We were looking at alternative solutions, so even one more:
3) netlink reader the Unix way
Why let our netlink reader bother about where he sends the event
messages. Just let him do his netlink receiption job and forward the
messages to stdout.
netlink reader:
set stdout to non blocking I/O
establish netlink socket
wait for event messages
gather event information
write message to stdout
hotplug startup code:
create a private pipe
spawn netlink reader, redirect stdout to write end of pipe
spawn fifosvd - xdev parser, redirect stdin from read end of pipe
close both pipe ends (write end open in netlink, read in fifosvd)
This way we can let the starting process decide which type of pipe we
use: private pipe for netlink, and named pipe for hotplug helper.
I think this is not far away from Laurent's (or Natanael's) solution, at
the only cost of a small long lived helper process, managing the on
demand handler startup and checking for failures. Small general purpose
daemon in the sense of supervisor daemons (e.g. tcpsvd), with generally
reusable function for other purposes.
... better?
Ok, but brings me to the message format in the pipe, I highly think, we
should use a textual format, but do required checks for control chars
and do some (shell compatible) quoting.
This would allow to do:
netlink reader >/dev/ttyX
(to display all device plug events on a console)
netlink reader >>/tmp/uevent.log
(append all event message to log file)
... and all such things.
I know, the parser needs to do some checking and unquoting, but we have
a single reader and it doesn't matter how much data it reads from the
pipe in a single hunck, as long as the writers assure, they are going to
write a single message with one write (atomicity). The parser assumes
reading text from stdin, with required checking and unquoting. This way
we get maximum compatibility and may easily replace every part with some
kind of script.
--
Harald
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