RFD: Rework/extending functionality of mdev

[Harald Becker]

On 13.03.2015 16:53, Natanael Copa wrote:
> I have the feeling (without digging into the busybox code) that making
> mdev suitable for a longlived daemon will not be that easy. I suspect
> there are lots of error handling that will just exit. A long lived
> daemon would need log and continue instead.

The major mdev part, will not be converted in a long lived daemon. That 
is more or less code in a process doing a job like cat, but still you 
are right. It will take some time and has to be done carefully. No doubt.

One of the reasons to have that "fifo supervisor" is the failure 
management. Even if the parser / handler process dies, this is catched 
and can fire up a failure script (something we do not have now).

The kernel hotplug handler stays as a normal process at all, does only 
need to write the gathered info to the pipe instead of a function to 
handle the event operation, so it is mainly replacing the handler 
function with a write to the named pipe, and on the other sid the 
hotplug gathering part is replaced by a read from stdin (pipe) with timeout.

So major work will go to the code resorting parser / device operation 
handler, but I expect the need of doing a parser rewrite (which is 
straight forward for that simple syntax).

Don't misunderstand, it will be an expensive piece of work, but compared 
to finding a specific bug in a 30000 line of a program from somebody 
else, the mdev code is simple ... and it's not my first Busybox hacking, 
it is only the first time do that as public discussion in this channel / 
list. I started creating specialized BB versions around 1995, so some 
experience now.

Did you note that pseudo code for the fifo supervisor? May be it hopped 
to the other thread. That is standard code, more or less, comparable to 
e.g. tcpsvd