dirstate-v2: Add a zero-size error type for dirstate v2 parse errors

This error should only happen if Mercurial is buggy or the file is corrupted.
It indicates for example that:

* A part of the file refers to another part, and the byte offset or item count
  would cause reading out of bounds, beyond the end of the file.
* The byte for an entry state has an invalid value

When parsing becomes lazy, many more functions will return a `Result` with
this error. Making it zero-size reduces the work that the `?` operator needs
to do to pass around the error value.

Differential Revision: https://phab.mercurial-scm.org/D10748

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

@@ -1,773 +1,773 b''
 use bytes_cast::BytesCast;
 use micro_timer::timed;
 use std::borrow::Cow;
 use std::convert::TryInto;
 use std::path::PathBuf;
 
 use super::on_disk;
 use super::path_with_basename::WithBasename;
 use crate::dirstate::parsers::pack_entry;
 use crate::dirstate::parsers::packed_entry_size;
 use crate::dirstate::parsers::parse_dirstate_entries;
 use crate::dirstate::parsers::Timestamp;
 use crate::matchers::Matcher;
 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<'on_disk> {
     /// Contents of the `.hg/dirstate` file
     pub(super) on_disk: &'on_disk [u8],
 
     pub(super) root: ChildNodes<'on_disk>,
 
     /// Number of nodes anywhere in the tree that have `.entry.is_some()`.
     pub(super) nodes_with_entry_count: u32,
 
     /// Number of nodes anywhere in the tree that have
     /// `.copy_source.is_some()`.
     pub(super) nodes_with_copy_source_count: u32,
 }
 
 /// 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 `HashMap` would waste time always re-hashing the same
 /// string prefix.
 pub(super) type NodeKey<'on_disk> = WithBasename<Cow<'on_disk, HgPath>>;
 
 pub(super) enum ChildNodes<'on_disk> {
     InMemory(FastHashMap<NodeKey<'on_disk>, Node<'on_disk>>),
 }
 
 pub(super) enum ChildNodesRef<'tree, 'on_disk> {
     InMemory(&'tree FastHashMap<NodeKey<'on_disk>, Node<'on_disk>>),
 }
 
 pub(super) enum NodeRef<'tree, 'on_disk> {
     InMemory(&'tree NodeKey<'on_disk>, &'tree Node<'on_disk>),
 }
 
 impl Default for ChildNodes<'_> {
     fn default() -> Self {
         ChildNodes::InMemory(Default::default())
     }
 }
 
 impl<'on_disk> ChildNodes<'on_disk> {
     pub(super) fn as_ref<'tree>(
         &'tree self,
     ) -> ChildNodesRef<'tree, 'on_disk> {
         match self {
             ChildNodes::InMemory(nodes) => ChildNodesRef::InMemory(nodes),
         }
     }
 
     pub(super) fn is_empty(&self) -> bool {
         match self {
             ChildNodes::InMemory(nodes) => nodes.is_empty(),
         }
     }
 
     pub(super) fn make_mut(
         &mut self,
     ) -> &mut FastHashMap<NodeKey<'on_disk>, Node<'on_disk>> {
         match self {
             ChildNodes::InMemory(nodes) => nodes,
         }
     }
 }
 
 impl<'tree, 'on_disk> ChildNodesRef<'tree, 'on_disk> {
     pub(super) fn get(
         &self,
         base_name: &HgPath,
     ) -> Option<NodeRef<'tree, 'on_disk>> {
         match self {
             ChildNodesRef::InMemory(nodes) => nodes
                 .get_key_value(base_name)
                 .map(|(k, v)| NodeRef::InMemory(k, v)),
         }
     }
 
     /// Iterate in undefined order
     pub(super) fn iter(
         &self,
     ) -> impl Iterator<Item = NodeRef<'tree, 'on_disk>> {
         match self {
             ChildNodesRef::InMemory(nodes) => {
                 nodes.iter().map(|(k, v)| NodeRef::InMemory(k, v))
             }
         }
     }
 
