[Libevent-users] Libevent, DBus, Netlink, peeking at packet lengths, etc.

[Philip Prindeville <philipp_subx@xxxxxxxxxxxxxxxxxxxxx>]

Hi,

I'm fairly new to using libevent, having done most of my event driven 
programming in C++ with either Boost or Perl, but needed to add multiple 
pending transaction support to a Tacacs+ proxy written in C so I decided 
to have a look at libevent.

So far, it all seems pretty straightforward and I'm getting the hang of it.

I did have a question about supporting other socket types besides the 
usual TCP or UDP transports.

Is there a preferred way of supporting Netlink or DBus traffic? I ask 
because I'm thinking of a long-running daemon on a laptop that might 
need to get notifications about roaming onto different network types via 
Netlink, for instance.  I did see this library out there but wasn't sure 
how complete and tested it was:

https://github.com/dex/dbus-service

Also, in the case of handling a protocol with fixed-size records (like 
Tacacs+) running over TCP, is there a preferred way to read a 
packet-at-a-time?  Since it's TCP, you have no record boundaries (unlike 
UDP or TP-4), so you have to find the packet length inside the header.

In my case, in my read callback, I'm doing this as:

      evbuf = bufferevent_get_input(bev);

      n = evbuffer_get_length(evbuf);

      /* evbuffer_pullup() returns NULL if there aren't enough bytes in
       * the buffer yet to pull-up as many as are requested.
       */
      start = evbuffer_pullup(evbuf, TAC_PLUS_HDR_SIZE);
      if (! start)
          return;

      th = (HDR *)start;

      /* should probably bcopy th->datalength to a word-aligned buffer
       * first, in case we're on a platform which doesn't handle word
       * fetches on unaligned addresses
       */

      length = ntohl(th->datalength) + TAC_PLUS_HDR_SIZE;

      /* if we're short, we'll get called again when more data arrives
       */
      if (n < length)
           return;

      u_char *pkt = malloc(length);

      /* copy out... */
      i = evbuffer_remove(evbuf, pkt, length);

      tac_parse_pkt(ctx->sess, ctx, pkt, length);

      free(pkt);
}

But wondered if there's a better or prototypical way of handling this?

Is it worth setting the socket low-water market to "length" so that we 
only get dispatched one more time if data trickles in?

Also, is there any interest in adding 'profiling' support to the event 
handlers to see how long they run (in real-time, probably using 
gettimeofday()) and generating warnings in debug- (or development-) mode 
if they exceed an upper threshold (say 10ms execution time)?

Thanks,

-Philip

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