Handle TFTP ERROR from client

If a PXE client only wants to find out the size of a file, it will
open the file and then abort the transfer by sending a TFTP ERROR packet.

The ERROR packet should cause qemu to terminate the session. If not,
the sessions will soon run out and cause timeouts in the client.

Also, if a TFTP session already exists with same IP/UDP port, it
should be terminated when a new RRQ is received, instead of creating a
duplicate (which will never be used).

A patch for gPXE to send the ERROR packet is also being submitted to
gPXE. Together they resolve slowness/hanging when booting pxegrub from
qemu's internal TFTP server. The patch from Milan Plzik to return
after sending OACK is also required for a complete fix.

Signed-off-by: Thomas Horsten <thomas@horsten.com>
Signed-off-by: Milan Plzik <milan.plzik@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

slirp/tftp.c

diff --git a/slirp/tftp.c b/slirp/tftp.c
index 082f5d0..12dd344 100644
--- a/slirp/tftp.c
+++ b/slirp/tftp.c
@@ -264,6 +264,12 @@
   size_t prefix_len;
   char *req_fname;
 
+  /* check if a session already exists and if so terminate it */
+  s = tftp_session_find(slirp, tp);
+  if (s >= 0) {
+    tftp_session_terminate(&slirp->tftp_sessions[s]);
+  }
+
   s = tftp_session_allocate(slirp, tp);
 
   if (s < 0) {
@@ -385,6 +391,19 @@
   }
 }
 
+static void tftp_handle_error(Slirp *slirp, struct tftp_t *tp, int pktlen)
+{
+  int s;
+
+  s = tftp_session_find(slirp, tp);
+
+  if (s < 0) {
+    return;
+  }
+
+  tftp_session_terminate(&slirp->tftp_sessions[s]);
+}
+
 void tftp_input(struct mbuf *m)
 {
   struct tftp_t *tp = (struct tftp_t *)m->m_data;
@@ -397,5 +416,9 @@
   case TFTP_ACK:
     tftp_handle_ack(m->slirp, tp, m->m_len);
     break;
+
+  case TFTP_ERROR:
+    tftp_handle_error(m->slirp, tp, m->m_len);
+    break;
   }
 }