4.x: Optimise msg_store recovery in case of large message file

deps/rabbit/src/rabbit_msg_store.erl

@@ -1515,28 +1515,38 @@ scan_data(<<Size:64, MsgIdAndMsg:Size/binary, 255, Rest/bits>> = Data,
         %% a remnant from a previous compaction, but it might
         %% simply be a coincidence. Try the next byte.
         #{MsgIdInt := true} ->
-            <<_, Rest2/bits>> = Data,
-            scan_data(Rest2, Fd, Fun, Offset + 1, FileSize, MsgIdsFound, Acc);
+            scan_next_byte(Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc);
         %% Data looks to be a message.
         _ ->
-            %% Avoid sub-binary construction.
-            MsgId = <<MsgIdInt:128>>,
             TotalSize = Size + 9,
-            case Fun({MsgId, TotalSize, Offset}) of
-                %% Confirmed to be a message by the provided fun.
-                {valid, Entry} ->
+            case check_msg(Fun, MsgIdInt, TotalSize, Offset, Acc) of
+                {continue, NewAcc} ->
                     scan_data(Rest, Fd, Fun, Offset + TotalSize, FileSize,
-                              MsgIdsFound#{MsgIdInt => true}, [Entry|Acc]);
-                %% Confirmed to be a message but we don't need it anymore.
-                previously_valid ->
-                    scan_data(Rest, Fd, Fun, Offset + TotalSize, FileSize,
-                              MsgIdsFound#{MsgIdInt => true}, Acc);
-                %% Not a message, try the next byte.
-                invalid ->
-                    <<_, Rest2/bits>> = Data,
-                    scan_data(Rest2, Fd, Fun, Offset + 1, FileSize, MsgIdsFound, Acc)
+                              MsgIdsFound#{MsgIdInt => true}, NewAcc);
+                try_next_byte ->
+                    scan_next_byte(Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc)
             end
     end;
+%% Large message alone in its own file
+scan_data(<<Size:64, MsgIdInt:128, _Rest/bits>> = Data, Fd, Fun, Offset, FileSize, _MsgIdsFound, _Acc)
+  when Offset == 0,
+       FileSize == Size + 9 ->
+    {ok, CurrentPos} = file:position(Fd, cur),
+    case file:pread(Fd, FileSize - 1, 1) of
+        {ok, <<255>>} ->
+            TotalSize = FileSize,
+            case check_msg(Fun, MsgIdInt, TotalSize, Offset, []) of
+                {continue, NewAcc} ->
+                    NewAcc;
+                try_next_byte ->
+                    {ok, _} = file:position(Fd, CurrentPos),
+                    scan_next_byte(Data, Fd, Fun, Offset, FileSize, #{}, [])
+            end;
+        _ ->
+            %% Wrong end marker
+            {ok, _} = file:position(Fd, CurrentPos),
+            scan_next_byte(Data, Fd, Fun, Offset, FileSize, #{}, [])
+    end;
 %% This might be the start of a message.
 scan_data(<<Size:64, Rest/bits>> = Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc)
           when byte_size(Rest) < Size + 1, Size < FileSize - Offset ->
@@ -1545,9 +1555,27 @@ scan_data(Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc)
           when byte_size(Data) < 8 ->
     scan(Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc);
 %% This is definitely not a message. Try the next byte.
-scan_data(<<_, Rest/bits>>, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc) ->
+scan_data(Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc) ->
+    scan_next_byte(Data, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc).
+
+scan_next_byte(<<_, Rest/bits>>, Fd, Fun, Offset, FileSize, MsgIdsFound, Acc) ->
     scan_data(Rest, Fd, Fun, Offset + 1, FileSize, MsgIdsFound, Acc).
 
+check_msg(Fun, MsgIdInt, TotalSize, Offset, Acc) ->
+    %% Avoid sub-binary construction.
+    MsgId = <<MsgIdInt:128>>,
+    case Fun({MsgId, TotalSize, Offset}) of
+        %% Confirmed to be a message by the provided fun.
+        {valid, Entry} ->
+            {continue, [Entry|Acc]};
+        %% Confirmed to be a message but we don't need it anymore.
+        previously_valid ->
+            {continue, Acc};
+        %% Not a message, try the next byte.
+        invalid ->
+            try_next_byte
+    end.
+
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