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charset=UTF-8 Content-Transfer-Encoding: quoted-printable Depending on the implemen= tation of the file system, openat vs open can be more efficient if there&rs= quo;s a lot of metadata locking for file creation.
Sent from my iPhone

On Apr 6, 2024, at 1:36 PM, ron = minnich <rminnich@gmail.com> wrote:


openat giv= es you the effect of 'cd path; open file' without having to cd. I d= on't see a lot of benefit to it unless you're opening a lot of file= s at that path. 

My first reaction, assuming you = have a lot of files in that directory, was something like
bind /d= ir /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 p= ath, make an '...at' version.
I am guessing so, though&nb= sp;I'm not sure it's as efficient.


On Fri, = Apr 5, 2024 at 8:19 PM <mo= ody@posixcafe.org> wrote:
My two cents on this:

What you _would_ want for this would be the ability to walk from the existi=
ng fd, however the limits of 9p walk make this a bit impossible to implemen=
t 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 sessio=
n 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 implemen=
t opanat() in any way that would be better than just batching the fd2path a=
nd 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 c=
hanged since the file has been opened so the same path may not work the sec=
ond time. So tagging the last directory Chan would be "more correct&qu=
ot;, 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=20 checking that a directory exists and doing something to a path=20 containing it. An additional motivator is providing the effect of=20 additional current working directories notably for Linux threads (which=20 presumably don't have their own. I think 'threads'  (proce= sses that=20 share memory) on Plan 9 do???).

Each process has it's own current working directory: % pwd /home/moody % @{cd /} % pwd % /home/moody 
T=
his 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 i= mplementation, I find that for more synthetic files they are indeed kept al= ive as long there is someone with a reference to the fid. This is identifiable if all the cleanup happens on clunks(destroyfid in lib= 9p), which only happen when a fid's refcount hits zero. For non-synthetic or more "disk" file servers the behavior can di= ffer. Cwfs does not keep the data around, readers that attempt to read a de= leted 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 = 9;something' >/tmp/test
win2% </tmp/test { sleep 10; ca= t }
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.
= --Apple-Mail-B2A64500-1842-46DE-9811-C4EDE0880C23--