This helps make it possible to use `SecureDir` (name pending) even
when we want to disable permissions checks. Otherwise, optional
permission checking would require users of this crate to maintain
separate code paths for the "check" and "don't check" cases.
This required a bit of poking through the `users` crate, to mess
with the user and group dbs. The original goal was to "trust the
group with the same name as us", but it turned into a bit of a
production, since:
* We want to take our own name from $USER, assuming that matches
our uid. (Otherwise we want to ask getpwuid_r().)
* We only want to trust the group if we are actually a member of
that group.
* We want to cache this information.
* We want to test this code.
Previously we would temporarily put self.resolved into an invalid
state by adding a path component that might be a symlink. With this
change, we create a new temporary path object (using Cow to avoid
unnecessary allocations) and only conditionally replace
self.resolved.
The only way to get a SecureDir is by having checked a directory.
Once you have one, it encourages you to open and create files and
directories with the right permissions, and checks them for you.
This crate is meant to solve #315 by giving a way to make sure that
a file or directory is only accessible by trusted users. I've tried
to explain carefully (in comments and documentation) what this crate
is doing and why, under the assumption that it will someday be read
by another person like me who does _not_ live and breathe unix file
permissions. The crate is still missing some key features, noted in
the TODO section.
It differs from the first version of the crate by taking a more
principled approach to directory checking: it emulates the path
lookup process (reading symlinks and all) one path change at a time,
thus ensuring that we check every directory which could enable
an untrusted user to get to our target file, _or_ which could
enable them to get to any symlink that would get them to the target
file.
The API is also slightly different: It separates the `Mistrust`
object (where you configure what you do or do not trust) from the
`Verifier` (where you set up a check that you want to perform on a
single object). Verifiers are set up to be a bit ephemeral,
so that it is hard to accidentally declare that _every_ object
is meant to be readable when you only mean that _some_ objects
may be readable.
Nick Mathewson