namec (was Re: [9fans] Strange date/time on some created files)

[rog@vitanuova.com]

this conversation reminded me of a little piece of
annoying behaviour under plan9. [please accept my apologies if
you've seen this before - it didn't seem to get through last time]

here's a demonstration:

cpu% pwd
/usr/rog/c
cpu% > /tmp/tst
cpu% cp /bin/echo 8.out
cpu% bind 8.out /tmp/tst
cpu% /tmp/tst -z
-z
cpu% cp /bin/sed 8.out
cpu% /tmp/tst -z
tst 424799: suicide: sys: trap: fault write addr=0x24fffbde pc=0x00017e7c

the problem seems to be that attachimage() relies on sysexec()
giving it a Chan with a currently valid qid.version, but when
the last component of the path is translated by a mount
point, namec returns the qid in the mount table's Chan,
which holds a stale version number, hence the old
shared text segment incorrectly used.

i can think of several ways of fixing it, but all of them
either slow things down slightly, or increase complexity.
(for instance, one doesn't really want to do a stat when the
version returned from namec *is* current, which is almost
all the time).