dirstate-tree: Fold "tracked descendants" counter update in main walk

For the purpose of implementing `has_tracked_dir` (which means "has tracked
descendants) without an expensive sub-tree traversal, we maintaing a counter
of tracked descendants on each "directory" node of the tree-shaped dirstate.

Before this changeset, mutating or inserting a node at a given path would
involve:

* Walking the tree from root through ancestors to find the node or the spot
  where to insert it
* Looking at the previous node if any to decide what counter update is needed
* Performing any node mutation
* Walking the tree *again* to update counters in ancestor nodes

When profiling `hg status` on a large repo, this second walk takes times
while loading a the dirstate from disk.

It turns out we have enough information to decide before he first tree walk
what counter update is needed. This changeset merges the two walks, gaining
~10% of the total time for `hg update` (in the same hyperfine benchmark as
the previous changeset).

---

Profiling was done by compiling with this `.cargo/config`:

    [profile.release]
    debug = true

then running with:

    py-spy record -r 500 -n -o /tmp/hg.json --format speedscope -- \
    ./hg status -R $REPO --config experimental.dirstate-tree.in-memory=1

then visualizing the recorded JSON file in https://www.speedscope.app/

Differential Revision: https://phab.mercurial-scm.org/D10554

rust/hg-core/src/dirstate_tree/dirstate_map.rs

@@ -1,653 +1,647 b''
 use bytes_cast::BytesCast;
 use micro_timer::timed;
 use std::convert::TryInto;
 use std::path::PathBuf;
 
 use super::path_with_basename::WithBasename;
 use crate::dirstate::parsers::clear_ambiguous_mtime;
 use crate::dirstate::parsers::pack_entry;
 use crate::dirstate::parsers::packed_entry_size;
 use crate::dirstate::parsers::parse_dirstate_entries;
 use crate::dirstate::parsers::parse_dirstate_parents;
 use crate::dirstate::parsers::Timestamp;
 use crate::matchers::Matcher;
 use crate::revlog::node::NULL_NODE;
 use crate::utils::hg_path::{HgPath, HgPathBuf};
 use crate::CopyMapIter;
 use crate::DirstateEntry;
 use crate::DirstateError;
 use crate::DirstateMapError;
 use crate::DirstateParents;
 use crate::DirstateStatus;
 use crate::EntryState;
 use crate::FastHashMap;
 use crate::PatternFileWarning;
 use crate::StateMapIter;
 use crate::StatusError;
 use crate::StatusOptions;
 
 pub struct DirstateMap {
     parents: Option<DirstateParents>,
     dirty_parents: bool,
     pub(super) root: ChildNodes,
 
     /// Number of nodes anywhere in the tree that have `.entry.is_some()`.
     nodes_with_entry_count: usize,
 
     /// Number of nodes anywhere in the tree that have
     /// `.copy_source.is_some()`.
     nodes_with_copy_source_count: usize,
 }
 
 /// Using a plain `HgPathBuf` of the full path from the repository root as a
 /// map key would also work: all paths in a given map have the same parent
 /// path, so comparing full paths gives the same result as comparing base
 /// names. However `BTreeMap` would waste time always re-comparing the same
 /// string prefix.
 pub(super) type ChildNodes = FastHashMap<WithBasename<HgPathBuf>, Node>;
 
 /// Represents a file or a directory
 #[derive(Default)]
 pub(super) struct Node {
     /// `None` for directories
     pub(super) entry: Option<DirstateEntry>,
 
     pub(super) copy_source: Option<HgPathBuf>,
 
     pub(super) children: ChildNodes,
 
     /// How many (non-inclusive) descendants of this node are tracked files
     tracked_descendants_count: usize,
 }
 
 impl Node {
-    /// Whether this node has a `DirstateEntry` with `.state.is_tracked()`
-    fn is_tracked_file(&self) -> bool {
-        if let Some(entry) = &self.entry {
-            entry.state.is_tracked()
-        } else {
-            false
-        }
-    }
-
     pub(super) fn state(&self) -> Option<EntryState> {
         self.entry.as_ref().map(|entry| entry.state)
     }
 }
 
