revset: add fork_point function

This can be used to find the fork point (best common ancestors) of a
revset with an arbitrary number of commits, which cannot be expressed
currently in the revset language.

CHANGELOG.md

@@ -38,6 +38,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   `--insert-before` options to customize the location of the duplicated
   revisions.
 
+* New `fork_point()` revset function can be used to obtain the fork point
+  of multiple commits.
+
 ### Fixed bugs
 
 ## [0.23.0] - 2024-11-06

docs/revsets.md

@@ -253,6 +253,11 @@ revsets (expressions) as arguments.
 * `latest(x[, count])`: Latest `count` commits in `x`, based on committer
   timestamp. The default `count` is 1.
 
+* `fork_point(x)`: The fork point of all commits in `x`. The fork point is the
+  common ancestor(s) of all commits in `x` which do not have any descendants
+  that are also common ancestors of all commits in `x`. If `x` resolves to a
+  single commit, `fork_point(x)` resolves to `x`.
+
 * `merges()`: Merge commits.
 
 * `description(pattern)`: Commits that have a description matching the given

lib/src/default_index/revset_engine.rs

@@ -953,6 +953,22 @@ impl EvaluationContext<'_> {
                 });
                 Ok(Box::new(EagerRevset { positions }))
             }
+            ResolvedExpression::ForkPoint(expression) => {
+                let expression_set = self.evaluate(expression)?;
+                let mut expression_positions_iter = expression_set.positions().attach(index);
+                let Some(position) = expression_positions_iter.next() else {
+                    return Ok(Box::new(EagerRevset::empty()));
+                };
+                let mut positions = vec![position?];
+                for position in expression_positions_iter {
+                    positions = index
+                        .common_ancestors_pos(&positions, [position?].as_slice())
+                        .into_iter()
+                        .collect_vec();
+                }
+                positions.reverse();
+                Ok(Box::new(EagerRevset { positions }))
+            }
             ResolvedExpression::Latest { candidates, count } => {
                 let candidate_set = self.evaluate(candidates)?;
                 Ok(Box::new(self.take_latest_revset(&*candidate_set, *count)?))

lib/src/revset.rs

@@ -249,6 +249,7 @@ pub enum RevsetExpression<St: ExpressionState> {
     },
     Heads(Rc<Self>),
     Roots(Rc<Self>),
+    ForkPoint(Rc<Self>),
     Latest {
         candidates: Rc<Self>,
         count: usize,
@@ -427,6 +428,11 @@ impl<St: ExpressionState> RevsetExpression<St> {
         })
     }
 
+    /// Fork point (best common ancestors) of `self`.
+    pub fn fork_point(self: &Rc<Self>) -> Rc<Self> {
+        Rc::new(Self::ForkPoint(self.clone()))
+    }
+
     /// Filter all commits by `predicate` in `self`.
     pub fn filtered(self: &Rc<Self>, predicate: RevsetFilterPredicate) -> Rc<Self> {
         self.intersection(&Self::filter(predicate))
@@ -615,6 +621,7 @@ pub enum ResolvedExpression {
     },
     Heads(Box<Self>),
     Roots(Box<Self>),
+    ForkPoint(Box<Self>),
     Latest {
         candidates: Box<Self>,
         count: usize,
@@ -800,6 +807,11 @@ static BUILTIN_FUNCTION_MAP: Lazy<HashMap<&'static str, RevsetFunction>> = Lazy:
         };
         Ok(candidates.latest(count))
     });
+    map.insert("fork_point", |diagnostics, function, context| {
+        let [expression_arg] = function.expect_exact_arguments()?;
+        let expression = lower_expression(diagnostics, expression_arg, context)?;
+        Ok(RevsetExpression::fork_point(&expression))
+    });
     map.insert("merges", |_diagnostics, function, _context| {
         function.expect_no_arguments()?;
         Ok(RevsetExpression::filter(
@@ -1247,6 +1259,9 @@ fn try_transform_expression<St: ExpressionState, E>(
             RevsetExpression::Roots(candidates) => {
                 transform_rec(candidates, pre, post)?.map(RevsetExpression::Roots)
             }
+            RevsetExpression::ForkPoint(expression) => {
+                transform_rec(expression, pre, post)?.map(RevsetExpression::ForkPoint)
+            }
             RevsetExpression::Latest { candidates, count } => transform_rec(candidates, pre, post)?
                 .map(|candidates| RevsetExpression::Latest {
                     candidates,
@@ -1437,6 +1452,10 @@ where
             let roots = folder.fold_expression(roots)?;
             RevsetExpression::Roots(roots).into()
         }
+        RevsetExpression::ForkPoint(expression) => {
+            let expression = folder.fold_expression(expression)?;
+            RevsetExpression::ForkPoint(expression).into()
+        }
         RevsetExpression::Latest { candidates, count } => {
             let candidates = folder.fold_expression(candidates)?;
             let count = *count;
@@ -2327,6 +2346,9 @@ impl VisibilityResolutionContext<'_> {
             RevsetExpression::Roots(candidates) => {
                 ResolvedExpression::Roots(self.resolve(candidates).into())
             }
+            RevsetExpression::ForkPoint(expression) => {
+                ResolvedExpression::ForkPoint(self.resolve(expression).into())
+            }
             RevsetExpression::Latest { candidates, count } => ResolvedExpression::Latest {
                 candidates: self.resolve(candidates).into(),
                 count: *count,
@@ -2431,6 +2453,7 @@ impl VisibilityResolutionContext<'_> {
             | RevsetExpression::Reachable { .. }
             | RevsetExpression::Heads(_)
             | RevsetExpression::Roots(_)
+            | RevsetExpression::ForkPoint(_)
             | RevsetExpression::Latest { .. } => {
                 ResolvedPredicateExpression::Set(self.resolve(expression).into())
             }

