fix(tsort): correct minimal cycle reporting and precise back-edge removal with iterative DFS (#8786)

- Implement iterative DFS to prevent stack overflows (from PR #8737)
- Fix minimal cycle reporting to show only actual cycle nodes
- Remove redundant back-edge from last cycle node to first
- Update test expectations for corrected cycle handling

src/uu/tsort/src/tsort.rs

@@ -236,8 +236,8 @@ impl<'input> Graph<'input> {
         for &node in &cycle {
             show!(LoopNode(node));
         }
-        let u = cycle[0];
-        let v = cycle[1];
+        let u = *cycle.last().expect("cycle must be non-empty");
+        let v = cycle[0];
         self.remove_edge(u, v);
         if self.indegree(v).unwrap() == 0 {
             frontier.push_back(v);
@@ -245,7 +245,6 @@ impl<'input> Graph<'input> {
     }
 
     fn detect_cycle(&self) -> Vec<&'input str> {
-        // Sort the nodes just to make this function deterministic.
         let mut nodes: Vec<_> = self.nodes.keys().collect();
         nodes.sort_unstable();
 
@@ -253,11 +252,8 @@ impl<'input> Graph<'input> {
         let mut stack = Vec::with_capacity(self.nodes.len());
         for node in nodes {
             if self.dfs(node, &mut visited, &mut stack) {
-                // last element in the stack appears twice: at the begin
-                // and at the end of the loop
                 let (loop_entry, _) = stack.pop().expect("loop is not empty");
 
-                // skip the prefix which doesn't belong to the loop
                 return stack
                     .into_iter()
                     .map(|(node, _)| node)

tests/by-util/test_tsort.rs

@@ -103,7 +103,7 @@ fn test_cycle() {
     new_ucmd!()
         .pipe_in("a b b c c d c b")
         .fails_with_code(1)
-        .stdout_is("a\nc\nd\nb\n")
+        .stdout_is("a\nb\nc\nd\n")
         .stderr_is("tsort: -: input contains a loop:\ntsort: b\ntsort: c\n");
 }
 
@@ -119,7 +119,7 @@ fn test_two_cycles() {
     new_ucmd!()
         .pipe_in("a b b c c b b d d b")
         .fails_with_code(1)
-        .stdout_is("a\nc\nd\nb\n")
+        .stdout_is("a\nb\nc\nd\n")
         .stderr_is("tsort: -: input contains a loop:\ntsort: b\ntsort: c\ntsort: -: input contains a loop:\ntsort: b\ntsort: d\n");
 }