This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
- yp_dblookup.c: Create non-DB specific database access functions.
Using these allows access to the underlying database functions without
needing explicit knowledge of Berkeley DB. (These are used only
when DB_CACHE is #defined. Other programs that use the non-caching
functions (yp_mkdb, ypxfr, yppush, rpc.yppasswdd) shouldn't notice
the difference.)
- yp_dnslookup: Implement async DNS lookups. We send our own DNS
requests using UDP and put the request in a queue. When the response
arrives, we use the ID in the header to find the corresponsing queue
entry and then send the response to the client. We can go about our
business and handle other YP requests in the meantime. This way, we
can deal with time consuming DNS requests without blocking and without
forking.
- yp_server.c: Convert to using new non-DB-specific database access
functions. This simplifies the code a bit and removes the need for
this module to know anything about Berkeley DB. Also convert the
ypproc_match_2_svc() function to use the async DNS lookup routines.
- yp_main.c: tweak yp_svc_run() to add the resolver socket to the
set of descriptors monitored in the select() loop. Also add a
timeout to select(); we may get stale DNS requests stuck in the
queue which we want to invalidate after a while. If the timeout
hits, we decrement the ttl on all pending DNS requests and nuke
those requests that aren't handled before ttl hits zero.
- yp_extern.h: Add prototypes for new stuff.
- yp_svc_udp.c (new file): The async resolver code needs to be able
to rummage around inside the RPC UDP transport handle in order to
work correcty. There's basically one transport handle, and each time
a request comes in, the transaction ID in the handle is changed.
This means that if we queue a DNS request, then we handle some other
unrelated requests, we will be unable to send the DNS response because
the transaction ID and remote address of the client that made the DNS
request will have been lost. What we need to do is save the client
address and transaction ID in the queue entry for the DNS request,
then put the transaction ID and address back in the transport handle
when we're ready to reply. (And then we have to undo the change so
as not to confuse any other part of the server.) The trouble is that
the transaction ID is hidden in an opaque part of the transport handle,
and only the code in the svc_udp module in the RPC library knows how
to handle it. This file contains a couple of functions that let us
read and set the transaction ID in spite of this. This is really a
dirty trick and I should be taken out and shot for even thinking about
it, but there's no other way to get this stuff to work.
- Makefile: add yp_svc_udp.c to SRCS.
