Here's an initial effort at adding an indirection layer to all I/O, so that it can be mocked or traced. The approach I'm using here is to put a trait object in the Config to handle all top-level filesystem interactions, and to make open file handles trait objects as well. There are currently two sets of implementations for these traits, one for the real operating system, and a simple mock file system for testing. The mocks lean heavily on mutexes for now, concurrent file operations are future work. strace reports that the test using mocked I/O does not touch the operating system! Tracing is not yet supported, but it would fit naturally as wrapper implementations of the I/O traits. Something in these changes seems to cause a significant performance penalty on the benchmarks, so non-test performance is an open issue too.

See also #1077. Some of the changes here also appear in #1108.