On Fri, Aug 28, 2020 at 08:37:07AM -0400, Hendrik Boom wrote:
> > >>>> Is there an establiched word in the Linux/Unix xommunity
> > >>>> for something which might be a file or a directory?

> > Therefore it looks to me more like an implementation
> > detail than a sensible concept.
>
> I do miss the ability to identify a directory independent of its name.
> It can be important if a directory is renamed and you have active data in it.
>
> There's one directory that works for -- the current directory. If you
> rename that directory you're still in it.
>
> In the 80's or 90's (I forget) there was a proposal to the ISO from
> Japan to establish a standard OS-independent OS interface for
> programmers to use. I lost against POSIX.
>
> But it had a machanism whereby opening a file could be done in two
> phases.
>
> First you get a "lock" on a file. That gives you a connection to that
> file. It's not an exclusive kind of lock (unless you ask it to be);
> it's just a way of unambiguously identifying the file as long as it
> exists and you keep the lock. It hangs on to the file even if someone
> renames it. (not sure what happens if the file is deleted, though)

_Any_ file descriptor does that. And a deleted file stays there, merely
with a 0 link count -- if there's an open descriptor, you can link the file
back into the filesystem. That's even the recommended way to create a new
file atomically -- instead of the old write+fsync+rename trick that leaves
junk upon a crash.

And if you don't need to do any I/O on the file, O_PATH won't waste time.

> Then afterward you can open it, close it, set exclusive
> read/write access, release exclusive access, etc, using the lock instead
> of the name.
>
> I've often regretted there's no such similar feature in Linux. The
> assurance that several uses of a file or directory are indeed to the
> same file or directory. You can do some of this by keeping a file open,
> but I know of nothing like that for directories.

man name_to_handle_at, open_by_handle_at. But this interface sucks and is
superfluous with regular openat() -- which is also portable.

So _always_ use openat() if you suspect something might happen to the
underlying directories.


Meow!
--
⢀⣴⠾⠻⢶⣦⠀
⣾⠁⢠⠒⠀⣿⡁
⢿⡄⠘⠷⠚⠋⠀ It's time to migrate your Imaginary Protocol from version 4i to 6i.
⠈⠳⣄⠀⠀⠀⠀