Обсуждение: LISTEN/NOTIFY problem

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LISTEN/NOTIFY problem

От
Dmitri Girski
Дата:
Hi everybody,

I've got a weird problem with LISTEN/NOTIFY.

My C++ app subscribes for the notifications, just like in libpq examples:

The only difference, that I am setting the timeout on select just to check if application wants to exit.

//open session

//subscribe

while(!exit)
{
    sock = PGsocket(conn)
     
    res = select(sock);

    if (res)

   //check if it timeout
   //or if there was a notification


}

The problem that I am facing is that after some time notifications stop coming.
select() returns on timeout and nothing else.

Just as a test a included UNLISTEN/LISTEN sequence into the loop, so after timeout or event application re-subscribes. And this helps.

The question is, what I am doing wrong with the code in the first place?

Any help is appreciated.

Cheers,
Dmitri.










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Re: LISTEN/NOTIFY problem

От
"Harvey, Allan AC"
Дата:
 
-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org]On Behalf Of Dmitri Girski
Sent: Monday, 23 March 2009 10:00 PM
To: pgsql-general@postgresql.org
Subject: [GENERAL] LISTEN/NOTIFY problem

Hi everybody,

I've got a weird problem with LISTEN/NOTIFY.

My C++ app subscribes for the notifications, just like in libpq examples:

The only difference, that I am setting the timeout on select just to check if application wants to exit.

//open session

//subscribe

while(!exit)
{
    sock = PGsocket(conn)
     
    res = select(sock);

    if (res)

   //check if it timeout
   //or if there was a notification


}

The problem that I am facing is that after some time notifications stop coming.
select() returns on timeout and nothing else.

Just as a test a included UNLISTEN/LISTEN sequence into the loop, so after timeout or event application re-subscribes. And this helps.

The question is, what I am doing wrong with the code in the first place?

Any help is appreciated.

Cheers,
Dmitri.










--
@Gmail
 
Cannot tell what is wrong with you piece of code.
But below is a routine I use for notifications.
No problems with it so far.
 
Hope it helps.
 
Allan
 
 /*
** Structure to hold the connection data
*/
typedef struct _condetails
        {
        char *pghost;
        char *pgport;
        char *pgoptions;
        char *pgtty;
        char *dbname;
        char *pguser;
        char *pgpswd;
        PGconn *conn;
        int bpid;
        } CONDETAILS;
 
 
/*
** Register for a database notification
*/
int reg_notification( CONDETAILS *cd, const char *notif )
{
                char *sqlnotify, sql[128];
                PGconn* conn;
                PGresult* res;
 
        conn = cd->conn;
 
        sqlnotify = "listen %s";
 
        /*
        ** check to see that the backend connection was successfully made
        */
        if ( PQstatus( conn ) == CONNECTION_BAD )
                {
                sysErr( "reg_notification(). %s", PQerrorMessage( conn ) );
                PQfinish( conn );
                condetails( cd );
                return -1;
                }
 
        /*
        ** Register
        */
        sprintf( sql, sqlnotify, notif );
        res = PQexec( conn, sql );
        if ( PQresultStatus( res ) != PGRES_COMMAND_OK )
                {
                sysErr( "reg_notification(). listen command failed. %s", PQerrorMessage( conn ) );
                PQclear( res );
                return -1;
                }
 
        PQclear( res );
 
        return 0;
}
/*
** Wait for a database notification or time out
** This is an async method.
**
** Return only the first notification not generated by me
** and flush the rest.
**
** Probly should do something smarter.
*/
int wait_db_notification( CONDETAILS *cd, char *notif, const long sec, const long usec )
{
                int sock, ret;
                fd_set input_mask;
                PGconn* conn;
                PGnotify *notify;
                struct timeval timeout;
 
        conn = cd->conn;
 
        /*
        ** check to see that the backend connection was successfully made
        */
        if ( PQstatus( conn ) == CONNECTION_BAD )
                {
                sysErr( "wait_db_notification(). %s", PQerrorMessage( conn ) );
                PQfinish( conn );
                condetails( cd );
                return -1;
                }
 
        sock = PQsocket( conn );
        if ( sock < 0 )
                {
                sysErr( "wait_db_notification(). Could not get socket descriptor. %s", PQerrorMessage( conn ) );
                return -1;
                }
 
        FD_ZERO( &input_mask );
        FD_SET( sock, &input_mask );
        timeout.tv_sec = sec;
        timeout.tv_usec = usec;
 
        ret = select( sock + 1, &input_mask, NULL, NULL, &timeout );
        if ( ret < 0 )
                {
                sysErr( "%d: %s", __LINE__, "wait_db_notification(). Select on db socket failed" );
 
                if ( errno != EINTR )
                        exit( 1 );
 
                /*
                ** Been interrupted by a trappable signal
                ** Force going to the top of the loop to handle it.
-                */
                return -1;
                }
        if ( ret == 0 )
                {
                /*
                ** Time out occurred.
                */
                return 0;
                }
 
        /*
        ** Some activity on the db
        */
        ret = -1;
        PQconsumeInput( conn );
        do
                {
                notify = PQnotifies( conn );
                if ( notify != NULL )
                        {
                        if ( cd->bpid != notify->be_pid )
                                {
                                strcpy( notif, notify->relname );
                                ret = 1;
                                }
 
                        PQfreemem( notify );
                        }
                }
        while ( notify != NULL );
 
        return ret;
}
 
 

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Re: LISTEN/NOTIFY problem

От
Dmitri Girski
Дата:
Thank you guys for your replies!

It is definitely something wrong with my code, here is the thread which does the listening:
http://mitek.id.au/flex/DBListener.cpp

The rest of application is fairly big and does not relate to the database.
For libpq calls I am using the SPTK (http://sptk.net) library. I've added additional wrappers for PGsocket, PQfreemem.

So the current code works without any problem, but as soon as I remove subcribe() function from the loop and call it just once, daemon eventually will stop getting notifications.
There is nothing wrong with the socket or db connections - all possible error will be logged.


Cheers,
Dmitri.


On Mon, Mar 23, 2009 at 10:00 PM, Dmitri Girski <mitek@lizardsw.com> wrote:
Hi everybody,

I've got a weird problem with LISTEN/NOTIFY.

My C++ app subscribes for the notifications, just like in libpq examples:

The only difference, that I am setting the timeout on select just to check if application wants to exit.

//open session

//subscribe

while(!exit)
{
    sock = PGsocket(conn)
     
    res = select(sock);

    if (res)

   //check if it timeout
   //or if there was a notification


}

The problem that I am facing is that after some time notifications stop coming.
select() returns on timeout and nothing else.

Just as a test a included UNLISTEN/LISTEN sequence into the loop, so after timeout or event application re-subscribes. And this helps.

The question is, what I am doing wrong with the code in the first place?

Any help is appreciated.

Cheers,
Dmitri.










--
@Gmail



--
@Gmail