added a race condition comment

packages/sync-service/lib/electric/shape_cache/file_storage/compaction.ex

@@ -6,6 +6,23 @@ defmodule Electric.ShapeCache.FileStorage.Compaction do
   alias Electric.ShapeCache.FileStorage.KeyIndex
   alias Electric.ShapeCache.FileStorage.ActionFile
 
+  # Compaction and race conditions
+  #
+  # `FileStorage` has a pointer to the last compacted offset (and the compacted log file name)
+  # which is updated atomically once the compaction is complete, so while it's ongoing, the
+  # pointer is not updated.
+  #
+  # While the log is compacted in place, it's actually a merged copy that's being
+  # compacted, not the original log. Original log is deleted after the compaction
+  # is complete and the pointer is updated.
+  #
+  # Any concurrent reads of the log that's being replaced are also OK: the `File.rename`
+  # on linux doesn't close the original file descriptor, so the reader will still see
+  # the original file, and we don't reuse file names. Any readers mid-file of the
+  # log that's being replaced but that read the chunk will continue from a correct chunk
+  # of the new file due to offset ordering being preserved. They might observe some updates
+  # more than once in a compacted form.
+
   @spec compact_in_place({String.t(), String.t(), String.t()}, non_neg_integer(), (any(), any() ->
                                                                                      any())) ::
           {String.t(), String.t(), String.t()}