[9fans] bind vs. 9660srv

[erik quanstrom <quanstro@quanstro.net>]

i mentioned a few days ago that bind and 9660srv were not
agreeing.  i tracked this down a bit further and am now
quite confused.  here's the initial symptom

	; 9660srv
	; mount /srv/9660 /n/cd0 /n/other/ftp/plan9.iso
	; du -s /n/cd0/386/bin
	80143	/n/cd1/386/bin

but then through if i bind over an entry served by 9660srv,
i see this error.

	; bind /386/bin/awk /n/cd0/386/bin/awk
	; du -s /n/cd0/386/bin
	du: /n/cd1/386/bin: seek in directory
	55724	/n/cd1/386/bin

a quick look shows that /sys/src/cmd/9660srv/9660srv.c:346 is the
source of the error and read(5) indicates that this really is an
error.  the bug must be induced by whoever is getting the offset
wrong, since directory offsets must either continue at the previous
offset or 0; seek is not allowed  (note: ramfs does not exhibit this
behavior.)

if i add a little debugging to 9660srv, i get

	du: /n/cd1/386/bin: seek in directory offset 8198 != dp->doffset 8190

and if i add a little more, i find that the closest doffset set
is at the entry "page"

	[PAGE] ip->doffset = offset 0 + rcnt 8190 = 8190

i'm quite puzzled by why binding in an entry (which 9660srv should
not know about) would cause the offset to be wrong.  this seems
like a bug in the kernel.  i'm fighting that conclusion, though.  one
would think that this problem would have reared its head before.

on a bit further inspection of sysfile:/^mountfix, i think the problem
might be that the winning bind spans buffers.

- erik

ps. this workaround, similar to the code in ramfs does "work"
but it contradicts read(5), so it must be wrong.

static long
ireaddir(Xfile *f, uchar *buf, long offset, long count)
{
	Isofile *ip = f->ptr;
	Dir d;
	char names[4*Maxname];
	uchar dbuf[256];
	Drec *drec = (Drec *)dbuf;
	int n, rcnt;

	if(offset==0 || offset != ip->doffset){
		ip->offset = 0;
		ip->doffset = offset;
	}else
		offset = 0;
	rcnt = 0;
	setnames(&d, names);
	while(rcnt < count && getdrec(f, drec) >= 0){
		if(drec->namelen == 1){
			if(drec->name[0] == 0)
				continue;
			if(drec->name[0] == 1)
				continue;
		}
		rzdir(f->xf, &d, ip->fmt, drec);
		d.qid.vers = f->qid.vers;
		if((n = convD2M(&d, buf+rcnt, count-rcnt)) <= BIT16SZ){
			ungetdrec(f);
			break;
		}
		if(offset > 0)
			offset -= n;
		if(offset <= 0)
			rcnt += n;
	}
	ip->doffset += rcnt;
	return rcnt;
}