[9fans] seeking for the truth

[rog@vitanuova.com]

i was caught up short just now when i tried to reverse
some lines in acme by selecting them and executing "|tail -r".

it didn't work.
the reason?

/sys/src/cmd/tail.c:94: 	seekable = seek(file,0L,0) == 0;

the seek() is succeeding, even though standard input
isn't actually seekable.

the trouble is, it can't do anything else.

we don't see this problem much because of the special case hack for
"#|" in sseek(), but the tail code is a common hack, and an insidious
one.  there are loads of special files around that don't allow
seeking, and many programs that require files that are seekable (diff
being the age-old example).

surely a better solution is possible?  in general, a fileserver knows
whether its files are seekable or not, because it determines how to
interpret the seek offset.

why not add a new bit to the file attributes, say DMNOSEEK (and
associated QTNOSEEK)?  if set, it would indicate that file offsets
when reading and writing the file will be ignored.  0x2 seems to be
available.

then the above line from tail.c could work, and finally it would be
possible to write programs that know definitively whether they are
able to seek on a particular file or not, rather than failing
strangely sometime later.

the amount of code involved would be tiny, and as far as i can see
it would be completely backwardly compatible.

thoughts?