oci: add handling for "/" in layer tars

So far we've been testing with local podman builds, which don't produce
these by default, but we need to support these, as they appear in some
bootc images.  It's a bit tricky because our root directory isn't an
Inode, so add a special path.

While we're at it, add some error context to the path where this was
failing before.

Signed-off-by: Allison Karlitskaya <[email protected]>

Author: allisonkarlitskaya

src/oci/image.rs

@@ -1,6 +1,6 @@
 use std::{ffi::OsStr, os::unix::ffi::OsStrExt, rc::Rc};
 
-use anyhow::Result;
+use anyhow::{ensure, Context, Result};
 use oci_spec::image::ImageConfiguration;
 
 use crate::{
@@ -20,6 +20,19 @@ pub fn process_entry<ObjectID: FsVerityHashValue>(
     filesystem: &mut FileSystem<ObjectID>,
     entry: TarEntry<ObjectID>,
 ) -> Result<()> {
+    if entry.path.file_name().is_none() {
+        // special handling for the root directory
+        ensure!(
+            matches!(entry.item, TarItem::Directory),
+            "Unpacking layer tar: filename {:?} must be a directory",
+            entry.path
+        );
+
+        // Update the stat, but don't do anything else
+        filesystem.set_root_stat(entry.stat);
+        return Ok(());
+    }
+
     let inode = match entry.item {
         TarItem::Directory => Inode::Directory(Box::from(Directory::new(entry.stat))),
         TarItem::Leaf(content) => Inode::Leaf(Rc::new(Leaf {
@@ -32,7 +45,15 @@ pub fn process_entry<ObjectID: FsVerityHashValue>(
         }
     };
 
-    let (dir, filename) = filesystem.root.split_mut(entry.path.as_os_str())?;
+    let (dir, filename) = filesystem
+        .root
+        .split_mut(entry.path.as_os_str())
+        .with_context(|| {
+            format!(
+                "Error unpacking container layer file {:?} {:?}",
+                entry.path, inode
+            )
+        })?;
 
     let bytes = filename.as_bytes();
     if let Some(whiteout) = bytes.strip_prefix(b".wh.") {