ShuffleHandler currently seems to create a map of mapId - mapInfo (file.out / index information) when it receives a message.
This should be caching map info across requests, so that the a scan of all directories is not required for each reducer fetching from the same map.
Also, the scan for each map output / index file is performed twice per mapId within a request. In populateHeaders - once in the call to getMapOutputInfo, and then directly in the method.
For an invocation where we do end up with more than 1000 (default) mapIds in a single call, and don't cache them in the map - the path constructed for such entries will be invalid. This is highly unlikely to be the case though, until there's proper caching.