This addresses a subtle deadlock where an error during a copy prevented pipe
closure to propagate correctly. By closing down the read end of the pipe rather
than the write end, the waiting writer is properly signaled. A nice side-effect
of this change is that errors encountered by io.Copy are no propagated to the
verifier's Write method.
A test to ensure validation errors for unsupported digest types has been added,
as well.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
8c254edb9a