Fix: don't break on empty car

with regression test and benchmark

Cargo.lock

···
 
 [[package]]
 name = "repo-stream"
-version = "0.2.0"
+version = "0.2.1"
 dependencies = [
  "bincode",
  "clap",

Cargo.toml

···
 [package]
 name = "repo-stream"
-version = "0.2.0"
+version = "0.2.1"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 description = "A robust CAR file -> MST walker for atproto"

benches/non-huge-cars.rs

···
 
 use criterion::{Criterion, criterion_group, criterion_main};
 
+const EMPTY_CAR: &'static [u8] = include_bytes!("../car-samples/empty.car");
 const TINY_CAR: &'static [u8] = include_bytes!("../car-samples/tiny.car");
 const LITTLE_CAR: &'static [u8] = include_bytes!("../car-samples/little.car");
 const MIDSIZE_CAR: &'static [u8] = include_bytes!("../car-samples/midsize.car");
···
         .build()
         .expect("Creating runtime failed");
 
+    c.bench_function("empty-car", |b| {
+        b.to_async(&rt).iter(async || drive_car(EMPTY_CAR).await)
+    });
     c.bench_function("tiny-car", |b| {
         b.to_async(&rt).iter(async || drive_car(TINY_CAR).await)
     });

src/walk.rs

···
 }
 
 fn push_from_node(stack: &mut Vec<Need>, node: &Node, parent_depth: Depth) -> Result<(), MstError> {
-    // empty nodes are not allowed in the MST
-    // ...except for a single one for empty MST, but we wouldn't be pushing that
+    // empty nodes are not allowed in the MST except in an empty MST
     if node.is_empty() {
-        return Err(MstError::EmptyNode);
+        if parent_depth == Depth::Root {
+            return Ok(()); // empty mst, nothing to push
+        } else {
+            return Err(MstError::EmptyNode);
+        }
     }
 
     let mut entries = Vec::with_capacity(node.entries.len());

tests/non-huge-cars.rs

···
 extern crate repo_stream;
 use repo_stream::Driver;
 
+const EMPTY_CAR: &'static [u8] = include_bytes!("../car-samples/empty.car");
 const TINY_CAR: &'static [u8] = include_bytes!("../car-samples/tiny.car");
 const LITTLE_CAR: &'static [u8] = include_bytes!("../car-samples/little.car");
 const MIDSIZE_CAR: &'static [u8] = include_bytes!("../car-samples/midsize.car");
 
-async fn test_car(bytes: &[u8], expected_records: usize, expected_sum: usize) {
+async fn test_car(
+    bytes: &[u8],
+    expected_records: usize,
+    expected_sum: usize,
+    expect_profile: bool,
+) {
     let mut driver = match Driver::load_car(bytes, |block| block.len(), 10 /* MiB */)
         .await
         .unwrap()
···
 
     assert_eq!(records, expected_records);
     assert_eq!(sum, expected_sum);
-    assert!(found_bsky_profile);
+    assert_eq!(found_bsky_profile, expect_profile);
+}
+
+#[tokio::test]
+async fn test_empty_car() {
+    test_car(EMPTY_CAR, 0, 0, false).await
 }
 
 #[tokio::test]
 async fn test_tiny_car() {
-    test_car(TINY_CAR, 8, 2071).await
+    test_car(TINY_CAR, 8, 2071, true).await
 }
 
 #[tokio::test]
 async fn test_little_car() {
-    test_car(LITTLE_CAR, 278, 246960).await
+    test_car(LITTLE_CAR, 278, 246960, true).await
 }
 
 #[tokio::test]
 async fn test_midsize_car() {
-    test_car(MIDSIZE_CAR, 11585, 3741393).await
+    test_car(MIDSIZE_CAR, 11585, 3741393, true).await
 }