Fix bug with topic edits on read messages.

If you read a message, then got a topic edit for it, we were
adding the message to our data structure of unread stream/topic
messages.

Now we guard against this in unread.update_unread_topics. I
no longer expose an update() method in unread_topic_counter,
since we really want to do the unread check at a higher level
to keep other data structures consistent.

frontend_tests/node_tests/unread.js

@@ -145,6 +145,26 @@ var zero_counts = {
     };
     unread.update_unread_topics(other_message, event);
 
+    // Update a message that was never marked as unread.
+    var sticky_message = {
+        id: 17,
+        type: 'stream',
+        stream_id: stream_id,
+        subject: 'sticky',
+    };
+
+    unread.process_loaded_messages([sticky_message]);
+    count = unread.num_unread_for_topic(stream_id, 'sticky');
+    assert.equal(count, 1);
+
+    unread.mark_as_read(sticky_message.id);
+    count = unread.num_unread_for_topic(stream_id, 'sticky');
+    assert.equal(count, 0);
+
+    unread.update_unread_topics(sticky_message, {subject: 'sticky'});
+    count = unread.num_unread_for_topic(stream_id, 'sticky');
+    assert.equal(count, 0);
+
     // cleanup
     unread.mark_as_read(message.id);
     count = unread.num_unread_for_topic(stream_id, 'dinner');
@@ -153,6 +173,9 @@ var zero_counts = {
     unread.mark_as_read(other_message.id);
     count = unread.num_unread_for_topic(stream_id, 'snack');
     assert.equal(count, 0);
+
+    // test coverage
+    unread.update_unread_topics(sticky_message, {});
 }());
 
 stream_data.get_stream_id = function () {

static/js/unread.js

@@ -164,11 +164,6 @@ exports.unread_topic_counter = (function () {
         bucketer.clear();
     };
 
-    self.update = function (msg_id, stream_id, new_topic) {
-        self.del(msg_id);
-        self.add(stream_id, new_topic, msg_id);
-    };
-
     self.add = function (stream_id, topic, msg_id) {
         bucketer.add({
             bucket_key: stream_id,
@@ -297,13 +292,23 @@ exports.id_flagged_as_unread = function (message_id) {
 };
 
 exports.update_unread_topics = function (msg, event) {
-    if (event.subject !== undefined) {
-        exports.unread_topic_counter.update(
-            msg.id,
-            msg.stream_id,
-            event.subject
-        );
+    if (event.subject === undefined) {
+        return;
     }
+
+    if (!unread_messages.has(msg.id)) {
+        return;
+    }
+
+    exports.unread_topic_counter.del(
+        msg.id
+    );
+
+    exports.unread_topic_counter.add(
+        msg.stream_id,
+        event.subject,
+        msg.id
+    );
 };
 
 exports.process_loaded_messages = function (messages) {