[exim] [PATCH] possible change to spamd_address behaviour

Hi,

In the interests of load-balancing and redundancy, we found it would be
helpful if there was more control over how the list of spamd servers in
spamd_addresses is used. The attached patch adds an MX-style priority as
an optional third argument to each ip/port combination, so that it is
possible to offload spam checking to a server or group of servers. and
fall back onto another server should none of these be contactable.

The general idea is that as spam processing becomes a greater drain on
system resources, it becomes practical to offload this burden onto a
dedicated machine (which can then become a cluster of machines once the
load becomes too high). This leaves the MX server free to concentrate on
mail delivery and processing ACL checks. However in the event of a
failure of the offload server, it would be useful to be able to fall
back onto a backup running elsewhere, for example on localhost.

This patch preserves the usual spamd_address behaviour at each level of
priority; the default priority for a server is 10 and a random(ish)
server will be chosen to begin with, then the list cycles in order.

I have tested it a little to ensure the functionality is correct, and
would be interested in any feedback and comments on the idea. The patch
is against the 4.63 source tree.

Cheers,
--alex
diff -ur exim-4.63/src/spam.c exim-4.63-spam-mx/src/spam.c

--- exim-4.63/src/spam.c    Mon Jul 31 15:19:48 2006
+++ exim-4.63-spam-mx/src/spam.c    Tue Aug 22 16:46:08 2006
@@ -20,6 +20,18 @@
 uschar prev_user_name[128] = "";
 int spam_ok = 0;
 int spam_rc = 0;
+int rand_server = 0;
+int num_servers = 0;
+
+int sort_by_prio(const void *ia, const void *ib) {
+   spamd_address_container * const *a = ia;
+   const spamd_address_container * const *b = ib;
+   if ((*a)->priority - (*b)->priority)
+     return (*a)->priority - (*b)->priority;
+   /* pseudo-randomisation of equal priorities
+    * will only change once per second, as before */
+   return ((*a)->pos + rand_server)%num_servers - ((*b)->pos + rand_server)%num_servers;
+}  

 int spam(uschar **listptr) {
   int sep = 0;
@@ -92,14 +104,14 @@
   /* socket does not start with '/' -> network socket */
   if (*spamd_address != '/') {
     time_t now = time(NULL);
-    int num_servers = 0;
     int current_server = 0;
-    int start_server = 0;
     uschar *address = NULL;
     uschar *spamd_address_list_ptr = spamd_address;
     uschar address_buffer[256];
     spamd_address_container * spamd_address_vector[32];

+    num_servers = 0;
+    
     /* Check how many spamd servers we have
        and register their addresses */
     while ((address = string_nextinlist(&spamd_address_list_ptr, &sep,
@@ -108,9 +120,17 @@

       spamd_address_container *this_spamd =
         (spamd_address_container *)store_get(sizeof(spamd_address_container));
+      
+      /* set a default spamd priority of 10 */
+      this_spamd->priority = 10;
+      /* store current server number for randomisation purposes */
+      this_spamd->pos      = num_servers;

       /* grok spamd address and port */
-      if( sscanf(CS address, "%s %u", this_spamd->tcp_addr, &(this_spamd->tcp_port)) != 2 ) {
+      if( sscanf(CS address, "%s %u %i", 
+                 this_spamd->tcp_addr,
+                 &(this_spamd->tcp_port), 
+                 &(this_spamd->priority)) < 2 ) {
         log_write(0, LOG_MAIN,
           "spam acl condition: warning - invalid spamd address: '%s'", address);
         continue;
@@ -130,13 +150,15 @@
       return DEFER;
     };

-    current_server = start_server = (int)now % num_servers;
+    rand_server = (int)now % num_servers;
+    qsort(spamd_address_vector, num_servers, sizeof(*spamd_address_vector), sort_by_prio);

     while (1) {

-      debug_printf("trying server %s, port %u\n",
+      debug_printf("trying server %s, port %u, priority %d\n",
                    spamd_address_vector[current_server]->tcp_addr,
-                   spamd_address_vector[current_server]->tcp_port);
+                   spamd_address_vector[current_server]->tcp_port,
+                   spamd_address_vector[current_server]->priority);

       /* contact a spamd */
       if ( (spamd_sock = ip_socket(SOCK_STREAM, AF_INET)) < 0) {
@@ -161,9 +183,7 @@
          spamd_address_vector[current_server]->tcp_port,
          strerror(errno));
       current_server++;
-      if (current_server >= num_servers)
-        current_server = 0;
-      if (current_server == start_server) {
+      if (current_server == num_servers) {
         log_write(0, LOG_MAIN|LOG_PANIC, "spam acl condition: all spamd servers failed");
         (void)fclose(mbox_file);
         (void)close(spamd_sock);
diff -ur exim-4.63/src/spam.h exim-4.63-spam-mx/src/spam.h
--- exim-4.63/src/spam.h    Mon Jul 31 15:19:48 2006
+++ exim-4.63-spam-mx/src/spam.h    Tue Aug 22 13:35:20 2006
@@ -25,6 +25,8 @@
 typedef struct spamd_address_container {
   uschar tcp_addr[24];
   unsigned int tcp_port;
+  int priority;
+  int pos;
 } spamd_address_container;

#endif