 /// `(full_path, entry, copy_source)`
 type NodeDataMut<'a> = (
     &'a WithBasename<HgPathBuf>,
     &'a mut Option<DirstateEntry>,
     &'a mut Option<HgPathBuf>,
 );
 
 impl DirstateMap {
     pub fn new() -> Self {
         Self {
             parents: None,
             dirty_parents: false,
             root: ChildNodes::default(),
             nodes_with_entry_count: 0,
             nodes_with_copy_source_count: 0,
         }
     }
 
     fn get_node(&self, path: &HgPath) -> Option<&Node> {
         let mut children = &self.root;
         let mut components = path.components();
         let mut component =
             components.next().expect("expected at least one components");
         loop {
             let child = children.get(component)?;
             if let Some(next_component) = components.next() {
                 component = next_component;
                 children = &child.children;
             } else {
                 return Some(child);
             }
         }
     }
 
     /// Returns a mutable reference to the node at `path` if it exists
     ///
     /// This takes `root` instead of `&mut self` so that callers can mutate
     /// other fields while the returned borrow is still valid
     fn get_node_mut<'tree>(
         root: &'tree mut ChildNodes,
         path: &HgPath,
     ) -> Option<&'tree mut Node> {
-        Self::each_and_get(root, path, |_| {})
+        Self::get_node_mut_tracing_ancestors(root, path, |_| {})
     }
 
-    /// Call `each` for each ancestor node of the one at `path` (not including
-    /// that node itself), starting from nearest the root.
+    /// Same as `get_node_mut`, and calls `each_ancestor` for each ancestor of
+    /// the node.
     ///
-    /// Panics (possibly after some calls to `each`) if there is no node at
-    /// `path`.
-    fn for_each_ancestor_node<'tree>(
-        &mut self,
-        path: &HgPath,
-        each: impl FnMut(&mut Node),
-    ) {
-        let parent = path.parent();
-        if !parent.is_empty() {
-            Self::each_and_get(&mut self.root, parent, each)
-                .expect("missing dirstate node");
-        }
-    }
-
-    /// Common implementation detail of `get_node_mut` and
-    /// `for_each_ancestor_node`
-    fn each_and_get<'tree>(
+    /// Note that `each_ancestor` may be called (with what would be ancestors)
+    /// even if it turns out there is no node at `path`.
+    fn get_node_mut_tracing_ancestors<'tree>(
         root: &'tree mut ChildNodes,
         path: &HgPath,
-        mut each: impl FnMut(&mut Node),
+        mut each_ancestor: impl FnMut(&mut Node),
     ) -> Option<&'tree mut Node> {
         let mut children = root;
         let mut components = path.components();
         let mut component =
             components.next().expect("expected at least one components");
         loop {
             let child = children.get_mut(component)?;
-            each(child);
             if let Some(next_component) = components.next() {
+                each_ancestor(child);
                 component = next_component;
                 children = &mut child.children;
             } else {
                 return Some(child);
             }
         }
     }
 
     fn get_or_insert_node<'tree>(
         root: &'tree mut ChildNodes,
         path: &HgPath,
     ) -> &'tree mut Node {
+        Self::get_or_insert_node_tracing_ancestors(root, path, |_| {})
+    }
+
+    fn get_or_insert_node_tracing_ancestors<'tree>(
+        root: &'tree mut ChildNodes,
+        path: &HgPath,
+        mut each_ancestor: impl FnMut(&mut Node),
+    ) -> &'tree mut Node {
         let mut child_nodes = root;
         let mut inclusive_ancestor_paths =
             WithBasename::inclusive_ancestors_of(path);
         let mut ancestor_path = inclusive_ancestor_paths
             .next()
             .expect("expected at least one inclusive ancestor");
         loop {
             // TODO: can we avoid allocating an owned key in cases where the
             // map already contains that key, without introducing double
             // lookup?
             let child_node =
                 child_nodes.entry(ancestor_path.to_owned()).or_default();
             if let Some(next) = inclusive_ancestor_paths.next() {
+                each_ancestor(child_node);
                 ancestor_path = next;
                 child_nodes = &mut child_node.children;
             } else {
                 return child_node;
             }
         }
     }
 
