Re: merge-similar-outgoing-queries.patch and lame servers

On Tue, Mar 03, 2009 at 02:01:53PM +0000, Gerrit Pape wrote:

> Hi, it looks like the patch doesn't handle responses from lame servers
> properly.  If such a single request gets a response from a lame server,
> it can happen that no further requests are sent.  To reproduce, look for
> a domain with a lame server, and do

Thanks for the bug report.

I believe what is happening is that dnscache is hanging in the iopause
call due to a bug in the qmerge patch. A patch (on top of qmerge) is
below. Let me know if it solves your problem.

-- >8 --
fix qmerge iopause hang

The qmerge layer works by sitting between the query and
dns_transmit layers and silently merging duplicate queries.
Because there are multiple queriers interested in the
answer, a qmerge query may be in a state where we have
received an answer, but not all queriers have consumed the
answer (qm->state == 2).

When we are asked to provide an iopause_fd for such a query
in such a state, we fill in "-1" for the fd and claim we are
not interested in either reading or writing. Then when the
querier eventually asks to read data, we feed them the
already-received answer.

However, if _all_ of the active queries are in this state,
then we will call to iopause without any actual I/O pending,
waiting for either a new query to come in or the default
120-second timeout to occur.

The solution is to drop the iopause timeout to zero when we
already have an answer waiting to be consumed.
---
 qmerge.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/qmerge.c b/qmerge.c
index 7c92299..be2fafd 100644
--- a/qmerge.c
+++ b/qmerge.c
@@ -90,11 +90,12 @@ void qmerge_io(struct qmerge *qm, iopause_fd *io, struct taia *deadline)
   if (qm->state == 0) {
     dns_transmit_io(&qm->dt, io, deadline);
     qm->state = 1;
+    return;
   }
-  else {
-    io->fd = -1;
-    io->events = 0;
-  }
+  if (qm->state == 2)
+    taia_now(deadline);
+  io->fd = -1;
+  io->events = 0;
 }
 
 int qmerge_get(struct qmerge **x, const iopause_fd *io, const struct taia *when)
-- 
1.6.2.rc2.330.gba39e