Thread

bug in libpgtcl listen

Massimo Dal Zotto <dz@cs.unitn.it> — 1999-01-08T15:41:48Z

Hi,

I found a bug in the notify handler of the libpgtcl interface.
If I have setup a notify handler and the backend dies, the tcl interpreter
will not remove the handler and loop forever with a bad fd in select().
I tested the bug with Tcl7.5 and the following code:

$ wish
% set tcl_version
7.5
% set tk_version
4.1
% source pg_test.tcl
% connect dz
pgsql4
% sql select 123
123
% listen
% notify
% --> callback test

# From another shell kill the backend:
$ kill 3937

# Now the wish shell is looping forever in the following syscall:
$ strace -p 3934
select(5, [0 3 4], [], [], NULL)        = -1 EBADF (Bad file number)
select(5, [0 3 4], [], [], NULL)        = -1 EBADF (Bad file number)
select(5, [0 3 4], [], [], NULL)        = -1 EBADF (Bad file number)
select(5, [0 3 4], [], [], NULL)        = -1 EBADF (Bad file number)
select(5, [0 3 4], [], [], NULL)        = -1 EBADF (Bad file number)
...

# The fd 4 doesn't exists anymore because it has been closed by pqReadData.
$ ls -l /proc/3934/fd
total 0
lrwx------   1 dz       users          64 Jan  8 16:00 0 -> [0801]:2081
lrwx------   1 dz       users          64 Jan  8 16:00 1 -> [0801]:2081
lrwx------   1 dz       users          64 Jan  8 16:00 2 -> [0801]:2081
lrwx------   1 dz       users          64 Jan  8 16:00 3 -> [0000]:12617

This is the test file loaded in the above example:

# pg_test.tcl -- 
#
# Test libpgtcl interface

load libpgtcl.so.2

proc connect {{db dz}} {
    global conn
    set conn [pg_connect $db]
}

proc sql {args} {
    global conn
    if {$args != {}} {
        set cmd $args
    } else {
        set cmd "select 1"
    }
    set res [pg_exec $conn $cmd]
    switch [pg_result $res -status] {
        PGRES_TUPLES_OK {
            for {set n 0} {$n < [pg_result $res -numTuples]} {incr n} {
                puts [pg_result $res -getTuple $n]
            }
        }
        PGRES_COMMAND_OK {
	}
        default {
            puts [pg_result $res -status]
        }
    }
    pg_result $res -clear
}

proc disconnect {} {
    global conn
    pg_disconnect $conn
    set conn {}
}

proc listen {{relname test}} {
    global conn
    pg_listen $conn $relname "callback $relname"
}

proc unlisten {{relname test}} {
    global conn
    pg_listen $conn $relname {}
}

proc notify {{relname test}} {
    global conn
    sql notify $relname
}

proc callback {args} {
    puts "--> callback $args"
}

# end of file


The following patch seems to fix the problem:

*** src/interfaces/libpgtcl/pgtclId.c.orig	Mon Sep 21 03:02:03 1998
--- src/interfaces/libpgtcl/pgtclId.c	Fri Jan  8 16:27:51 1999
***************
*** 638,649 ****
  Pg_Notify_FileHandler (ClientData clientData, int mask)
  {
  	Pg_ConnectionId *connid = (Pg_ConnectionId *) clientData;
  
  	/*
  	 * Consume any data available from the SQL server (this just buffers
  	 * it internally to libpq; but it will clear the read-ready condition).
  	 */
! 	PQconsumeInput(connid->conn);
  
  	/* Transfer notify events from libpq to Tcl event queue. */
  	PgNotifyTransferEvents(connid);
--- 638,656 ----
  Pg_Notify_FileHandler (ClientData clientData, int mask)
  {
  	Pg_ConnectionId *connid = (Pg_ConnectionId *) clientData;
+ 	int status;
  
  	/*
  	 * Consume any data available from the SQL server (this just buffers
  	 * it internally to libpq; but it will clear the read-ready condition).
+ 	 *
+ 	 * Must check the connection status and stop the event source for
+ 	 * closed connection. -- dz
  	 */
! 	status = PQconsumeInput(connid->conn);
! 	if (status == 0) {
! 		PgStopNotifyEventSource(connid);
! 	}
  
  	/* Transfer notify events from libpq to Tcl event queue. */
  	PgNotifyTransferEvents(connid);

-- 
Massimo Dal Zotto

+----------------------------------------------------------------------+
|  Massimo Dal Zotto               email: dz@cs.unitn.it               |
|  Via Marconi, 141                phone: ++39-0461534251              |
|  38057 Pergine Valsugana (TN)      www: http://www.cs.unitn.it/~dz/  |
|  Italy                             pgp: finger dz@tango.cs.unitn.it  |
+----------------------------------------------------------------------+