Could repeatably read zeros.

With the handle position caching in, avoiding seeks opened a lovely opportunity for reading zeros. The read handle had read_ahead turned on, so even though the write handle was being sync'd correctly, the next read was reading cached data. Consequently, reading zeros instead of the real message.

This makes a valuable point which is that with read_ahead turned on, the seeks on every read, as was happening previously, were evicting the read_ahead cache, even if the seek was in effect a noop. Thus read_ahead, in combination with the seeks, made no performance gain.

Consequently, turning read_ahead off and only seeking when necessary is a performance gain, over (pointlessly) having read_ahead on and seeking every time. Turning read_ahead off also solves this reading zeros bug.

Finally, corrected some maths so that we now check to see if the sync offset is < the offset we will reach rather than <= the offset we read from. Because the offsets should only ever be on message boundaries, both will work, but the new version is more intuitively correct.

1 files changed, 3 insertions, 4 deletions

diff --git a/src/rabbit_disk_queue.erl b/src/rabbit_disk_queue.erl
index d19469d682..344aff9181 100644
--- a/src/rabbit_disk_queue.erl
+++ b/src/rabbit_disk_queue.erl
@@ -761,18 +761,17 @@ get_read_handle(File, Offset, TotalSize, State =
                            current_dirty = IsDirty,
                            last_sync_offset = SyncOffset
                          }) ->
-    State1 = if CurName =:= File andalso IsDirty andalso Offset >= SyncOffset ->
+    NewOffset = Offset + TotalSize + ?FILE_PACKING_ADJUSTMENT,
+    State1 = if CurName =:= File andalso IsDirty andalso NewOffset > SyncOffset ->
                      sync_current_file_handle(State);
                 true -> State
              end,
     Now = now(),
-    NewOffset = Offset + TotalSize + ?FILE_PACKING_ADJUSTMENT,
     {FileHdl, OldOffset, ReadHdls1, ReadHdlsAge1} =
         case dict:find(File, ReadHdls) of
             error ->
                 {ok, Hdl} = file:open(form_filename(File),
-                                      [read, raw, binary,
-                                       read_ahead]),
+                                      [read, raw, binary]),
                 case dict:size(ReadHdls) < ReadFileHandlesLimit of
                     true ->
                         {Hdl, 0, ReadHdls, ReadHdlsAge};