csplit: create final empty file with --suppress-matched to match GNU (fixes #7286)

Problem
- With , uutils csplit failed to create the final empty output file when the last split point consumed the trailing input.
- GNU csplit always creates a final segment after processing patterns; with , that empty final segment is elided.
- This broke the GNU test (tests/csplit/csplit-suppress-matched.pl) and the scenario:

Root cause
- Final-file creation was conditional on there being remaining input after pattern processing. If none remained, no final file was created, contrary to GNU semantics.

Fix (minimal, targeted)
- In , after :
  - If there is remaining input, always create a final split and copy the remainder, then finish.
  - Else, if all patterns were integer-based and  is set, create a final (possibly empty) split and finish;  elides it when  is set.

Tests (Rust integration)
- Added two Rust tests under  to lock down GNU-compatible behavior:
  -  (expects sizes 2,2,2,0 and a final empty )
  -  (final empty file is correctly elided)

Verification
- All  tests pass locally. The originally reported case now matches GNU.

Relation to PR #7806
- #7806 proposes a broader refactor to fix multiple issues, but remains a draft and notes remaining GNU suppress-matched differences.
- This PR provides a small, reviewable fix specifically for #7286, plus precise integration tests to safeguard behavior.

Fixes #7286

Author: naoNao89

src/uu/csplit/src/csplit.rs

@@ -120,19 +120,28 @@ where
         .enumerate();
     let mut input_iter = InputSplitter::new(enumerated_input_lines);
     let mut split_writer = SplitWriter::new(options);
-    let patterns: Vec<patterns::Pattern> = patterns::get_patterns(patterns)?;
-    let ret = do_csplit(&mut split_writer, patterns, &mut input_iter);
+    let patterns_vec: Vec<patterns::Pattern> = patterns::get_patterns(patterns)?;
+    let all_up_to_line = patterns_vec
+        .iter()
+        .all(|p| matches!(p, patterns::Pattern::UpToLine(_, _)));
+    let ret = do_csplit(&mut split_writer, patterns_vec, &mut input_iter);
 
     // consume the rest, unless there was an error
     if ret.is_ok() {
         input_iter.rewind_buffer();
         if let Some((_, line)) = input_iter.next() {
+            // There is remaining input: create a final split and copy remainder
             split_writer.new_writer()?;
             split_writer.writeln(&line?)?;
             for (_, line) in input_iter {
                 split_writer.writeln(&line?)?;
             }
             split_writer.finish_split();
+        } else if all_up_to_line && options.suppress_matched {
+            // GNU semantics for integer patterns with --suppress-matched:
+            // even if no remaining input, create a final (possibly empty) split
+            split_writer.new_writer()?;
+            split_writer.finish_split();
         }
     }
     // delete files on error by default

tests/by-util/test_csplit.rs

@@ -12,6 +12,44 @@ fn generate(from: u32, to: u32) -> String {
     (from..to).fold(String::new(), |acc, v| format!("{acc}{v}\n"))
 }
 
+#[test]
+fn test_line_numbers_suppress_matched_final_empty() {
+    // Repro for #7286
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.args(&["--suppress-matched", "-", "2", "4", "6"]) // stdin, split at 2/4/6
+        .pipe_in("1\n2\n3\n4\n5\n6\n")
+        .succeeds()
+        .stdout_only("2\n2\n2\n0\n");
+
+    // Expect four files: xx00:"1\n", xx01:"3\n", xx02:"5\n", xx03:""
+    let count = glob(&at.plus_as_string("xx*"))
+        .expect("there should be splits created")
+        .count();
+    assert_eq!(count, 4);
+    assert_eq!(at.read("xx00"), "1\n");
+    assert_eq!(at.read("xx01"), "3\n");
+    assert_eq!(at.read("xx02"), "5\n");
+    assert_eq!(at.read("xx03"), "");
+}
+
+#[test]
+fn test_line_numbers_suppress_matched_final_empty_elided_with_z() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.args(&["--suppress-matched", "-z", "-", "2", "4", "6"]) // elide empty
+        .pipe_in("1\n2\n3\n4\n5\n6\n")
+        .succeeds()
+        .stdout_only("2\n2\n2\n");
+
+    // Expect three files: xx00:"1\n", xx01:"3\n", xx02:"5\n"
+    let count = glob(&at.plus_as_string("xx*"))
+        .expect("there should be splits created")
+        .count();
+    assert_eq!(count, 3);
+    assert_eq!(at.read("xx00"), "1\n");
+    assert_eq!(at.read("xx01"), "3\n");
+    assert_eq!(at.read("xx02"), "5\n");
+}
+
 #[test]
 fn test_invalid_arg() {
     new_ucmd!().arg("--definitely-invalid").fails_with_code(1);