oci: Switch to tar-core crate

Working on direct import from containers-storage: I hit on
the fact that our tar parsing was very hand-rolled and incomplete.
We didn't handle PAX overrides correctly, there was a big `todo!`
for global headers where we'd panic, etc.

Recently I did a new tar-core crate which is now part of
the composefs org - it's "sans-io" so works cleanly
with both sync and async parsing.

Update the code to use it.

The tar crate is retained as a dev-dependency for tar::Builder in
tests.

Assisted-by: OpenCode (Claude claude-opus-4-6)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/composefs-oci/Cargo.toml

@@ -24,12 +24,13 @@ rustix = { version = "1.0.0", features = ["fs"] }
 serde = { version = "1.0", default-features = false, features = ["derive"] }
 serde_json = { version = "1.0", default-features = false, features = ["std"] }
 sha2 = { version = "0.10.1", default-features = false }
-tar = { version = "0.4.38", default-features = false }
+tar-core = "0.1.0"
 tokio = { version = "1.24.2", features = ["rt-multi-thread"] }
 tokio-util = { version = "0.7", default-features = false, features = ["io"] }
 
 [dev-dependencies]
 similar-asserts = "1.7.0"
+tar = { version = "0.4.38", default-features = false }
 composefs = { workspace = true, features = ["test"] }
 once_cell = "1.21.3"
 proptest = "1"

crates/composefs-oci/src/lib.rs

@@ -115,7 +115,7 @@ pub async fn import_layer<ObjectID: FsVerityHashValue>(
     repo: &Arc<Repository<ObjectID>>,
     diff_id: &str,
     name: Option<&str>,
-    tar_stream: impl tokio::io::AsyncBufRead + Unpin,
+    tar_stream: impl tokio::io::AsyncRead + Unpin,
 ) -> Result<(ObjectID, ImportStats)> {
     let content_identifier = layer_identifier(diff_id);
 

crates/composefs-oci/src/skopeo.rs

@@ -196,7 +196,7 @@ impl<ObjectID: FsVerityHashValue> ImageOp<ObjectID> {
             let media_type = descriptor.media_type();
             let (object_id, layer_stats) = if is_tar_media_type(media_type) {
                 // Tar layers: decompress and split into a splitstream
-                let reader: Box<dyn tokio::io::AsyncBufRead + Unpin + Send> = match media_type {
+                let reader: Box<dyn tokio::io::AsyncRead + Unpin + Send> = match media_type {
                     MediaType::ImageLayer | MediaType::ImageLayerNonDistributable => {
                         Box::new(BufReader::with_capacity(IO_BUF_CAPACITY, progress))
                     }