Enforce the per-vhost server limit. Note that this is per-vhost, not
per-directory (since it works by reading the scoreboard), so we need to add
per-server configuration data.

Index: httpd-2.2.11/server/mpm/experimental/itk/itk.c
===================================================================
--- httpd-2.2.11.orig/server/mpm/experimental/itk/itk.c	2009-03-21 13:03:45.000000000 +0100
+++ httpd-2.2.11/server/mpm/experimental/itk/itk.c	2009-03-21 13:03:50.000000000 +0100
@@ -159,6 +159,11 @@
     char *username;
 } itk_per_dir_conf;
 
+typedef struct
+{
+    int max_clients_vhost;
+} itk_server_conf;
+
 module AP_MODULE_DECLARE_DATA mpm_itk_module;
 
 #ifdef GPROF
@@ -1383,6 +1388,26 @@
     gid_t wanted_gid;
     const char *wanted_username;
     int err = 0;
+    
+    itk_server_conf *sconf =
+        (itk_server_conf *) ap_get_module_config(r->server->module_config, &mpm_itk_module);
+
+    /* Enforce MaxClientsVhost. */
+    if (sconf->max_clients_vhost > 0) {
+        int i, num_other_servers = 0;
+        for (i = 0; i < ap_daemons_limit; ++i) {
+            worker_score *ws = &ap_scoreboard_image->servers[i][0];
+            if (ws->status >= SERVER_BUSY_READ && strncmp(ws->vhost, r->server->server_hostname, 31) == 0)
+                ++num_other_servers;
+        }
+
+        if (num_other_servers > sconf->max_clients_vhost) {
+            ap_log_error(APLOG_MARK, APLOG_WARNING, 0, NULL, \
+                "MaxClientsVhost reached for %s, refusing client.",
+                r->server->server_hostname);
+            return HTTP_SERVICE_UNAVAILABLE;
+        }
+    }
 
     itk_per_dir_conf *dconf =
         (itk_per_dir_conf *) ap_get_module_config(r->per_dir_config, &mpm_itk_module);
@@ -1568,6 +1593,14 @@
     return NULL;
 }
 
+static const char *set_max_clients_vhost (cmd_parms *cmd, void *dummy, const char *arg)   
+{
+    itk_server_conf *sconf =
+        (itk_server_conf *) ap_get_module_config(cmd->server->module_config, &mpm_itk_module);
+    sconf->max_clients_vhost = atoi(arg);
+    return NULL;
+}
+
 static const command_rec itk_cmds[] = {
 UNIX_DAEMON_COMMANDS,
 LISTEN_COMMANDS,
@@ -1583,6 +1616,8 @@
               "Maximum value of MaxClients for this run of Apache"),
 AP_INIT_TAKE2("AssignUserID", assign_user_id, NULL, RSRC_CONF|ACCESS_CONF,
               "Tie a virtual host to a specific child process."),
+AP_INIT_TAKE1("MaxClientsVHost", set_max_clients_vhost, NULL, RSRC_CONF,
+              "Maximum number of children alive at the same time for this virtual host."),
 AP_GRACEFUL_SHUTDOWN_TIMEOUT_COMMAND,
 { NULL }
 };
@@ -1596,12 +1631,21 @@
     return c;
 }
 
+/* == allocate a private server config structure == */
+static void *itk_create_server_config(apr_pool_t *p, server_rec *s)
+{
+    itk_server_conf *c = (itk_server_conf *)
+        apr_pcalloc(p, sizeof(itk_server_conf));
+    c->max_clients_vhost = -1;
+    return c;
+}
+
 module AP_MODULE_DECLARE_DATA mpm_itk_module = {
     MPM20_MODULE_STUFF,
     ap_mpm_rewrite_args,        /* hook to run before apache parses args */
     itk_create_dir_config,      /* create per-directory config structure */
     NULL,                       /* merge per-directory config structures */
-    NULL,                       /* create per-server config structure */
+    itk_create_server_config,   /* create per-server config structure */
     NULL,                       /* merge per-server config structures */
     itk_cmds,                   /* command apr_table_t */
     itk_hooks,                  /* register hooks */