rust-discovery: using the children cache in add_missing

The DAG range computation often needs to get back to very old
revisions, and turns out to be disproportionately long, given
that the end goal is to remove the descendents of the given
missing revisons from the undecided set.

The fast iteration capabilities available in the Rust case make
it possible to avoid the DAG range entirely, at the cost of
precomputing the children cache, and to simply iterate on
children of the given missing revisions.

This is a case where staying on the same side of the interface
between the two languages has clear benefits.

On discoveries with initial undecided sets
small enough to bypass sampling entirely, the total cost of
computing the children cache and the subsequent iteration
becomes better than the Python + C counterpart, which relies on
reachableroots2.

For example, on a repo with more than one million revisions with
an initial undecided set of 11 elements, we get these figures:

Rust version with simple iteration
  addcommons: 57.287us
  first undecided computation: 184.278334ms
  first children cache computation: 131.056us
  addmissings iteration: 42.766us
  first addinfo total: 185.24 ms

Python + C version
  first addcommons: 0.29 ms
  addcommons 0.21 ms
  first undecided computation 191.35 ms
  addmissings 45.75 ms
  first addinfo total: 237.77 ms

On discoveries with large undecided sets, the initial price paid
makes the first addinfo slower than the Python + C version,
but that's more than compensated by the gain in sampling and
subsequent iterations.
Here's an extreme example with an undecided set of a million revisions:

Rust version:
  first undecided computation: 293.842629ms
  first children cache computation: 407.911297ms
  addmissings iteration: 34.312869ms
  first addinfo total: 776.02 ms
  taking initial sample
  query 2: sampling time: 1318.38 ms
  query 2; still undecided: 1005013, sample size is: 200
  addmissings: 143.062us

Python + C version:
  first undecided computation 298.13 ms
  addmissings 80.13 ms
  first addinfo total: 399.62 ms
  taking initial sample
  query 2: sampling time: 3957.23 ms
  query 2; still undecided: 1005013, sample size is: 200
  addmissings 52.88 ms

Differential Revision: https://phab.mercurial-scm.org/D6428

rust/hg-core/src/discovery.rs

@@ -233,20 +233,47 b' impl<G: Graph + Clone> PartialDiscovery<'
     }
 
     /// Register revisions known as being missing
+    ///
+    /// # Performance note
+    ///
+    /// Except in the most trivial case, the first call of this method has
+    /// the side effect of computing `self.undecided` set for the first time,
+    /// and the related caches it might need for efficiency of its internal
+    /// computation. This is typically faster if more information is
+    /// available in `self.common`. Therefore, for good performance, the
+    /// caller should avoid calling this too early.
     pub fn add_missing_revisions(
         &mut self,
         missing: impl IntoIterator<Item = Revision>,
     ) -> Result<(), GraphError> {
-        self.ensure_undecided()?;
-        let range = dagops::range(
-            &self.graph,
-            missing,
-            self.undecided.as_ref().unwrap().iter().cloned(),
-        )?;
+        self.ensure_children_cache()?;
+        self.ensure_undecided()?; // for safety of possible future refactors
+        let children = self.children_cache.as_ref().unwrap();
+        let mut seen: HashSet<Revision> = HashSet::new();
+        let mut tovisit: VecDeque<Revision> = missing.into_iter().collect();
         let undecided_mut = self.undecided.as_mut().unwrap();
-        for missrev in range {
-            self.missing.insert(missrev);
-            undecided_mut.remove(&missrev);
+        while let Some(rev) = tovisit.pop_front() {
+            if !self.missing.insert(rev) {
+                // either it's known to be missing from a previous
+                // invocation, and there's no need to iterate on its
+                // children (we now they are all missing)
+                // or it's from a previous iteration of this loop
+                // and its children have already been queued
+                continue;
+            }
+            undecided_mut.remove(&rev);
+            match children.get(&rev) {
+                None => {
+                    continue;
+                }
+                Some(this_children) => {
+                    for child in this_children.iter().cloned() {
+                        if seen.insert(child) {
+                            tovisit.push_back(child);
+                        }
+                    }
+                }
+            }
         }
         Ok(())
     }
@@ -547,6 +574,22 b' mod tests {'
     }
 
     #[test]
+    fn test_add_missing_early_continue() -> Result<(), GraphError> {
+        eprintln!("test_add_missing_early_stop");
+        let mut disco = full_disco();
+        disco.add_common_revisions(vec![13, 3, 4])?;
+        disco.ensure_children_cache()?;
+        // 12 is grand-child of 6 through 9
+        // passing them in this order maximizes the chances of the
+        // early continue to do the wrong thing
+        disco.add_missing_revisions(vec![6, 9, 12])?;
+        assert_eq!(sorted_undecided(&disco), vec![5, 7, 10, 11]);
+        assert_eq!(sorted_missing(&disco), vec![6, 9, 12]);
+        assert!(!disco.is_complete());
+        Ok(())
+    }
+
+    #[test]
     fn test_limit_sample_no_need_to() {
         let sample = vec![1, 2, 3, 4];
         assert_eq!(full_disco().limit_sample(sample, 10), vec![1, 2, 3, 4]);