Small Bug in pgstat display during recovery conflict resolution

Hi Simon, Hi all,


if (!logged && (wait_s > 0 || wait_us > 500000))
{       const char *oldactivitymsg;       int                     len;
       oldactivitymsg = get_ps_display(&len);       snprintf(waitactivitymsg, sizeof(waitactivitymsg),
     "waiting for max_standby_delay (%u s)",                        MaxStandbyDelay);
set_ps_display(waitactivitymsg,false);       if (len > 100)               len = 100;       memcpy(waitactivitymsg,
oldactivitymsg,len);
 
       pgstat_report_waiting(true);
       logged = true;
}
..
if (logged)
{       set_ps_display(waitactivitymsg, false);       pgstat_report_waiting(false);
}

That doesnt work because get_ps_display returns the internal buffer. This 
leads to the situation that after conflict resolution the 
"waiting for max_standby_delay ..."
message is displayed until the next segment starts where its replaced
again by the 
"... recovering ..." line.

Additionally the old code may print unintialized memory if get_ps_displayreturns a string without a \0 terminator.

The attached patch fixes that.

Andres