     /// Iterate in parallel in undefined order
     pub(super) fn par_iter(
         &self,
     ) -> impl rayon::iter::ParallelIterator<Item = NodeRef<'tree, 'on_disk>>
     {
         use rayon::prelude::*;
         match self {
             ChildNodesRef::InMemory(nodes) => {
                 nodes.par_iter().map(|(k, v)| NodeRef::InMemory(k, v))
             }
         }
     }
 
     pub(super) fn sorted(&self) -> Vec<NodeRef<'tree, 'on_disk>> {
         match self {
             ChildNodesRef::InMemory(nodes) => {
                 let mut vec: Vec<_> = nodes
                     .iter()
                     .map(|(k, v)| NodeRef::InMemory(k, v))
                     .collect();
                 // `sort_unstable_by_key` doesn’t allow keys borrowing from the
                 // value: https://github.com/rust-lang/rust/issues/34162
                 vec.sort_unstable_by(|a, b| a.base_name().cmp(b.base_name()));
                 vec
             }
         }
     }
 }
 
 impl<'tree, 'on_disk> NodeRef<'tree, 'on_disk> {
     pub(super) fn full_path(&self) -> &'tree HgPath {
         match self {
             NodeRef::InMemory(path, _node) => path.full_path(),
         }
     }
 
     /// Returns a `Cow` that can borrow 'on_disk but is detached from 'tree
     pub(super) fn full_path_cow(&self) -> Cow<'on_disk, HgPath> {
         match self {
             NodeRef::InMemory(path, _node) => path.full_path().clone(),
         }
     }
 
     pub(super) fn base_name(&self) -> &'tree HgPath {
         match self {
             NodeRef::InMemory(path, _node) => path.base_name(),
         }
     }
 
     pub(super) fn children(&self) -> ChildNodesRef<'tree, 'on_disk> {
         match self {
             NodeRef::InMemory(_path, node) => node.children.as_ref(),
         }
     }
 
     pub(super) fn copy_source(&self) -> Option<&'tree HgPath> {
         match self {
             NodeRef::InMemory(_path, node) => {
                 node.copy_source.as_ref().map(|s| &**s)
             }
         }
     }
 
     pub(super) fn has_entry(&self) -> bool {
         match self {
             NodeRef::InMemory(_path, node) => node.entry.is_some(),
         }
     }
 
     pub(super) fn entry(&self) -> Option<DirstateEntry> {
         match self {
             NodeRef::InMemory(_path, node) => node.entry,
         }
     }
     pub(super) fn state(&self) -> Option<EntryState> {
         match self {
             NodeRef::InMemory(_path, node) => {
                 node.entry.as_ref().map(|entry| entry.state)
             }
         }
     }
 
     pub(super) fn tracked_descendants_count(&self) -> u32 {
         match self {
             NodeRef::InMemory(_path, node) => node.tracked_descendants_count,
         }
     }
 }
 
 /// Represents a file or a directory
 #[derive(Default)]
 pub(super) struct Node<'on_disk> {
     /// `None` for directories
     pub(super) entry: Option<DirstateEntry>,
 
     pub(super) copy_source: Option<Cow<'on_disk, HgPath>>,
 
     pub(super) children: ChildNodes<'on_disk>,
 
     /// How many (non-inclusive) descendants of this node are tracked files
     pub(super) tracked_descendants_count: u32,
 }
 
 impl<'on_disk> DirstateMap<'on_disk> {
     pub(super) fn empty(on_disk: &'on_disk [u8]) -> Self {
         Self {
             on_disk,
             root: ChildNodes::default(),
             nodes_with_entry_count: 0,
             nodes_with_copy_source_count: 0,
         }
     }
 
     #[timed]
     pub fn new_v2(
         on_disk: &'on_disk [u8],
     ) -> Result<(Self, Option<DirstateParents>), DirstateError> {
-        on_disk::read(on_disk)
+        Ok(on_disk::read(on_disk)?)
     }
 
     #[timed]
     pub fn new_v1(
         on_disk: &'on_disk [u8],
     ) -> Result<(Self, Option<DirstateParents>), DirstateError> {
         let mut map = Self::empty(on_disk);
         if map.on_disk.is_empty() {
             return Ok((map, None));
         }
 
         let parents = parse_dirstate_entries(
             map.on_disk,
             |path, entry, copy_source| {
                 let tracked = entry.state.is_tracked();
                 let node = Self::get_or_insert_node(
                     &mut map.root,
                     path,
                     WithBasename::to_cow_borrowed,
                     |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(Cow::Borrowed);
                 map.nodes_with_entry_count += 1;
                 if copy_source.is_some() {
                     map.nodes_with_copy_source_count += 1
                 }
             },
         )?;
         let parents = Some(parents.clone());
 
