Update AsynchronousReadIndirectBufferFromRemoteFS.cpp

src/Disks/IO/AsynchronousReadIndirectBufferFromRemoteFS.cpp

@@ -133,8 +133,18 @@ void AsynchronousReadIndirectBufferFromRemoteFS::prefetch()
 
 void AsynchronousReadIndirectBufferFromRemoteFS::setReadUntilPosition(size_t position)
 {
+    /// Do not reinitialize internal state in case the new end of range is already included.
+    /// Actually it is likely that we will anyway reinitialize it as seek method is called after
+    /// changing end position, but seek avoiding feature might help to avoid reinitialization,
+    /// so this check is useful to save the prefetch for the time when we try to avoid seek by
+    /// reading and ignoring some data.
     if (!read_until_position || position > *read_until_position)
     {
+        /// We must wait on future and reset the prefetch here, because otherwise there might be
+        /// a race between reading the data in the threadpool and impl->setReadUntilPosition()
+        /// which reinitializes internal remote read buffer (because if we have a new read range
+        /// then we need a new range request) and in case of reading from cache we need to request
+        /// and hold more file segment ranges from cache.
         if (prefetch_future.valid())
         {
             ProfileEvents::increment(ProfileEvents::RemoteFSCancelledPrefetches);