splitstream: Add a test case for previous change

I verified this passes now and fails without the
previous commit.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/composefs-oci/src/tar.rs

@@ -284,7 +284,8 @@ pub fn get_entry<R: Read, ObjectID: FsVerityHashValue>(
 mod tests {
     use super::*;
     use composefs::{
-        fsverity::Sha256HashValue, repository::Repository, splitstream::SplitStreamReader,
+        fsverity::Sha256HashValue, generic_tree::LeafContent, repository::Repository,
+        splitstream::SplitStreamReader,
     };
     use std::{io::Cursor, path::Path, sync::Arc};
     use tar::Builder;
@@ -618,6 +619,39 @@ mod tests {
         assert_eq!(entries[0].path, Path::new(&abspath));
     }
 
+    #[test]
+    fn test_gnu_longlink() {
+        let very_long_path = format!(
+            "very/long/path/that/exceeds/the/normal/tar/header/limit/{}",
+            "x".repeat(120)
+        );
+
+        // Use append_data to create a tar with a very long filename that triggers GNU extensions
+        let mut tar_data = Vec::new();
+        {
+            let mut builder = Builder::new(&mut tar_data);
+            let mut header = tar::Header::new_gnu();
+            header.set_mode(0o777);
+            header.set_entry_type(EntryType::Symlink);
+            header.set_size(0);
+            header.set_uid(0);
+            header.set_gid(0);
+            builder
+                .append_link(&mut header, "long-symlink", &very_long_path)
+                .unwrap();
+            builder.finish().unwrap();
+        };
+
+        let entries = read_all_via_splitstream(tar_data).unwrap();
+        assert_eq!(entries.len(), 1);
+        match &entries[0].item {
+            TarItem::Leaf(LeafContent::Symlink(ref target)) => {
+                assert_eq!(&**target, OsStr::new(&very_long_path));
+            }
+            _ => unreachable!(),
+        };
+    }
+
     /// Compare a tar::Header with a composefs Stat structure for equality
     fn assert_header_stat_equal(header: &tar::Header, stat: &Stat, msg_prefix: &str) {
         assert_eq!(