The Rust standard library does not (yet) offer at-style filesystem calls as a
core feature. For instance mkdirat. These calls are essential for writing
race-free filesystem code, since otherwise the state of the filesystem path that
operations are executed against can change silently, leading to TOC-TOU race
conditions. For Unix these calls are readily available in the libc crate, but
for Windows some more plumbing is needed. This crate provides a unified Rust-y
and safe interface to these calls.
Not all platforms behave identically in their underlying syscalls, and this crate doesn't abstract over fundamental differences, but it does attempt to provide consistent errors for key scenarios. As a concrete example creating a directory at the path of an existing link with follow disabled errors with AlreadyExists.
On Linux this is achieved by reading back the path that was requested, as
atomic mkdir isn't yet available. mkdirat is used so the parent directory
is reliable, but the presence of a link pointing to another part of the file
system cannot be precluded.
On Windows this same scenario will either result in fs_at receiving a
NotADirectory error from NtCreateFile, or the open succeeding but a
race-free detection of the presence of the link is done using
DeviceIoControl. Both cases are reported as AlreadyExists. The two
codepaths exist because on Windows symlinks can themselves be files or
directories, and the kernel type-checks some operations such as creating a
directory or truncating a file at both the link target and the link source.
Truncate+nofollow also varies by platform: See OpenOptions::truncate.
I'll keep this compiling against older rusts as long as it is easy, but not at the expense of a lot of code golf, or past CVEs in old releases of dependencies. Currently MSRV is 1.62. If there is a lot of interest in older versions I'm open to patches.
See the crate docs. But in short: use
fs_at::OpenOptions, similar to std::fs::OpenOptions.
openat is a nice wrapper around the Unix *at facilities. It doesn't offer Windows support, and it also requires adoption of a new Dir struct which owns the fd - which adds friction for interop with the rest of std.
cap_std is a lovely rethink of many system interactions as operations on capabilities. Even more than openat, it steps away from the familiar std APIs and instead provides its own comprehensive ecosystem.
Unfortunately that doesn't use the full capabilities of the underlying OS - it layers on top of Rust's own IO stack in some cases (e.g. Windows, some non-Linux), leading to TOCTOU concerns. That is obviously fixable over time - if you want a high level API that will make insecure usage hard, I think cap-std is perfect.
The goal of fs_at isn't to reframe how we do IO though - but just to surface these important calls in an ergonomic way. Perhaps cap_std could layer on fs_at when it is finished.
PR's as normal on Github.
Coverage - consider grcov.
export RUSTFLAGS="-Cinstrument-coverage"
export LLVM_PROFILE_FILE="fs-at-%p-%m.profraw"
cargo test && grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing -o ./target/debug/cove
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