-    /// The meaning of `new_copy_source` is:
-    ///
-    /// * `Some(Some(x))`: set `Node::copy_source` to `Some(x)`
-    /// * `Some(None)`: set `Node::copy_source` to `None`
-    /// * `None`: leave `Node::copy_source` unchanged
-    fn add_file_node(
+    fn add_or_remove_file(
         &mut self,
         path: &HgPath,
+        old_state: EntryState,
         new_entry: DirstateEntry,
-        new_copy_source: Option<Option<HgPathBuf>>,
     ) {
-        let node = Self::get_or_insert_node(&mut self.root, path);
-        if node.entry.is_none() {
-            self.nodes_with_entry_count += 1
-        }
-        if let Some(source) = &new_copy_source {
-            if node.copy_source.is_none() && source.is_some() {
-                self.nodes_with_copy_source_count += 1
-            }
-            if node.copy_source.is_some() && source.is_none() {
-                self.nodes_with_copy_source_count -= 1
-            }
-        }
         let tracked_count_increment =
-            match (node.is_tracked_file(), new_entry.state.is_tracked()) {
+            match (old_state.is_tracked(), new_entry.state.is_tracked()) {
                 (false, true) => 1,
                 (true, false) => -1,
                 _ => 0,
             };
 
-        node.entry = Some(new_entry);
-        if let Some(source) = new_copy_source {
-            node.copy_source = source
-        }
-        // Borrow of `self.root` through `node` ends here
-
+        let node = Self::get_or_insert_node_tracing_ancestors(
+            &mut self.root,
+            path,
+            |ancestor| {
+                // We can’t use `+= increment` because the counter is unsigned,
+                // and we want debug builds to detect accidental underflow
+                // through zero
         match tracked_count_increment {
-            1 => self.for_each_ancestor_node(path, |node| {
-                node.tracked_descendants_count += 1
-            }),
-            // We can’t use `+= -1` because the counter is unsigned
-            -1 => self.for_each_ancestor_node(path, |node| {
-                node.tracked_descendants_count -= 1
-            }),
+                    1 => ancestor.tracked_descendants_count += 1,
+                    -1 => ancestor.tracked_descendants_count -= 1,
             _ => {}
         }
+            },
+        );
+        if node.entry.is_none() {
+            self.nodes_with_entry_count += 1
+        }
+        node.entry = Some(new_entry)
     }
 
     fn iter_nodes<'a>(
         &'a self,
     ) -> impl Iterator<Item = (&'a WithBasename<HgPathBuf>, &'a Node)> + 'a
     {
         // Depth first tree traversal.
         //
         // If we could afford internal iteration and recursion,
         // this would look like:
         //
         // ```
         // fn traverse_children(
         //     children: &ChildNodes,
         //     each: &mut impl FnMut(&Node),
         // ) {
         //     for child in children.values() {
         //         traverse_children(&child.children, each);
         //         each(child);
         //     }
         // }
         // ```
         //
         // However we want an external iterator and therefore can’t use the
         // call stack. Use an explicit stack instead:
         let mut stack = Vec::new();
         let mut iter = self.root.iter();
         std::iter::from_fn(move || {
             while let Some((key, child_node)) = iter.next() {
                 // Pseudo-recursion
                 let new_iter = child_node.children.iter();
                 let old_iter = std::mem::replace(&mut iter, new_iter);
                 stack.push((key, child_node, old_iter));
             }
             // Found the end of a `children.iter()` iterator.
             if let Some((key, child_node, next_iter)) = stack.pop() {
                 // "Return" from pseudo-recursion by restoring state from the
                 // explicit stack
                 iter = next_iter;
 
                 Some((key, child_node))
             } else {
                 // Reached the bottom of the stack, we’re done
                 None
             }
         })
     }
 
