commit: Skip bind mounts too

This fixes a failure I was seeing using `buildah` in Gitlab CI;
it seems like `/run/secrets` ends up as a bind mount here, and
because it's a bind mount it has the same device node.

With new enough Linux there's a handy attribute from the `statx`
system call that will tell us whether a given path is a mount
point root.  Let's use that to start.

(The other alternatives are much uglier; see the linked systemd
 git source code)

We may need to end up doing some of that if we find we
keep hitting this with systems that have older kernels.

lib/src/commit.rs

@@ -9,6 +9,7 @@ use camino::Utf8Path;
 use cap_std::fs::Dir;
 use cap_std_ext::cap_std;
 use cap_std_ext::dirext::CapStdExtDirExt;
+use io_lifetimes::AsFd;
 use rustix::fs::MetadataExt;
 use std::borrow::Cow;
 use std::convert::TryInto;
@@ -101,16 +102,42 @@ fn remove_all_on_mount_recurse(root: &Dir, rootdev: u64, path: &Path) -> Result<
     Ok(skipped)
 }
 
+/// Try to (heuristically) determine if the provided path is a mount root.
+fn is_mountpoint(root: &Dir, path: &Path) -> bool {
+    // https://github.com/systemd/systemd/blob/8fbf0a214e2fe474655b17a4b663122943b55db0/src/basic/mountpoint-util.c#L176
+    use rustix::fs::{AtFlags, StatxFlags};
+
+    let attr_mount_root: u32 = libc::STATX_ATTR_MOUNT_ROOT.try_into().unwrap();
+    let flags: StatxFlags = StatxFlags::from_bits(attr_mount_root).unwrap();
+    if let Ok(meta) = rustix::fs::statx(
+        root.as_fd(),
+        path,
+        AtFlags::NO_AUTOMOUNT | AtFlags::SYMLINK_NOFOLLOW,
+        flags,
+    ) {
+        (meta.stx_attributes & attr_mount_root as u64) > 0
+    } else {
+        return false;
+    }
+}
+
 fn clean_subdir(root: &Dir, rootdev: u64) -> Result<()> {
     for entry in root.entries()? {
         let entry = entry?;
         let metadata = entry.metadata()?;
         let dev = metadata.dev();
+        let path = PathBuf::from(entry.file_name());
         // Ignore other filesystem mounts, e.g. podman injects /run/.containerenv
         if dev != rootdev {
+            tracing::trace!("Skipping entry in foreign dev {path:?}");
+            continue;
+        }
+        // Also ignore bind mounts, if we have a new enough kernel with statx()
+        // that will tell us.
+        if is_mountpoint(root, &path) {
+            tracing::trace!("Skipping mount point {path:?}");
             continue;
         }
-        let path = PathBuf::from(entry.file_name());
         if metadata.is_dir() {
             remove_all_on_mount_recurse(root, rootdev, &path)?;
         } else {