Re: ftrig pipe naming convention

From: Ihor Antonov <>
Date: Sun, 18 Sep 2022 14:11:16 -0700

On 2022-09-18 20:38, Laurent Bercot wrote:
> > I wonder what is the reason behind the naming convention? What is the
> > downside of simply writing to any present fifo file ?
> It could work like you're suggesting. But :
> - checking the type of a file is an additional fstat() system call
> - there may be reasons in the future to store other files in the
> fifodir that do not receive the event
> - it is nice to detect stale fifos, if any, and delete them as soon
> as you can (#L39), and you don't want to delete unrelated files
> - but most importantly: creating a fifo in a fifodir that allows you to
> receive events without a race condition, which is the whole point of the
> ftrig library, is slightly more complex to do safely than just "mkfifo
> event/foobar", and I don't want people to think that this is the API.
> No, the API is ftrigr_subscribe(), and everything under it is
> implementation details. Restricting the naming is a way of ensuring
> (as much as possible) that the fifos were indeed created by the
> appropriate programs.

Could you please elaborate on the possible race condition?
This is simply for curiosity and educational purposes. It feels like a
lot of thought was put into s6 codebase, and a lot of ideas are not
immediatedly obvious for people not intimately familiar with OS

> Don't create fifos willy-nilly in a fifodir, and since you found the
> naming convention, don't use it to work around the check to create your
> fifos outside of ftrigr_subscribe(). If you do, it will work, until the
> time when it doesn't, and it will be a complete PITA to debug.

This does make the most sense. We could say that pipes are also
implementation detail. There could be a socket, or, say, a regular file
(aka event log). So hiding this behind an interface is very reasonable.

> --
> Laurent
Received on Sun Sep 18 2022 - 23:11:16 CEST

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