coalesce drop notifications

lib/floof.ex

@@ -94,6 +94,11 @@ defmodule Floof do
 
           {backlog, sesskey}
 
+        {:fwddrop, dcdhash} ->
+          to_drop = coalesce_fwddrops(MapSet.new(dcdhash))
+          :ok = send_summary(client, :drop, to_drop)
+          {backlog, sesskey}
+
         {:SessionPushed, sesskey} ->
           sesskeys = coalesce_sesspushes([sesskey])
           [sesskey | _] = sesskeys
@@ -180,9 +185,9 @@ defmodule Floof do
 
       {:xfer, xf} ->
         mhash = Floof.Message.emit(xf)
-        # notify the remote end that we successfully consumed the message
-        # non-fatal if this fails
-        send_summary(client, :drop, [mhash])
+        # the following indirection is necessary
+        # to reduce the amount of 'drop' notifications
+        send(self(), {:fwddrop, mhash})
         {backlog, sesskey}
     end
   end
@@ -210,6 +215,14 @@ defmodule Floof do
     end
   end
 
+  defp coalesce_fwddrops(to_drop) do
+    receive do
+      {:fwddrop, dcdhash} -> coalesce_fwddrops(MapSet.put(to_drop, dcdhash))
+    after
+      10 -> to_drop
+    end
+  end
+
   defp partition_hashes(xs) do
     xs
     |> Floof.DistributorSeen.have_we_multi()