Use counter in client to server messages to avoid duplicate handling (#11733)

An server message id counter is included in every client to server message and an
expected id is included in every server to client message. This makes it safe to
re-send any message when the client is not 100% sure the server has received the
message, without having to worry about double handling on the server side.

Change-Id: I840cc04829fc2491f35a0e6f98f07eaf46b1ea42

3 files changed, 88 insertions, 4 deletions

diff --git a/server/src/com/vaadin/server/communication/ServerRpcHandler.java b/server/src/com/vaadin/server/communication/ServerRpcHandler.java
index 65fb144810..85a8b341d0 100644
--- a/server/src/com/vaadin/server/communication/ServerRpcHandler.java
+++ b/server/src/com/vaadin/server/communication/ServerRpcHandler.java
@@ -77,6 +77,7 @@ public class ServerRpcHandler implements Serializable {
         private final int syncId;
         private final JsonObject json;
         private final boolean resynchronize;
+        private final int clientToServerMessageId;
 
         public RpcRequest(String jsonString, VaadinRequest request) {
             json = JsonUtil.parse(jsonString);
@@ -106,7 +107,14 @@ public class ServerRpcHandler implements Serializable {
             } else {
                 resynchronize = false;
             }
-
+            if (json.hasKey(ApplicationConstants.CLIENT_TO_SERVER_ID)) {
+                clientToServerMessageId = (int) json
+                        .getNumber(ApplicationConstants.CLIENT_TO_SERVER_ID);
+            } else {
+                getLogger()
+                        .warning("Server message without client id received");
+                clientToServerMessageId = -1;
+            }
             invocations = json.getArray(ApplicationConstants.RPC_INVOCATIONS);
         }
 
@@ -149,6 +157,15 @@ public class ServerRpcHandler implements Serializable {
         }
 
         /**
+         * Gets the id of the client to server message
+         * 
+         * @return the server message id
+         */
+        public int getClientToServerId() {
+            return clientToServerMessageId;
+        }
+
+        /**
          * Gets the entire request in JSON format, as it was received from the
          * client.
          * <p>
@@ -199,8 +216,41 @@ public class ServerRpcHandler implements Serializable {
                 rpcRequest.getCsrfToken())) {
             throw new InvalidUIDLSecurityKeyException("");
         }
-        handleInvocations(ui, rpcRequest.getSyncId(),
-                rpcRequest.getRpcInvocationsData());
+
+        int expectedId = ui.getLastProcessedClientToServerId() + 1;
+        if (rpcRequest.getClientToServerId() != -1
+                && rpcRequest.getClientToServerId() != expectedId) {
+            // Invalid message id, skip RPC processing but force a full
+            // re-synchronization of the client as it might have not received
+            // the previous response (e.g. due to a bad connection)
+
+            // Must resync also for duplicate messages because the server might
+            // have generated a response for the first message but the response
+            // did not reach the client. When the client re-sends the message,
+            // it would only get an empty response (because the dirty flags have
+            // been cleared on the server) and would be out of sync
+            ui.getSession().getCommunicationManager().repaintAll(ui);
+
+            if (rpcRequest.getClientToServerId() < expectedId) {
+                // Just a duplicate message due to a bad connection or similar
+                // It has already been handled by the server so it is safe to
+                // ignore
+                getLogger().fine(
+                        "Ignoring old message from the client. Expected: "
+                                + expectedId + ", got: "
+                                + rpcRequest.getClientToServerId());
+            } else {
+                getLogger().warning(
+                        "Unexpected message id from the client. Expected: "
+                                + expectedId + ", got: "
+                                + rpcRequest.getClientToServerId());
+            }
+        } else {
+            // Message id ok, process RPCs
+            ui.setLastProcessedClientToServerId(expectedId);
+            handleInvocations(ui, rpcRequest.getSyncId(),
+                    rpcRequest.getRpcInvocationsData());
+        }
 
         ui.getConnectorTracker().cleanConcurrentlyRemovedConnectorIds(
                 rpcRequest.getSyncId());
diff --git a/server/src/com/vaadin/server/communication/UidlWriter.java b/server/src/com/vaadin/server/communication/UidlWriter.java
index a4797e49aa..3d52230927 100644
--- a/server/src/com/vaadin/server/communication/UidlWriter.java
+++ b/server/src/com/vaadin/server/communication/UidlWriter.java
@@ -130,7 +130,10 @@ public class UidlWriter implements Serializable {
                     .getCurrentSyncId() : -1;
             writer.write("\"" + ApplicationConstants.SERVER_SYNC_ID + "\": "
                     + syncId + ", ");
-
+            int nextClientToServerMessageId = ui
+                    .getLastProcessedClientToServerId() + 1;
+            writer.write("\"" + ApplicationConstants.CLIENT_TO_SERVER_ID
+                    + "\": " + nextClientToServerMessageId + ", ");
             writer.write("\"changes\" : ");
 
             JsonPaintTarget paintTarget = new JsonPaintTarget(manager, writer,
diff --git a/server/src/com/vaadin/ui/UI.java b/server/src/com/vaadin/ui/UI.java
index 2129db614b..28f18a6b92 100644
--- a/server/src/com/vaadin/ui/UI.java
+++ b/server/src/com/vaadin/ui/UI.java
@@ -260,6 +260,12 @@ public abstract class UI extends AbstractSingleComponentContainer implements
             this);
 
     /**
+     * Tracks which message from the client should come next. First message from
+     * the client has id 0.
+     */
+    private int lastProcessedClientToServerId = -1;
+
+    /**
      * Creates a new empty UI without a caption. The content of the UI must be
      * set by calling {@link #setContent(Component)} before using the UI.
      */
@@ -1691,4 +1697,29 @@ public abstract class UI extends AbstractSingleComponentContainer implements
     public String getEmbedId() {
         return embedId;
     }
+
+    /**
+     * Gets the last processed server message id.
+     * 
+     * Used internally for communication tracking.
+     * 
+     * @return lastProcessedServerMessageId the id of the last processed server
+     *         message
+     */
+    public int getLastProcessedClientToServerId() {
+        return lastProcessedClientToServerId;
+    }
+
+    /**
+     * Sets the last processed server message id.
+     * 
+     * Used internally for communication tracking.
+     * 
+     * @param lastProcessedServerMessageId
+     *            the id of the last processed server message
+     */
+    public void setLastProcessedClientToServerId(
+            int lastProcessedClientToServerId) {
+        this.lastProcessedClientToServerId = lastProcessedClientToServerId;
+    }
 }