lib/tests/test_revset.rs

@@ -2388,6 +2388,187 @@ fn test_evaluate_expression_latest() {
     );
 }
 
+#[test]
+fn test_evaluate_expression_fork_point() {
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+
+    // 5 6
+    // |/|
+    // 4 |
+    // | |
+    // 1 2 3
+    // | |/
+    // |/
+    // 0
+    let mut tx = repo.start_transaction(&settings);
+    let mut_repo = tx.repo_mut();
+    let mut graph_builder = CommitGraphBuilder::new(&settings, mut_repo);
+    let root_commit = repo.store().root_commit();
+    let commit1 = graph_builder.initial_commit();
+    let commit2 = graph_builder.initial_commit();
+    let commit3 = graph_builder.initial_commit();
+    let commit4 = graph_builder.commit_with_parents(&[&commit1]);
+    let commit5 = graph_builder.commit_with_parents(&[&commit4]);
+    let commit6 = graph_builder.commit_with_parents(&[&commit4, &commit2]);
+
+    assert_eq!(resolve_commit_ids(mut_repo, "fork_point(none())"), vec![]);
+    assert_eq!(
+        resolve_commit_ids(mut_repo, "fork_point(root())"),
+        vec![root_commit.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &format!("fork_point({})", commit1.id())),
+        vec![commit1.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &format!("fork_point({})", commit2.id())),
+        vec![commit2.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &format!("fork_point({})", commit3.id())),
+        vec![commit3.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &format!("fork_point({})", commit4.id())),
+        vec![commit4.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &format!("fork_point({})", commit5.id())),
+        vec![commit5.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &format!("fork_point({})", commit6.id())),
+        vec![commit6.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit1.id(), commit2.id())
+        ),
+        vec![root_commit.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit2.id(), commit3.id())
+        ),
+        vec![root_commit.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!(
+                "fork_point({} | {} | {})",
+                commit1.id(),
+                commit2.id(),
+                commit3.id()
+            )
+        ),
+        vec![root_commit.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit1.id(), commit4.id())
+        ),
+        vec![commit1.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit2.id(), commit5.id())
+        ),
+        vec![root_commit.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit3.id(), commit6.id())
+        ),
+        vec![root_commit.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit1.id(), commit5.id())
+        ),
+        vec![commit1.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit4.id(), commit5.id())
+        ),
+        vec![commit4.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit5.id(), commit6.id())
+        ),
+        vec![commit4.id().clone()]
+    );
+}
+
+#[test]
+fn test_evaluate_expression_fork_point_criss_cross() {
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+
+    // 3 4
+    // |X|
+    // 1 2
+    // |/
+    // 0
+    let mut tx = repo.start_transaction(&settings);
+    let mut_repo = tx.repo_mut();
+    let mut graph_builder = CommitGraphBuilder::new(&settings, mut_repo);
+    let commit1 = graph_builder.initial_commit();
+    let commit2 = graph_builder.initial_commit();
+    let commit3 = graph_builder.commit_with_parents(&[&commit1, &commit2]);
+    let commit4 = graph_builder.commit_with_parents(&[&commit1, &commit2]);
+
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit3.id(), commit4.id())
+        ),
+        vec![commit2.id().clone(), commit1.id().clone()]
+    );
+}
+
+#[test]
+fn test_evaluate_expression_fork_point_merge_with_ancestor() {
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+
+    // 4   5
+    // |\ /|
+    // 1 2 3
+    //  \|/
+    //   0
+    let mut tx = repo.start_transaction(&settings);
+    let mut_repo = tx.repo_mut();
+    let mut graph_builder = CommitGraphBuilder::new(&settings, mut_repo);
+    let commit1 = graph_builder.initial_commit();
+    let commit2 = graph_builder.initial_commit();
+    let commit3 = graph_builder.initial_commit();
+    let commit4 = graph_builder.commit_with_parents(&[&commit1, &commit2]);
+    let commit5 = graph_builder.commit_with_parents(&[&commit2, &commit3]);
+
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo,
+            &format!("fork_point({} | {})", commit4.id(), commit5.id())
+        ),
+        vec![commit2.id().clone()]
+    );
+}
+
 #[test]
 fn test_evaluate_expression_merges() {
     let settings = testutils::user_settings();