tar: Handle absolute hardlinked paths to sysroot

See containers/bootc#856

Basically we need to handle absolute paths here too.

This currently fails tests as we need to normalize in multiple
places.

lib/src/tar/write.rs

@@ -159,6 +159,23 @@ fn remap_etc_path(path: &Utf8Path) -> Cow<Utf8Path> {
     }
 }
 
+/// Checks whether the given path has `prefix` as a prefix, ignoring or relative path
+/// prefixes. The input paths are also normalized.
+fn path_starts_with_ignoring_absolute(
+    path: impl AsRef<Utf8Path>,
+    prefix: impl AsRef<Utf8Path>,
+) -> bool {
+    fn filter_prefix(c: &Utf8Component) -> bool {
+        !matches!(
+            c,
+            camino::Utf8Component::RootDir | camino::Utf8Component::CurDir,
+        )
+    }
+    let path = path.as_ref().components().filter(filter_prefix);
+    let prefix = prefix.as_ref().components().filter(filter_prefix);
+    path.zip(prefix).all(|(a, b)| a == b)
+}
+
 fn normalize_validate_path<'a>(
     path: &'a Utf8Path,
     config: &'_ TarImportConfig,
@@ -266,7 +283,7 @@ pub(crate) fn filter_tar(
 
         let is_modified = header.mtime().unwrap_or_default() > 0;
         let is_regular = header.entry_type() == tar::EntryType::Regular;
-        if path.strip_prefix(crate::tar::REPO_PREFIX).is_ok() {
+        if path_starts_with_ignoring_absolute(&path, crate::tar::REPO_PREFIX) {
             // If it's a modified file in /sysroot, it may be a target for future hardlinks.
             // In that case, we copy the data off to a temporary file.  Then the first hardlink
             // to it becomes instead the real file, and any *further* hardlinks refer to that
@@ -294,7 +311,7 @@ pub(crate) fn filter_tar(
                 .ok_or_else(|| anyhow!("Invalid empty hardlink"))?;
             let target: &Utf8Path = (&*target).try_into()?;
             // If this is a hardlink into /sysroot...
-            if target.strip_prefix(crate::tar::REPO_PREFIX).is_ok() {
+            if path_starts_with_ignoring_absolute(&target, crate::tar::REPO_PREFIX) {
                 // And we found a previously processed modified file there
                 if let Some((mut header, data)) = changed_sysroot_objects.remove(target) {
                     tracing::debug!("Making {path} canonical for sysroot link {target}");
@@ -509,6 +526,33 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_path_match_ignoring_absolute() {
+        for (path, prefix) in [
+            ("foo", "foo"),
+            ("foo", "/"),
+            ("foo/bar/baz", "foo"),
+            ("/foo", "foo"),
+            ("/foo/bar", "foo"),
+            ("/foo/bar", "/foo"),
+            ("/foo/bar", "/foo/bar"),
+            ("///foo", "foo"),
+            ("///foo", "foo"),
+        ] {
+            assert!(path_starts_with_ignoring_absolute(path, prefix));
+        }
+
+        for (path, prefix) in [
+            ("bar", "foo"),
+            ("foo/bar/baz", "foo/baz"),
+            ("/bar", "foo"),
+            ("/foo/bar", "baz"),
+            ("/foo/bar", "/baz"),
+        ] {
+            assert!(!path_starts_with_ignoring_absolute(path, prefix));
+        }
+    }
+
     #[test]
     fn test_normalize_path() {
         let imp_default = &TarImportConfig {

lib/tests/it/main.rs

@@ -1132,7 +1132,16 @@ async fn test_container_var_content() -> Result<()> {
 }
 
 #[tokio::test]
-async fn test_container_etc_hardlinked() -> Result<()> {
+async fn test_container_etc_hardlinked_absolute() -> Result<()> {
+    test_container_etc_hardlinked(true).await
+}
+
+#[tokio::test]
+async fn test_container_etc_hardlinked_relative() -> Result<()> {
+    test_container_etc_hardlinked(false).await
+}
+
+async fn test_container_etc_hardlinked(absolute: bool) -> Result<()> {
     let fixture = Fixture::new_v1()?;
 
     let imgref = fixture.export_container().await.unwrap().0;
@@ -1186,7 +1195,13 @@ async fn test_container_etc_hardlinked() -> Result<()> {
             h.set_entry_type(tar::EntryType::Link);
             h.set_mtime(42);
             h.set_size(0);
-            layer_tar.append_link(&mut h.clone(), "sysroot/ostree/repo/objects/45/7279b28b541ca20358bec8487c81baac6a3d5ed3cea019aee675137fab53cb.file", "etc/dnf.conf")?;
+            let path = "sysroot/ostree/repo/objects/45/7279b28b541ca20358bec8487c81baac6a3d5ed3cea019aee675137fab53cb.file";
+            let path = if absolute {
+                Cow::Owned(format!("/{path}"))
+            } else {
+                Cow::Borrowed(path)
+            };
+            layer_tar.append_link(&mut h.clone(), &*path, "etc/dnf.conf")?;
             layer_tar.finish()?;
             Ok(())
         },