACE FUNCTION dblink_exists(text)
RETURNS bool AS $$  SELECT COALESCE($1 = ANY (dblink_get_connections()), false)
$$ LANGUAGE sql;



-- 
Tommy Gildseth