     /// Mutable iterator for the `(entry, copy source)` of each node.
     ///
     /// It would not be safe to yield mutable references to nodes themeselves
     /// with `-> impl Iterator<Item = &mut Node>` since child nodes are
     /// reachable from their ancestor nodes, potentially creating multiple
     /// `&mut` references to a given node.
     fn iter_node_data_mut<'a>(
         &'a mut self,
     ) -> impl Iterator<Item = NodeDataMut<'a>> + 'a {
         // Explict stack for pseudo-recursion, see `iter_nodes` above.
         let mut stack = Vec::new();
         let mut iter = self.root.iter_mut();
         std::iter::from_fn(move || {
             while let Some((key, child_node)) = iter.next() {
                 // Pseudo-recursion
                 let data =
                     (key, &mut child_node.entry, &mut child_node.copy_source);
                 let new_iter = child_node.children.iter_mut();
                 let old_iter = std::mem::replace(&mut iter, new_iter);
                 stack.push((data, old_iter));
             }
             // Found the end of a `children.values_mut()` iterator.
             if let Some((data, next_iter)) = stack.pop() {
                 // "Return" from pseudo-recursion by restoring state from the
                 // explicit stack
                 iter = next_iter;
 
                 Some(data)
             } else {
                 // Reached the bottom of the stack, we’re done
                 None
             }
         })
     }
 }
 
 impl super::dispatch::DirstateMapMethods for DirstateMap {
     fn clear(&mut self) {
         self.set_parents(&DirstateParents {
             p1: NULL_NODE,
             p2: NULL_NODE,
         });
         self.root.clear();
         self.nodes_with_entry_count = 0;
         self.nodes_with_copy_source_count = 0;
     }
 
     fn add_file(
         &mut self,
         filename: &HgPath,
-        _old_state: EntryState,
+        old_state: EntryState,
         entry: DirstateEntry,
     ) -> Result<(), DirstateMapError> {
-        self.add_file_node(filename, entry, None);
+        self.add_or_remove_file(filename, old_state, entry);
         Ok(())
     }
 
     fn remove_file(
         &mut self,
         filename: &HgPath,
-        _old_state: EntryState,
+        old_state: EntryState,
         size: i32,
     ) -> Result<(), DirstateMapError> {
         let entry = DirstateEntry {
             state: EntryState::Removed,
             mode: 0,
             size,
             mtime: 0,
         };
-        self.add_file_node(filename, entry, None);
+        self.add_or_remove_file(filename, old_state, entry);
         Ok(())
     }
 
     fn drop_file(
         &mut self,
         filename: &HgPath,
-        _old_state: EntryState,
+        old_state: EntryState,
     ) -> Result<bool, DirstateMapError> {
-        if let Some(node) = Self::get_node_mut(&mut self.root, filename) {
-            let was_tracked = node.is_tracked_file();
+        let was_tracked = old_state.is_tracked();
+        if let Some(node) = Self::get_node_mut_tracing_ancestors(
+            &mut self.root,
+            filename,
+            |ancestor| {
+                if was_tracked {
+                    ancestor.tracked_descendants_count -= 1
+                }
+            },
+        ) {
             let had_entry = node.entry.is_some();
             let had_copy_source = node.copy_source.is_some();
 
             // TODO: this leaves in the tree a "non-file" node. Should we
             // remove the node instead, together with ancestor nodes for
             // directories that become empty?
             node.entry = None;
             node.copy_source = None;
 
             if had_entry {
                 self.nodes_with_entry_count -= 1
             }
             if had_copy_source {
                 self.nodes_with_copy_source_count -= 1
             }
-            if was_tracked {
-                self.for_each_ancestor_node(filename, |node| {
-                    node.tracked_descendants_count -= 1
-                })
-            }
             Ok(had_entry)
         } else {
+            assert!(!was_tracked);
             Ok(false)
         }
     }
 
     fn clear_ambiguous_times(&mut self, filenames: Vec<HgPathBuf>, now: i32) {
         for filename in filenames {
             if let Some(node) = Self::get_node_mut(&mut self.root, &filename) {
                 if let Some(entry) = node.entry.as_mut() {
                     clear_ambiguous_mtime(entry, now);
                 }
             }
         }
     }
 
     fn non_normal_entries_contains(&mut self, key: &HgPath) -> bool {
         self.get_node(key)
             .and_then(|node| node.entry.as_ref())
             .map_or(false, DirstateEntry::is_non_normal)
     }
 
     fn non_normal_entries_remove(&mut self, _key: &HgPath) {
         // Do nothing, this `DirstateMap` does not have a separate "non normal
         // entries" set that need to be kept up to date
     }
 
