avoid incrementing index size twice for a single message

lib/hk-util.js

@@ -300,6 +300,7 @@ var trimMapByOffset = function (map, offset) {
     * the fix is to use callbacks and implement queueing for writes
       * to guarantee that offset computation is always atomic with writes
 */
+// FIXME 'optionalMessageHash' is always supplied, so we could consider renaming it for clarity
 const storeMessage = function (Env, channel, msg, isCp, optionalMessageHash, cb) {
     const id = channel.id;
     const Log = Env.Log;
@@ -338,6 +339,12 @@ const storeMessage = function (Env, channel, msg, isCp, optionalMessageHash, cb)
                     cb();
                     return void next();
                 }
+
+                if (optionalMessageHash && typeof(index.offsetByHash[optionalMessageHash]) === 'number') {
+                    cb();
+                    return void next();
+                }
+
                 if (typeof (index.line) === "number") { index.line++; }
                 if (isCp) {
                     index.cpIndex = sliceCpIndex(index.cpIndex, index.line || 0);
@@ -352,7 +359,7 @@ const storeMessage = function (Env, channel, msg, isCp, optionalMessageHash, cb)
     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') {
+                if (optionalMessageHash /* && typeof(index.offsetByHash[optionalMessageHash]) === 'undefined' */) {
                     index.offsetByHash[optionalMessageHash] = index.size;
                     index.offsets++;
                 }