Variable: process-file-side-effects

Whether a call of `process-file' changes remote files.

By default, this variable is always set to `t', meaning that a
call of `process-file' could potentially change any file on a
remote host. When set to `nil', a file handler could optimize
its behavior with respect to remote file attribute caching.

You should only ever change this variable with a let-binding; never with `setq'.