dir: Add an is_mountpoint API

Cargo.toml

@@ -16,6 +16,7 @@ cap-primitives = "3"
 
 [target.'cfg(not(windows))'.dependencies]
 rustix = { version = "0.38", features = ["fs", "procfs", "process", "pipe"] }
+libc = "0.2"
 
 [dev-dependencies]
 anyhow = "1.0"

src/dirext.rs

@@ -119,6 +119,14 @@ pub trait CapStdExtDirExt {
         contents: impl AsRef<[u8]>,
         perms: cap_std::fs::Permissions,
     ) -> Result<()>;
+
+    #[cfg(any(target_os = "android", target_os = "linux"))]
+    /// Returns `Some(true)` if the target is known to be a mountpoint, or
+    /// `Some(false)` if the target is definitively known not to be a mountpoint.
+    ///
+    /// In some scenarios (such as an older kernel) this currently may not be possible
+    /// to determine, and `None` will be returned in those cases.
+    fn is_mountpoint(&self, path: impl AsRef<Path>) -> Result<Option<bool>>;
 }
 
 #[cfg(feature = "fs_utf8")]
@@ -300,6 +308,28 @@ fn subdir_of<'d, 'p>(d: &'d Dir, p: &'p Path) -> io::Result<(DirOwnedOrBorrowed<
     Ok((r, name))
 }
 
+fn is_mountpoint_impl_statx(root: &Dir, path: &Path) -> Result<Option<bool>> {
+    // https://github.com/systemd/systemd/blob/8fbf0a214e2fe474655b17a4b663122943b55db0/src/basic/mountpoint-util.c#L176
+    use rustix::fs::{AtFlags, StatxFlags};
+    use std::os::fd::AsFd;
+
+    // SAFETY(unwrap): We can infallibly convert an i32 into a u64.
+    let mountroot_flag: u64 = libc::STATX_ATTR_MOUNT_ROOT.try_into().unwrap();
+    match rustix::fs::statx(
+        root.as_fd(),
+        path,
+        AtFlags::NO_AUTOMOUNT | AtFlags::SYMLINK_NOFOLLOW,
+        StatxFlags::empty(),
+    ) {
+        Ok(r) => {
+            let present = (r.stx_attributes_mask & mountroot_flag) > 0;
+            Ok(present.then_some(r.stx_attributes & mountroot_flag > 0))
+        }
+        Err(e) if e == rustix::io::Errno::NOSYS => Ok(None),
+        Err(e) => Err(e.into()),
+    }
+}
+
 impl CapStdExtDirExt for Dir {
     fn open_optional(&self, path: impl AsRef<Path>) -> Result<Option<File>> {
         map_optional(self.open(path.as_ref()))
@@ -452,6 +482,10 @@ impl CapStdExtDirExt for Dir {
             Ok(())
         })
     }
+
+    fn is_mountpoint(&self, path: impl AsRef<Path>) -> Result<Option<bool>> {
+        is_mountpoint_impl_statx(self, path.as_ref()).map_err(Into::into)
+    }
 }
 
 // Implementation for the Utf8 variant of Dir. You shouldn't need to add

tests/it/main.rs

@@ -383,3 +383,12 @@ fn test_rootdir_entries() -> Result<()> {
     assert_eq!(ents.len(), 2);
     Ok(())
 }
+
+#[test]
+fn test_mountpoint() -> Result<()> {
+    let root = &Dir::open_ambient_dir("/", cap_std::ambient_authority())?;
+    assert_eq!(root.is_mountpoint(".").unwrap(), Some(true));
+    let td = &cap_tempfile::TempDir::new(cap_std::ambient_authority())?;
+    assert_eq!(td.is_mountpoint(".").unwrap(), Some(false));
+    Ok(())
+}