Re: Automatic refresh for local Maildir's mail counts -- an attempt based on named pipes

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Linux-Fan writes:

> The actual error (why I initially got endless loops when trying to do t=
his), =20
> was with my understanding of named pipes: What actually caused the endl=
ess =20
> loop was that whenever I notified Cone from offlineimap, the named pipe =
got =20
> closed in the process, causing POLLHUP to be returned (immediately) on =
all =20
> subsequent poll invocations.

Closing and reopening is fine, but the usual approach here is to open the =20
named pipe twice, both for reading and writing, and having the named pipe =20
opened for writing will prevent this POLLHUP. But, again, this is fine. T=
he =20
only problem this might potentially cause is that there's a brief period =
of =20
time when the named pipe is closed, and anything that happens to open it =
for =20
writing, at that time, will fail unless it's specifically coded to handle =20
this eventuality.

>
> I have now fixed this by re-opening the file descriptor once POLLHUP is =20
> returned and now it works as expected. See attached =20
> `patch_cone_refresh_with_named_pipe_2.diff`.

I took a closer look at this code.

The named pipe should have the FD_CLOEXEC flag set, to prevent it from be=
ing =20
inherited by child processes.

You also place the file descriptor into the poll vector even if it's not =20
used, if the file descriptor is -1. I was actually surprised to learn tha=
t =20
this is not an error, and this is actually documented as a no-op, in the =20
poll() manually page. You got lucky there=E2=80=A6

If you'd like to tweak this further and set FD_CLOEXEC on the pipe, you c=
an =20
go ahead and do this, but this looks ok, as is.


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