     fn non_normal_or_other_parent_paths(
         &mut self,
     ) -> Box<dyn Iterator<Item = &HgPathBuf> + '_> {
         Box::new(self.iter_nodes().filter_map(|(path, node)| {
             node.entry
                 .as_ref()
                 .filter(|entry| {
                     entry.is_non_normal() || entry.is_from_other_parent()
                 })
                 .map(|_| path.full_path())
         }))
     }
 
     fn set_non_normal_other_parent_entries(&mut self, _force: bool) {
         // Do nothing, this `DirstateMap` does not have a separate "non normal
         // entries" and "from other parent" sets that need to be recomputed
     }
 
     fn iter_non_normal_paths(
         &mut self,
     ) -> Box<dyn Iterator<Item = &HgPathBuf> + Send + '_> {
         self.iter_non_normal_paths_panic()
     }
 
     fn iter_non_normal_paths_panic(
         &self,
     ) -> Box<dyn Iterator<Item = &HgPathBuf> + Send + '_> {
         Box::new(self.iter_nodes().filter_map(|(path, node)| {
             node.entry
                 .as_ref()
                 .filter(|entry| entry.is_non_normal())
                 .map(|_| path.full_path())
         }))
     }
 
     fn iter_other_parent_paths(
         &mut self,
     ) -> Box<dyn Iterator<Item = &HgPathBuf> + Send + '_> {
         Box::new(self.iter_nodes().filter_map(|(path, node)| {
             node.entry
                 .as_ref()
                 .filter(|entry| entry.is_from_other_parent())
                 .map(|_| path.full_path())
         }))
     }
 
     fn has_tracked_dir(
         &mut self,
         directory: &HgPath,
     ) -> Result<bool, DirstateMapError> {
         if let Some(node) = self.get_node(directory) {
             // A node without a `DirstateEntry` was created to hold child
             // nodes, and is therefore a directory.
             Ok(node.entry.is_none() && node.tracked_descendants_count > 0)
         } else {
             Ok(false)
         }
     }
 
     fn has_dir(
         &mut self,
         directory: &HgPath,
     ) -> Result<bool, DirstateMapError> {
         if let Some(node) = self.get_node(directory) {
             // A node without a `DirstateEntry` was created to hold child
             // nodes, and is therefore a directory.
             Ok(node.entry.is_none())
         } else {
             Ok(false)
         }
     }
 
     fn parents(
         &mut self,
         file_contents: &[u8],
     ) -> Result<&DirstateParents, DirstateError> {
         if self.parents.is_none() {
             let parents = if !file_contents.is_empty() {
                 parse_dirstate_parents(file_contents)?.clone()
             } else {
                 DirstateParents {
                     p1: NULL_NODE,
                     p2: NULL_NODE,
                 }
             };
             self.parents = Some(parents);
         }
         Ok(self.parents.as_ref().unwrap())
     }
 
     fn set_parents(&mut self, parents: &DirstateParents) {
         self.parents = Some(parents.clone());
         self.dirty_parents = true;
     }
 
     #[timed]
     fn read<'a>(
         &mut self,
         file_contents: &'a [u8],
     ) -> Result<Option<&'a DirstateParents>, DirstateError> {
         if file_contents.is_empty() {
             return Ok(None);
         }
 
         let parents = parse_dirstate_entries(
             file_contents,
             |path, entry, copy_source| {
-                self.add_file_node(
+                let tracked = entry.state.is_tracked();
+                let node = Self::get_or_insert_node_tracing_ancestors(
+                    &mut self.root,
                     path,
-                    *entry,
-                    Some(copy_source.map(HgPath::to_owned)),
-                )
+                    |ancestor| {
+                        if tracked {
+                            ancestor.tracked_descendants_count += 1
+                        }
+                    },
+                );
+                assert!(
+                    node.entry.is_none(),
+                    "duplicate dirstate entry in read"
+                );
+                assert!(
+                    node.copy_source.is_none(),
+                    "duplicate dirstate entry in read"
+                );
+                node.entry = Some(*entry);
+                node.copy_source = copy_source.map(HgPath::to_owned);
+                self.nodes_with_entry_count += 1;
+                if copy_source.is_some() {
+                    self.nodes_with_copy_source_count += 1
+                }
             },
         )?;
 
