file_get_file_references() can cause PHP out of memory errors

Problem/Motivation

If you have a managed file that is referenced by 200k entities then file_get_file_references() will cause PHP out of memory errors. It uses an unlimited entity field query and unlimited queries are bad.

Steps to reproduce

Create a managed file and reference it from 200k nodes.

Then invoke hook_file_download() for example from entity_metadata_file_access().

You get a PHP memory error.

Proposed resolution

Limit the query to just 1 result. This is not ideal and could cause problems, but in our use case a file is always used in the same field type. That has no access implications for us.

Remaining tasks

Review patch, come up with a better idea?