fix serverside index corruption

lib/hk-util.js

@@ -325,6 +325,9 @@ const storeMessage = function (Env, channel, msg, isCp, optionalMessageHash) {
                 }
             }));
         }).nThen((waitFor) => {
+/*  TODO we can skip updating the index if there's nobody in the channel.
+    Populating it might actually be the cause of a memory leak.
+*/
             getIndex(Env, id, waitFor((err, index) => {
                 if (err) {
                     Log.warn("HK_STORE_MESSAGE_INDEX", err.stack);
@@ -340,7 +343,12 @@ const storeMessage = function (Env, channel, msg, isCp, optionalMessageHash) {
                         line: ((index.line || 0) + 1)
                     });
                 }
-                if (optionalMessageHash) {
+/*  This 'getIndex' call will construct a new index if one does not already exist.
+    If that is the case then our message will already be present and updating our offset map
+    can actually cause it to become incorrect, leading to incorrect behaviour when clients connect
+    with a lastKnownHash. We avoid this by only assigning new offsets to the map.
+*/
+                if (optionalMessageHash && typeof(index.offsetByHash[optionalMessageHash]) === 'undefined') {
                     index.offsetByHash[optionalMessageHash] = index.size;
                     index.offsets++;
                 }