Fixed #1593 - Missing changes on changes feed (Mac <- Android) pull replication

Same issue with https://github.com/couchbase/couchbase-lite-java-core/issues/1495
Referenced previous PR: #1521 , but this PR caused another another issue with `/_changes` REST API.

Notes:
- In case of `/_changes?feed=longpoll` REST API, there is case document is inserted or updated between calling `changesSince()` method and starting to observe Database change event. It could causing unhandled documents. This usually is caused by calling POST/PUT /{db}/{docID} and calling POST/GET /{db}/_changes?feed=longpool concurrently.
- To avoid above scenario, calling changesSince() method in DatabaseListener changed() callback method to make sure if any document changes occurs between them.
- Fixed the case startTimeout() is not called at line 1737.
- Make sure to set correct `since` value by deleting the method `sendLongpollChanges(rev)` that uses 0 as since value.

Author: hideki

src/main/java/com/couchbase/lite/router/Router.java

@@ -114,6 +114,9 @@ private enum TDContentOptions {
     private URL source = null;
 
     // _changes request:
+    private boolean filled = false;
+    private long changesSince = 0;
+    private ChangesOptions changesOptions = null;
     private ReplicationFilter changesFilter;
     Map<String, Object> changesFilterParams = null;
     private boolean longpoll = false;
@@ -1644,10 +1647,6 @@ private void sendContinuousChangeLastSequenceAndFinish(long lastSeq) {
         }
     }
 
-    private void sendLongpollChanges(List<RevisionInternal> revs) {
-        sendLongpollChanges(revs, 0);
-    }
-
     private void sendLongpollChanges(List<RevisionInternal> revs, long since) {
         // Ensure that the content type is application/json:
         connection.getResHeader().add("Content-Type", CONTENT_TYPE_JSON);
@@ -1700,6 +1699,19 @@ public void changed(Database.ChangeEvent event) {
             // Stop timeout timer:
             stopTimeout();
 
+            // In race condition, new doc or update doc is fired before starting to observe the
+            // DatabaseChangeEvent, it allows to skip few document changes with /_changes REST API.
+            // Make sure all document changes are tread by /_changes REST API.
+            if (!filled) {
+                filled = true;
+                RevisionList changes = db.changesSince(changesSince, changesOptions,
+                        changesFilter, changesFilterParams);
+                if (changes.size() > 0) {
+                    sendLongpollChanges(changes, changesSince);
+                    return;
+                }
+            }
+
             List<RevisionInternal> revs = new ArrayList<RevisionInternal>();
             List<DocumentChange> changes = event.getChanges();
             for (DocumentChange change : changes) {
@@ -1734,13 +1746,11 @@ else if (!winningRevID.equals(rev.getRevID())) {
                 timeoutLastSeqence = rev.getSequence();
             }
 
-            if (longpoll) {
-                if (revs.size() > 0)
-                    sendLongpollChanges(revs);
-            } else {
+            if (longpoll && revs.size() > 0)
+                sendLongpollChanges(revs, changesSince);
+            else
                 // Restart timeout timer for continuous feed request:
                 startTimeout();
-            }
         }
     }
 
@@ -1820,6 +1830,15 @@ private Status doChanges(Database db) {
                     timeoutLastSeqence = rev.getSequence();
                 }
             }
+
+            // set following parameters to call Database.changesSince(...) in changed(...) callback
+            // to fill the gap between changesSince() and changed() callbacks.
+            // https://github.com/couchbase/couchbase-lite-java-core/issues/1495
+            filled = false;
+            changesSince = since;
+            changesOptions = options;
+
+            // To observe database change event
             db.addChangeListener(this);
 
             //timeout