Make append_paths preserve stream if no additions

1 files changed, 12 insertions, 3 deletions

diff --git a/src/path_list_editor.rs b/src/path_list_editor.rs
index 1ae4816..d605d25 100644
--- a/src/path_list_editor.rs
+++ b/src/path_list_editor.rs
@@ -25,8 +25,7 @@ const NUL: &[u8] = b"\0";
 ///
 /// This function is defensive about unterminated input: if `input` is non-empty and does not end
 /// with a NUL byte, a NUL byte is inserted before appended paths are written. If `new_paths` is
-/// empty, this still has the effect of repairing a non-empty unterminated input stream into a
-/// NUL-terminated one.
+/// empty, the input stream is passed through unchanged.
 ///
 /// This function does not normalize, deduplicate, or validate paths.
 pub fn append_paths<R: Read, W: Write>(
@@ -36,7 +35,7 @@ pub fn append_paths<R: Read, W: Write>(
 ) -> std::io::Result<()> {
     let last_byte_in_input = copy_while_tracking_last_byte(input, &mut output)?;
     // Defensively ensure non-empty input is NUL-terminated.
-    if matches!(last_byte_in_input, Some(byte) if byte != 0) {
+    if !new_paths.is_empty() && matches!(last_byte_in_input, Some(byte) if byte != 0) {
         output.write_all(NUL)?;
     }
     for path in new_paths {
@@ -219,6 +218,16 @@ mod tests {
     }
 
     #[test]
+    fn append_paths_does_not_insert_nul_when_no_new_paths_and_input_not_terminated() {
+        let input = b"a.rs";
+        let mut output = Vec::new();
+
+        append_paths(&input[..], &mut output, &[]).unwrap();
+
+        assert_eq!(output, b"a.rs");
+    }
+
+    #[test]
     fn prepend_paths_works_with_empty_stdin_and_args() {
         let input = b"";
         let mut output = Vec::new();