[RFC] Proof-of-concept for netlink listener for mdev -i

[Natanael Copa]

On Sun, 15 Mar 2015 02:30:33 +0100
Harald Becker <ralda at gmx.de> wrote:

> Hi Isaac !
> 
> On 14.03.2015 08:10, Isaac Dunham wrote:
> > Basic concept is that it creates a fifo, and treats the success of mkfifo
> > as a lock to determine whether it's the daemon or a writer.
> > If it's the daemon, it will
> > fork;
> > in the child:
> > 	exec the parser with the fifo as stdin
> > in the parent:
> > 	set a SIGCHLD handler that respawns the parser if the fifo is not empty,
> > 	open the fifo to write,
> > 	dump the environment into the fifo
> > 	sleep
> 
> Congratulations, you implemented the base functionality, I intended, 
> except I use some command line parameter to distinguished between 
> hotplug helper and parser. This is mainly for increased stability and 
> flexibility with no extra cost (due to BB startup / option handling less 
> code than fifo check).
> 
> ... beside not (yet) implemented failure management
> 
> ... has your approach a problem: The write side of the pipe (fifo) is 
> closed as soon as you exit the hotplug helper, which let the parser die 
> immediately too (except other hotplug run at exactly the same moment). 
> This way you get extra parser starts when events have slight delays. 
> With the possibility of race conditions (which need complicated failure 
> check and handling).
> 
> Therefore it's better to let a small process hold the fifo open and 
> available (skips need to recreate on every event). For no extra cost 
> this process can do the parser startup, signal management and failure 
> checks, as this automatically shares code otherwise also required in the 
> netlink part.
> 
> ... and you got my "fifo manager".

But this means that you have replaced the tiny always-in-memory netlink
socket activator with a bigger always-in-memory "fifo manager".

I don't see how that improves anything.

-nc