Index: lib/accesslib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/lib/accesslib.php,v
retrieving revision 1.421.2.101
diff -u -r1.421.2.101 accesslib.php
--- lib/accesslib.php	25 Mar 2009 02:37:27 -0000	1.421.2.101
+++ lib/accesslib.php	1 Apr 2009 12:03:34 -0000
@@ -160,6 +160,10 @@
 
 require_once($CFG->dirroot.'/group/lib.php');
 
+if (!defined('MAX_CONTEXT_CACHE_SIZE')) { 
+    define('MAX_CONTEXT_CACHE_SIZE', 5000);
+}
+
 $context_cache    = array();    // Cache of all used context objects for performance (by level and instance)
 $context_cache_id = array();    // Index to above cache by id
 
@@ -167,6 +171,28 @@
 $ACCESS = array(); // cache of caps for cron user switching and has_capability for other users (==not $USER)
 $RDEFS = array(); // role definitions cache - helps a lot with mem usage in cron
 
+/**
+ * Called after adding items to the context cache. Makes sure the context cache
+ * is not getting too large. This deals with situations where the system 
+ * accidentally ends up caching every context in the system and running out
+ * of memory.
+ */
+function check_context_cache_size() {
+    global $context_cache, $context_cache_id;
+
+    // If size is within the allowed limit, return
+    if (count($context_cache_id) < MAX_CONTEXT_CACHE_SIZE) {
+        return;
+    }
+
+    // OK, remove 1/4 the items from cache
+    for ($remove = MAX_CONTEXT_CACHE_SIZE/4; $remove > 0;  $remove--) {
+        $context = reset($context_cache_id);
+        unset($context_cache_id[$context->id]);
+        unset($context_cache[$context->contextlevel][$context->instanceid]);
+    }
+}
+
 function get_role_context_caps($roleid, $context) {
     //this is really slow!!!! - do not use above course context level!
     $result = array();
@@ -2467,6 +2493,7 @@
     }
     rs_close($rs);
     $preloadedcourses[$courseid] = true;
+    check_context_cache_size();
 }
 
 /**
@@ -2517,6 +2544,7 @@
         if (!empty($context)) {
             $context_cache[$contextlevel][$instance] = $context;    // Cache it for later
             $context_cache_id[$context->id]          = $context;    // Cache it for later
+            check_context_cache_size();
         }
 
         return $context;
@@ -2561,6 +2589,7 @@
             if (!empty($context)) {
                 $context_cache[$contextlevel][$instance] = $context;    // Cache it for later
                 $context_cache_id[$context->id] = $context;             // Cache it for later
+                check_context_cache_size();
             }
 
             $result[$instance] = $context;
@@ -2591,6 +2620,7 @@
     if ($context = get_record('context', 'id', $id)) {   // Update the cache and return
         $context_cache[$context->contextlevel][$context->instanceid] = $context;
         $context_cache_id[$context->id] = $context;
+        check_context_cache_size();
         return $context;
     }
 
Index: course/lib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/course/lib.php,v
retrieving revision 1.538.2.69
diff -u -r1.538.2.69 lib.php
--- course/lib.php	14 Jan 2009 04:47:21 -0000	1.538.2.69
+++ course/lib.php	1 Apr 2009 12:03:33 -0000
@@ -21,6 +21,9 @@
 define('MOD_CLASS_ACTIVITY', 0);
 define('MOD_CLASS_RESOURCE', 1);
 
+if (!defined('MAX_MODINFO_CACHE_SIZE')) { 
+    define('MAX_MODINFO_CACHE_SIZE', 10);
+}
 
 function make_log_url($module, $url) {
     switch ($module) {
@@ -1176,6 +1179,11 @@
     unset($cache[$course->id]); // prevent potential reference problems when switching users
     $cache[$course->id] = $modinfo;
 
+    // Ensure cache does not use too much RAM
+    if (count($cache) > MAX_MODINFO_CACHE_SIZE) {
+        unset($cache[reset($cache)->id]);
+    }
+
     return $cache[$course->id];
 }
 
Index: mod/forum/lib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/mod/forum/lib.php,v
retrieving revision 1.609.2.90
diff -u -r1.609.2.90 lib.php
--- mod/forum/lib.php	20 Mar 2009 13:38:27 -0000	1.609.2.90
+++ mod/forum/lib.php	1 Apr 2009 12:03:35 -0000
@@ -5448,21 +5448,21 @@
     switch ($context->contextlevel) {
 
         case CONTEXT_SYSTEM:   // For the whole site
-             if ($courses = get_records('course')) {
-                 foreach ($courses as $course) {
-                     $subcontext = get_context_instance(CONTEXT_COURSE, $course->id);
-                     forum_add_user_default_subscriptions($userid, $subcontext);
-                 }
+             $rs = get_recordset('course', '', '', '', 'id');
+             while ($course = rs_fetch_next_record($rs)) {
+                 $subcontext = get_context_instance(CONTEXT_COURSE, $course->id);
+                 forum_add_user_default_subscriptions($userid, $subcontext);
              }
+             rs_close($rs);
              break;
 
         case CONTEXT_COURSECAT:   // For a whole category
-             if ($courses = get_records('course', 'category', $context->instanceid)) {
-                 foreach ($courses as $course) {
-                     $subcontext = get_context_instance(CONTEXT_COURSE, $course->id);
-                     forum_add_user_default_subscriptions($userid, $subcontext);
-                 }
-             }
+            $rs = get_recordset('course', 'category', $context->instanceid, '', 'id');
+            while ($course = rs_fetch_next_record($rs)) {
+                $subcontext = get_context_instance(CONTEXT_COURSE, $course->id);
+                forum_add_user_default_subscriptions($userid, $subcontext);
+            }
+            rs_close($rs);
              if ($categories = get_records('course_categories', 'parent', $context->instanceid)) {
                  foreach ($categories as $category) {
                      $subcontext = get_context_instance(CONTEXT_COURSECAT, $category->id);