sort: fix incorrect sort ordering/output corruption with long line inputs + add a test

src/uu/sort/src/chunks.rs

@@ -380,14 +380,15 @@ fn read_to_buffer<T: Read>(
                         }
                     }
 
+                    let search_start = newline_search_offset;
                     let mut sep_iter =
-                        memchr_iter(separator, &buffer[newline_search_offset..buffer.len()]).rev();
+                        memchr_iter(separator, &buffer[search_start..buffer.len()]).rev();
                     newline_search_offset = buffer.len();
                     if let Some(last_line_end) = sep_iter.next() {
                         if found_newline || sep_iter.next().is_some() {
                             // We read enough lines.
                             // We want to include the separator here, because it shouldn't be carried over.
-                            return Ok((last_line_end + 1, true));
+                            return Ok((search_start + last_line_end + 1, true));
                         }
                         found_newline = true;
                     }

tests/by-util/test_sort.rs

@@ -1102,6 +1102,34 @@ fn test_merge_interleaved() {
         .stdout_only_fixture("merge_ints_interleaved.expected");
 }
 
+#[test]
+fn test_merge_preserves_long_lines() {
+    use std::fmt::Write;
+
+    const N_ROWS: usize = 3;
+    const LINE_LEN: usize = 32_000;
+    const LINE_VALUES: [&str; N_ROWS] = ["a", "b", "c"];
+    // Exercise merge reads where long lines span internal chunk boundaries.
+    let input = LINE_VALUES.into_iter().fold(
+        String::with_capacity(N_ROWS * (LINE_LEN + 1)),
+        |mut acc, value| {
+            writeln!(acc, "{}", value.repeat(LINE_LEN)).unwrap();
+            acc
+        },
+    );
+
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write("long-lines.txt", &input);
+
+    let result = ucmd.arg("-m").arg("long-lines.txt").succeeds();
+    result.no_stderr();
+
+    let stdout = result.stdout_move_bytes();
+    assert_eq!(bytecount::count(&stdout, b'\n'), N_ROWS);
+    assert_eq!(stdout.len(), input.len());
+    assert_eq!(stdout.as_slice(), input.as_bytes());
+}
+
 #[test]
 fn test_merge_unique() {
     new_ucmd!()