Hi!
On Wed, Jun 28, 2023 at 01:38:53PM +0200, Jan Kara wrote:
> On Tue 27-06-23 22:50:46, Ahelenia Ziemiańska wrote:
> > Always generate modify out, access in for splice;
> > this gets automatically merged with no ugly special cases.
> >
> > No changes to 2/3 or 3/3.
> Thanks for the patches Ahelena! The code looks fine to me but to be honest
> I still have one unresolved question so let me think about it loud here for
> documentation purposes :). Do we want fsnotify (any filesystem
> notification framework like inotify or fanotify) to actually generate
> events on FIFOs? FIFOs are virtual objects and are not part of the
> filesystem as such (well, the inode itself and the name is), hence
> *filesystem* notification framework does not seem like a great fit to watch
> for changes or accesses there. And if we say "yes" for FIFOs, then why not
> AF_UNIX sockets? Where do we draw the line? And is it all worth the
> trouble?
As a relative outsider (I haven't used inotify before this, and have not
been subjected to it or its peripheries before),
I interpreted inotify as being the Correct solution for:
1. stuff you can find in a normal
(non-/dev, you don't want to touch devices)
filesystem traversal
2. stuff you can open
where, going down the list in inode(7):
S_IFSOCK can't open
S_IFLNK can't open
S_IFREG yes!
S_IFBLK it's a device
S_IFDIR yes!
S_IFCHR it's a device
S_IFIFO yes!
It appears that I'm not the only one who's interpreted it that way,
especially since neither regular files nor pipes are pollable.
(Though, under that same categorisation, I wouldn't be surprised
if anonymous pipes had been refused, for example, since those are
conventionally unnameable.)
To this end, I'd say we're leaving the line precisely where it was drawn
before, even if by accident.
> I understand the convenience of inotify working on FIFOs for the "tail -f"
> usecase but then wouldn't this better be fixed in tail(1) itself by using
> epoll(7) for FIFOs which, as I've noted in my other reply, does not have
> the problem that poll(2) has when there are no writers?
Yes, epoll in ET mode returns POLLHUP only once, but you /also/ need the
inotify anyway for regular files, which epoll refuses
(and, with -F, you may want both epoll for a pipe and inotify for the
directory it's contained in).
Is it possible to do? yes. Is it more annoying than just having pipes
report when they were written to? very much so.
inotify actually working(*) is presumably why coreutils tail doesn't use
epoll ‒ inotify already provides all required events(*), you can use the
same code for regular files and fifos, and with one fewer level of
indirection: there's just no need(*).
(*: except with a magic syscall only I use apparently)
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