deltas: fold the update: command back into an empty delta: command.

Also: address parts of #6897, primarily:

* remove the problematic aspect of bumping the last wid in our
TileData, when this could trigger a re-send of a previously
sent delta, causing tile corruption.
* instead append an empty wid entry.
* as an optimization - if the last entry is empty update
the wid - since re-sending an empty delta is of no
concern.

* simplify a number of code-paths that special-case zero length
deltas. All deltas now commence with 'D'.

* still track updates in the JS - by detecting empty deltas.

* shares more code and simplifies various paths.

Signed-off-by: Michael Meeks <michael.meeks@collabora.com>
Change-Id: I02af6d4b152524c201b6985b7a3497da7f08a517

1 files changed, 3 insertions, 2 deletions

diff --git a/kit/Delta.hpp b/kit/Delta.hpp
index 4f88540e42..da4803848c 100644
--- a/kit/Delta.hpp
+++ b/kit/Delta.hpp
@@ -450,8 +450,9 @@ class DeltaGenerator {
         {
             // The tile content is identical to what the client already has, so skip it
             LOG_TRC("Identical / un-changed tile");
-            // Return a zero byte image to inform WSD we didn't need that.
-            // This allows WSD side TileCache to free up waiting subscribers.
+            // Return a zero length delta to inform WSD we didn't need that.
+            // This allows WSD side TileCache to send updates to waiting subscribers.
+            outStream.push_back('D');
             return true;
         }