fix(buffer_worker): avoid setting flush timer when inflight is full

Fixes https://emqx.atlassian.net/browse/EMQX-9902

When the buffer worker inflight window is full, we don’t need to set a
timer to flush the messages again because there’s no more room, and
one of the inflight windows will flush the buffer worker by calling
`flush_worker`.

Currently, we do set the timer on such situation, and this fact
combined with the default batch time of 0 yields a busy loop situation
where the CPU spins a lot while inflight messages do not return.

apps/emqx_resource/src/emqx_resource.app.src

@@ -1,7 +1,7 @@
 %% -*- mode: erlang -*-
 {application, emqx_resource, [
     {description, "Manager for all external resources"},
-    {vsn, "0.1.15"},
+    {vsn, "0.1.16"},
     {registered, []},
     {mod, {emqx_resource_app, []}},
     {applications, [

apps/emqx_resource/src/emqx_resource_buffer_worker.erl

@@ -495,8 +495,7 @@ flush(Data0) ->
             {keep_state, Data1};
         {_, true} ->
             ?tp(buffer_worker_flush_but_inflight_full, #{}),
-            Data2 = ensure_flush_timer(Data1),
+            {keep_state, Data1};
-            {keep_state, Data2};
         {_, false} ->
             ?tp(buffer_worker_flush_before_pop, #{}),
             {Q1, QAckRef, Batch} = replayq:pop(Q0, #{count_limit => BatchSize}),

changes/ce/fix-10717.en.md

@@ -0,0 +1 @@
+Fixed an issue where the buffering layer processes could use a lot of CPU when inflight window is full.