tsort: use iterative dfs to prevent stack overflows

src/uu/tsort/src/tsort.rs

@@ -4,7 +4,8 @@
 // file that was distributed with this source code.
 //spell-checker:ignore TAOCP indegree
 use clap::{Arg, Command};
-use std::collections::{HashMap, HashSet, VecDeque};
+use std::collections::hash_map::Entry;
+use std::collections::{HashMap, VecDeque};
 use std::ffi::OsString;
 use std::path::Path;
 use thiserror::Error;
@@ -34,13 +35,15 @@ enum TsortError {
     /// The graph contains a cycle.
     #[error("{input}: {message}", input = .0, message = translate!("tsort-error-loop"))]
     Loop(String),
-
-    /// A particular node in a cycle. (This is mainly used for printing.)
-    #[error("{0}")]
-    LoopNode(String),
 }
 
+// Auxiliary struct, just for printing loop nodes via show! macro
+#[derive(Debug, Error)]
+#[error("{0}")]
+struct LoopNode<'a>(&'a str);
+
 impl UError for TsortError {}
+impl UError for LoopNode<'_> {}
 
 #[uucore::main]
 pub fn uumain(args: impl uucore::Args) -> UResult<()> {
@@ -131,6 +134,12 @@ struct Graph<'input> {
     nodes: HashMap<&'input str, Node<'input>>,
 }
 
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum VisitedState {
+    Opened,
+    Closed,
+}
+
 impl<'input> Graph<'input> {
     fn new(name: String) -> Graph<'input> {
         Self {
@@ -224,8 +233,8 @@ impl<'input> Graph<'input> {
     fn find_and_break_cycle(&mut self, frontier: &mut VecDeque<&'input str>) {
         let cycle = self.detect_cycle();
         show!(TsortError::Loop(self.name.clone()));
-        for node in &cycle {
-            show!(TsortError::LoopNode((*node).to_string()));
+        for &node in &cycle {
+            show!(LoopNode(node));
         }
         let u = cycle[0];
         let v = cycle[1];
@@ -240,41 +249,76 @@ impl<'input> Graph<'input> {
         let mut nodes: Vec<_> = self.nodes.keys().collect();
         nodes.sort_unstable();
 
-        let mut visited = HashSet::new();
+        let mut visited = HashMap::new();
         let mut stack = Vec::with_capacity(self.nodes.len());
         for node in nodes {
-            if !visited.contains(node) && self.dfs(node, &mut visited, &mut stack) {
-                return stack;
+            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)
+                    .skip_while(|&node| node != loop_entry)
+                    .collect();
             }
         }
-        unreachable!();
+        unreachable!("detect_cycle is expected to be called only on graphs with cycles");
     }
 
-    fn dfs(
-        &self,
+    fn dfs<'a>(
+        &'a self,
         node: &'input str,
-        visited: &mut HashSet<&'input str>,
-        stack: &mut Vec<&'input str>,
+        visited: &mut HashMap<&'input str, VisitedState>,
+        stack: &mut Vec<(&'input str, &'a [&'input str])>,
     ) -> bool {
-        if stack.contains(&node) {
-            return true;
-        }
-        if visited.contains(&node) {
+        stack.push((
+            node,
+            self.nodes.get(node).map_or(&[], |n| &n.successor_names),
+        ));
+        let state = *visited.entry(node).or_insert(VisitedState::Opened);
+
+        if state == VisitedState::Closed {
             return false;
         }
 
-        visited.insert(node);
-        stack.push(node);
-
-        if let Some(successor_names) = self.nodes.get(node).map(|n| &n.successor_names) {
-            for &successor in successor_names {
-                if self.dfs(successor, visited, stack) {
-                    return true;
+        while let Some((node, pending_successors)) = stack.pop() {
+            let Some((&next_node, pending)) = pending_successors.split_first() else {
+                // no more pending successors in the list -> close the node
+                visited.insert(node, VisitedState::Closed);
+                continue;
+            };
+
+            // schedule processing for the pending part of successors for this node
+            stack.push((node, pending));
+
+            match visited.entry(next_node) {
+                Entry::Vacant(v) => {
+                    // It's a first time we enter this node
+                    v.insert(VisitedState::Opened);
+                    stack.push((
+                        next_node,
+                        self.nodes
+                            .get(next_node)
+                            .map_or(&[], |n| &n.successor_names),
+                    ));
+                }
+                Entry::Occupied(o) => {
+                    if *o.get() == VisitedState::Opened {
+                        // we are entering the same opened node again -> loop found
+                        // stack contains it
+                        //
+                        // But part of the stack may not be belonging to this loop
+                        // push found node to the stack to be able to trace the beginning of the loop
+                        stack.push((next_node, &[]));
+                        return true;
+                    }
                 }
             }
         }
 
-        stack.pop();
         false
     }
 }

tests/by-util/test_tsort.rs

@@ -122,3 +122,34 @@ fn test_two_cycles() {
         .stdout_is("a\nc\nd\nb\n")
         .stderr_is("tsort: -: input contains a loop:\ntsort: b\ntsort: c\ntsort: -: input contains a loop:\ntsort: b\ntsort: d\n");
 }
+
+#[test]
+fn test_long_loop_no_stack_overflow() {
+    use std::fmt::Write;
+    const N: usize = 100_000;
+    let mut input = String::new();
+    for v in 0..N {
+        let next = (v + 1) % N;
+        let _ = write!(input, "{v} {next} ");
+    }
+    new_ucmd!()
+        .pipe_in(input)
+        .fails_with_code(1)
+        .stderr_contains("tsort: -: input contains a loop");
+}
+
+#[test]
+fn test_loop_for_iterative_dfs_correctness() {
+    let input = r"
+        A B
+        B C
+        C B
+        C D
+        D A
+    ";
+
+    new_ucmd!()
+        .pipe_in(input)
+        .fails_with_code(1)
+        .stderr_contains("tsort: -: input contains a loop:\ntsort: B\ntsort: C");
+}