Don't write metadata updates to the log if it doesn't change the value

lib/metadata.js

@@ -28,10 +28,14 @@ commands.ADD_OWNERS = function (meta, args) {
         throw new Error("METADATA_NONSENSE_OWNERS");
     }
 
+    var changed = false;
     args.forEach(function (owner) {
         if (meta.owners.indexOf(owner) >= 0) { return; }
         meta.owners.push(owner);
+        changed = true;
     });
+
+    return changed;
 };
 
 // ["RM_OWNERS", ["CrufexqXcY-z+eKJlEbNELVy5Sb7E-EAAEFI8GnEtZ0="], 1561623439989]
@@ -45,13 +49,17 @@ commands.RM_OWNERS = function (meta, args) {
         throw new Error("METADATA_NONSENSE_OWNERS");
     }
 
+    var changed = false;
     // remove owners one by one
     // we assume there are no duplicates
     args.forEach(function (owner) {
         var index = meta.owners.indexOf(owner);
         if (index < 0) { return; }
         meta.owners.splice(index, 1);
+        changed = true;
     });
+
+    return changed;
 };
 
 // ["ADD_PENDING_OWNERS", ["7eEqelGso3EBr5jHlei6av4r9w2B9XZiGGwA1EgZ-5I="], 1561623438989]
@@ -67,16 +75,20 @@ commands.ADD_PENDING_OWNERS = function (meta, args) {
         throw new Error("METADATA_NONSENSE_PENDING_OWNERS");
     }
 
+    var changed = false;
     // Add pending_owners array if it doesn't exist
     if (!meta.pending_owners) {
         meta.pending_owners = deduplicate(args);
-        return;
+        return true;
     }
     // or fill it
     args.forEach(function (owner) {
         if (meta.pending_owners.indexOf(owner) >= 0) { return; }
         meta.pending_owners.push(owner);
+        changed = true;
     });
+
+    return changed;
 };
 
 // ["RM_PENDING_OWNERS", ["CrufexqXcY-z+eKJlEbNELVy5Sb7E-EAAEFI8GnEtZ0="], 1561623439989]
@@ -90,13 +102,17 @@ commands.RM_PENDING_OWNERS = function (meta, args) {
         throw new Error("METADATA_NONSENSE_PENDING_OWNERS");
     }
 
+    var changed = false;
     // remove owners one by one
     // we assume there are no duplicates
     args.forEach(function (owner) {
         var index = meta.pending_owners.indexOf(owner);
         if (index < 0) { return; }
         meta.pending_owners.splice(index, 1);
+        changed = true;
     });
+
+    return changed;
 };
 
 // ["RESET_OWNERS", ["7eEqelGso3EBr5jHlei6av4r9w2B9XZiGGwA1EgZ-5I="], 1561623439989]
@@ -112,6 +128,7 @@ commands.RESET_OWNERS = function (meta, args) {
 
     // overwrite the existing owners with the new one
     meta.owners = deduplicate(args);
+    return true;
 };
 
 commands.UPDATE_EXPIRATION = function () {

rpc.js

@@ -340,6 +340,7 @@ var getMetadata = function (Env, channel, cb) {
         value: value
     }
 */
+// XXX global saferphore may cause issues here, a queue "per channel" is probably better
 var metadataSem = Saferphore.create(1);
 var setMetadata = function (Env, data, unsafeKey, cb) {
     var channel = data.channel;
@@ -382,13 +383,19 @@ var setMetadata = function (Env, data, unsafeKey, cb) {
 
             // Add the new metadata line
             var line = [command, data.value, +new Date()];
+            var changed = false;
             try {
-                Meta.handleCommand(metadata, line);
+                changed = Meta.handleCommand(metadata, line);
             } catch (e) {
                 g();
                 return void cb(e);
             }
 
+            // if your command is valid but it didn't result in any change to the metadata,
+            // call back now and don't write any "useless" line to the log
+            if (!changed) {
+                return void cb(void 0, metadata);
+            }
             Env.msgStore.writeMetadata(channel, JSON.stringify(line), function (e) {
                 g();
                 if (e) {