Small Bug in GetConflictingVirtualXIDs

Hi Simon, Hi all,


HS currently does the following in GetConflictingVirtualXIDs


TransactionId pxmin = proc->xmin;

/*
 * We ignore an invalid pxmin because this means that backend
 * has no snapshot and cannot get another one while we hold exclusive lock.
 */
if (TransactionIdIsValid(pxmin) && !TransactionIdFollows(pxmin, limitXmin))
{
	VirtualTransactionId vxid;

	GET_VXID_FROM_PGPROC(vxid, *proc);
	if (VirtualTransactionIdIsValid(vxid))
		vxids[count++] = vxid;
}


The logic behind this seems fine except in the case of dropping a database. 
There you very well might have a open connection without an open snapshot.

This leads to nice errors when youre connected to a database on the slave 
without having a in progress transaction while dropping the database on the 
primary:


CET FATAL:  terminating connection due to administrator command
CET LOG:  shutting down
CET LOG:  restartpoint starting: shutdown immediate
CET FATAL:  could not open file "base/16604/1259": No such file or directory
CET CONTEXT:  writing block 5 of relation base/16604/1259
CET WARNING:  could not write block 5 of base/16604/1259
CET DETAIL:  Multiple failures --- write error might be permanent.
CET CONTEXT:  writing block 5 of relation base/16604/1259

Should easily be solvable through an extra parameter for 
GetConflictingVirtualXIDs. Want me to prepare a patch?

Andres