tools: start fs submodule, add read_subdir

This creates an iterator relative to a RawFd via
nix::dir::Dir over nix::dir::Entrys.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/tools.rs

@@ -25,6 +25,7 @@ pub mod wrapped_reader_stream;
 pub mod common_regex;
 pub mod ticket;
 pub mod borrow;
+pub mod fs;
 
 /// The `BufferedReader` trait provides a single function
 /// `buffered_read`. It returns a reference to an internal buffer. The

src/tools/fs.rs

@@ -0,0 +1,37 @@
+//! File system helper utilities.
+
+use std::os::unix::io::RawFd;
+
+use failure::Error;
+use nix::dir;
+use nix::dir::Dir;
+
+use crate::tools::borrow::Tied;
+
+// Since Tied<T, U> implements Deref to U, a Tied<Dir, Iterator> already implements Iterator.
+// This is simply a wrapper with a shorter type name mapping nix::Error to failure::Error.
+/// Wrapper over a pair of `nix::dir::Dir` and `nix::dir::Iter`, returned by `read_subdir()`.
+pub struct ReadDir {
+    iter: Tied<Dir, Iterator<Item = nix::Result<dir::Entry>>>,
+}
+
+impl Iterator for ReadDir {
+    type Item = Result<dir::Entry, Error>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter.next().map(|res| res.map_err(|e| Error::from(e)))
+    }
+}
+
+/// Create an iterator over sub directory entries.
+/// This uses `openat` on `dirfd`, so `path` can be relative to that or an absolute path.
+pub fn read_subdir<P: ?Sized + nix::NixPath>(dirfd: RawFd, path: &P) -> Result<ReadDir, Error> {
+    use nix::fcntl::OFlag;
+    use nix::sys::stat::Mode;
+
+    let dir = Dir::openat(dirfd, path, OFlag::O_RDONLY, Mode::empty())?;
+    let iter = Tied::new(dir, |dir| {
+        Box::new(unsafe { (*dir).iter() }) as Box<Iterator<Item = nix::Result<dir::Entry>>>
+    });
+    Ok(ReadDir { iter })
+}