Re: [9fans] 9fs/9auth for FreeBSD

[Alexander Viro <viro@math.psu.edu>]

On Wed, 28 Mar 2001, Lyndon Nerenberg wrote:

>     Russ> the biggest shortcoming
>     Russ> is the lack of a portable way to implement append-only
>     Russ> files.  freebsd (and presumably the other unix clones) take
>     Russ> append-only to mean "only appends succeed" rather than "all
>     Russ> writes are treated as appends", and there's no standard way
>     Russ> to enable even this.
 
> On 4.4BSD-derived systems you should be able to get the behaviour you
> want with chflags(path, UF_APPEND) after creating the file.  During
> subsequent opens you would have to check for this flag, and open with
> O_APPEND if it is present (to force the writes to always be treated as
> appends). Did you try this? (I realize it's not very portable.)
 
> Without chflags() you would have to hijack on of the mode bits, which
> is downright evil.

In case of Linux append-only is "all writes are treated as appends"
(see mm/filemap.c::generic_file_write()), so no wanking with O_APPEND
on subsequent open() is needed, but marking file append-only
_is_ downright evil - ioctl(fd, EXT2_IOC_SETFLAGS, EXT2_APPEND_FL).
chflags() had been discussed many times, there are several patches
of varying age that implement it, but nobody cared enough to get
them into the tree.