openat gives you the effect of 'cd path; open file' without having to cd. I
don't see a lot of benefit to it unless you're opening a lot of files at
that path.
My first reaction, assuming you have a lot of files in that directory, was
something like
bind /dir /n/x and then just open /n/x/file... for a lot of files.
This would work for any system call that takes a path.
The question I had was, can I get the benefit of *at without doing what
linux is doing, namely, for all system calls with a path, make an '...at'
version.
I am guessing so, though I'm not sure it's as efficient.
On Fri, Apr 5, 2024 at 8:19 PM <mo...@posixcafe.org> wrote:
> My two cents on this:
>
> What you _would_ want for this would be the ability to walk from the existing
> fd, however the limits of 9p walk make this a bit impossible to implement in
> a great way in my opinion. From walk(5):
>
> The fid must represent a directory unless zero path name elements(for just
> cloning the fid) are specified. The fid must be valid in the current session
> and must not have been opened for I/O by an open or create message.
>
> Since not every fid is eligible for being walked from, in order to implement
> opanat() in any way that would be better than just batching the fd2path and
> open would be to keep a "last directory" associated, like what we do with the
> string used to open it. Also worth mentioning that fd2path is not without its
> own problems, it's possible that the namespace has changed since the file has
> been opened so the same path may not work the second time. So tagging the
> last directory Chan would be "more correct", but I am not sure how useful
> this is.
>
> Answering some other comments made:
>
> As I understand it from the rationale section on the linux man page, the
> call exists to avoid a race condition between checking that a directory
> exists and doing something to a path containing it. An additional motivator
> is providing the effect of additional current working directories notably
> for Linux threads (which presumably don't have their own. I think
> 'threads' (processes that share memory) on Plan 9 do???).
>
>
> Each process has it's own current working directory:
>
> % pwd
> /home/moody
> % @{cd /}
> % pwd
> % /home/moody
>
> This is all based on the assumption that holding a file/directory open keeps
> it alive and in existence... which on Plan 9, I think it doesn't, does it? As
> I understand it, remove can remove a file or directory that is open, which is
> not like UNIX/Linux...
>
>
> Depends on the file server implementation, I find that for more synthetic
> files they are indeed kept alive as long there is someone with a reference
> to the fid. This is identifiable if all the cleanup happens on
> clunks(destroyfid in lib9p), which only happen when a fid's refcount hits
> zero. For non-synthetic or more "disk" file servers the behavior can
> differ. Cwfs does not keep the data around, readers that attempt to read a
> deleted file, even if they already have a reference open to it will result
> in a phase error. However 9front's ramfs keeps the data around.
>
> My test for this was as follows:
> win1% echo 'something' >/tmp/test
> win2% </tmp/test { sleep 10; cat }
> win1% rm /tmp/test # observe the error(if any) on win2
>
> So you really can't assume either case.
>
>
> Thanks,
> moody
>
> P.S.
> I apologize for the formatting, it seems my emails are not making it to
> the list when I attempt to send them from my mail server so I had to copy
> this response in to the web form. I would like to figure out why if
> possible.
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