         if !self.dirty_parents {
             self.set_parents(parents);
         }
 
         Ok(Some(parents))
     }
 
     fn pack(
         &mut self,
         parents: DirstateParents,
         now: Timestamp,
     ) -> Result<Vec<u8>, DirstateError> {
         // Optizimation (to be measured?): pre-compute size to avoid `Vec`
         // reallocations
         let mut size = parents.as_bytes().len();
         for (path, node) in self.iter_nodes() {
             if node.entry.is_some() {
                 size += packed_entry_size(
                     path.full_path(),
                     node.copy_source.as_ref(),
                 )
             }
         }
 
         let mut packed = Vec::with_capacity(size);
         packed.extend(parents.as_bytes());
 
         let now: i32 = now.0.try_into().expect("time overflow");
         for (path, opt_entry, copy_source) in self.iter_node_data_mut() {
             if let Some(entry) = opt_entry {
                 clear_ambiguous_mtime(entry, now);
                 pack_entry(
                     path.full_path(),
                     entry,
                     copy_source.as_ref(),
                     &mut packed,
                 );
             }
         }
         self.dirty_parents = false;
         Ok(packed)
     }
 
     fn set_all_dirs(&mut self) -> Result<(), DirstateMapError> {
         // Do nothing, this `DirstateMap` does not a separate `all_dirs` that
         // needs to be recomputed
         Ok(())
     }
 
     fn set_dirs(&mut self) -> Result<(), DirstateMapError> {
         // Do nothing, this `DirstateMap` does not a separate `dirs` that needs
         // to be recomputed
         Ok(())
     }
 
     fn status<'a>(
         &'a mut self,
         matcher: &'a (dyn Matcher + Sync),
         root_dir: PathBuf,
         ignore_files: Vec<PathBuf>,
         options: StatusOptions,
     ) -> Result<(DirstateStatus<'a>, Vec<PatternFileWarning>), StatusError>
     {
         super::status::status(self, matcher, root_dir, ignore_files, options)
     }
 
     fn copy_map_len(&self) -> usize {
         self.nodes_with_copy_source_count
     }
 
     fn copy_map_iter(&self) -> CopyMapIter<'_> {
         Box::new(self.iter_nodes().filter_map(|(path, node)| {
             node.copy_source
                 .as_ref()
                 .map(|copy_source| (path.full_path(), copy_source))
         }))
     }
 
     fn copy_map_contains_key(&self, key: &HgPath) -> bool {
         if let Some(node) = self.get_node(key) {
             node.copy_source.is_some()
         } else {
             false
         }
     }
 
     fn copy_map_get(&self, key: &HgPath) -> Option<&HgPathBuf> {
         self.get_node(key)?.copy_source.as_ref()
     }
 
     fn copy_map_remove(&mut self, key: &HgPath) -> Option<HgPathBuf> {
         let count = &mut self.nodes_with_copy_source_count;
         Self::get_node_mut(&mut self.root, key).and_then(|node| {
             if node.copy_source.is_some() {
                 *count -= 1
             }
             node.copy_source.take()
         })
     }
 
     fn copy_map_insert(
         &mut self,
         key: HgPathBuf,
         value: HgPathBuf,
     ) -> Option<HgPathBuf> {
         let node = Self::get_or_insert_node(&mut self.root, &key);
         if node.copy_source.is_none() {
             self.nodes_with_copy_source_count += 1
         }
         node.copy_source.replace(value)
     }
 
     fn len(&self) -> usize {
         self.nodes_with_entry_count
     }
 
     fn contains_key(&self, key: &HgPath) -> bool {
         self.get(key).is_some()
     }
 
     fn get(&self, key: &HgPath) -> Option<&DirstateEntry> {
         self.get_node(key)?.entry.as_ref()
     }
 
     fn iter(&self) -> StateMapIter<'_> {
         Box::new(self.iter_nodes().filter_map(|(path, node)| {
             node.entry.as_ref().map(|entry| (path.full_path(), entry))
         }))
     }
 }