sort: Fix crash on empty final buffer

rust/src/utils/sort.rs

@@ -237,8 +237,12 @@ where
             .par_bridge()
             .map(|buffer: RefCell<Vec<(usize, usize)>>| {
                 let mut buffer = buffer.borrow_mut();
-                let sorted_iterator = flush(temp_dir, &mut (*buffer)[..])?;
-                Ok(vec![sorted_iterator])
+                if buffer.len() > 0 {
+                    let sorted_iterator = flush(temp_dir, &mut (*buffer)[..])?;
+                    Ok(vec![sorted_iterator])
+                } else {
+                    Ok(vec![])
+                }
             })
             .collect::<Vec<_>>()
             .into_iter(),

rust/tests/sort.rs

+/*
+ * Copyright (C) 2023  The Software Heritage developers
+ * See the AUTHORS file at the top-level directory of this distribution
+ * License: GNU General Public License version 3, or any later version
+ * See top-level LICENSE file for more information
+ */
+
+use anyhow::{Context, Result};
+use rayon::prelude::*;
+
+use swh_graph::utils::sort::*;
+
+#[test]
+fn test_par_sort_arcs() -> Result<()> {
+    let tempdir = tempfile::tempdir().context("temp dir")?;
+    assert_eq!(
+        par_sort_arcs(
+            tempdir.path(),
+            100,
+            vec![(1, 10), (2, 1), (1, 5), (2, 10)].into_par_iter(),
+            |buf, arc| {
+                buf.push(arc);
+                Ok(())
+            }
+        )
+        .context("par_sort_arcs")?
+        .collect::<Vec<_>>(),
+        vec![(1, 5), (1, 10), (2, 1), (2, 10)],
+    );
+
+    Ok(())
+}
+
+#[test]
+fn test_par_sort_arcs_empty_buffer() -> Result<()> {
+    let tempdir = tempfile::tempdir().context("temp dir")?;
+    assert_eq!(
+        par_sort_arcs(
+            tempdir.path(),
+            0, // Causes buffers to be flushed immediately, so the last batch will be empty
+            vec![(1, 10), (2, 1), (1, 5), (2, 10)].into_par_iter(),
+            |buf, arc| {
+                buf.push(arc);
+                Ok(())
+            }
+        )
+        .context("par_sort_arcs")?
+        .collect::<Vec<_>>(),
+        vec![(1, 5), (1, 10), (2, 1), (2, 10)],
+    );
+
+    Ok(())
+}