         Ok((map, parents))
     }
 
     fn get_node<'tree>(
         &'tree self,
         path: &HgPath,
     ) -> Option<NodeRef<'tree, 'on_disk>> {
         let mut children = self.root.as_ref();
         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<'on_disk>,
         path: &HgPath,
     ) -> Option<&'tree mut Node<'on_disk>> {
         let mut children = root;
         let mut components = path.components();
         let mut component =
             components.next().expect("expected at least one components");
         loop {
             let child = children.make_mut().get_mut(component)?;
             if let Some(next_component) = components.next() {
                 component = next_component;
                 children = &mut child.children;
             } else {
                 return Some(child);
             }
         }
     }
 
     fn get_or_insert_node<'tree, 'path>(
         root: &'tree mut ChildNodes<'on_disk>,
         path: &'path HgPath,
         to_cow: impl Fn(
             WithBasename<&'path HgPath>,
         ) -> WithBasename<Cow<'on_disk, HgPath>>,
         mut each_ancestor: impl FnMut(&mut Node),
     ) -> &'tree mut Node<'on_disk> {
         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
                 .make_mut()
                 .entry(to_cow(ancestor_path))
                 .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;
             }
         }
     }
 
     fn add_or_remove_file(
         &mut self,
         path: &HgPath,
         old_state: EntryState,
         new_entry: DirstateEntry,
     ) {
         let tracked_count_increment =
             match (old_state.is_tracked(), new_entry.state.is_tracked()) {
                 (false, true) => 1,
                 (true, false) => -1,
                 _ => 0,
             };
 
         let node = Self::get_or_insert_node(
             &mut self.root,
             path,
             WithBasename::to_cow_owned,
             |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 => 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<'tree>(
         &'tree self,
     ) -> impl Iterator<Item = NodeRef<'tree, 'on_disk>> + 'tree {
         // 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.as_ref().iter();
         std::iter::from_fn(move || {
             while let Some(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((child_node, old_iter));
             }
             // Found the end of a `children.iter()` iterator.
             if let Some((child_node, next_iter)) = stack.pop() {
                 // "Return" from pseudo-recursion by restoring state from the
                 // explicit stack
                 iter = next_iter;
 
                 Some(child_node)
             } else {
                 // Reached the bottom of the stack, we’re done
                 None
             }
         })
     }
 
     fn clear_known_ambiguous_mtimes(&mut self, paths: &[impl AsRef<HgPath>]) {
         for path in paths {
             if let Some(node) =
                 Self::get_node_mut(&mut self.root, path.as_ref())
             {
                 if let Some(entry) = node.entry.as_mut() {
                     entry.clear_mtime();
                 }
             }
         }
     }
 }
 
 impl<'on_disk> super::dispatch::DirstateMapMethods for DirstateMap<'on_disk> {
     fn clear(&mut self) {
         self.root = Default::default();
         self.nodes_with_entry_count = 0;
         self.nodes_with_copy_source_count = 0;
     }
 
     fn add_file(
         &mut self,
         filename: &HgPath,
         old_state: EntryState,
         entry: DirstateEntry,
     ) -> Result<(), DirstateMapError> {
         self.add_or_remove_file(filename, old_state, entry);
         Ok(())
     }
 
     fn remove_file(
         &mut self,
         filename: &HgPath,
         old_state: EntryState,
         size: i32,
     ) -> Result<(), DirstateMapError> {
         let entry = DirstateEntry {
             state: EntryState::Removed,
             mode: 0,
             size,
             mtime: 0,
         };
         self.add_or_remove_file(filename, old_state, entry);
         Ok(())
     }
 
     fn drop_file(
         &mut self,
         filename: &HgPath,
         old_state: EntryState,
     ) -> Result<bool, DirstateMapError> {
         struct Dropped {
             was_tracked: bool,
             had_entry: bool,
             had_copy_source: bool,
         }
         fn recur(nodes: &mut ChildNodes, path: &HgPath) -> Option<Dropped> {
             let (first_path_component, rest_of_path) =
                 path.split_first_component();
             let node = nodes.make_mut().get_mut(first_path_component)?;
             let dropped;
             if let Some(rest) = rest_of_path {
                 dropped = recur(&mut node.children, rest)?;
                 if dropped.was_tracked {
                     node.tracked_descendants_count -= 1;
                 }
             } else {
                 dropped = Dropped {
                     was_tracked: node
                         .entry
                         .as_ref()
                         .map_or(false, |entry| entry.state.is_tracked()),
                     had_entry: node.entry.take().is_some(),
                     had_copy_source: node.copy_source.take().is_some(),
                 };
             }
             // After recursion, for both leaf (rest_of_path is None) nodes and
             // parent nodes, remove a node if it just became empty.
             if node.entry.is_none()
                 && node.copy_source.is_none()
                 && node.children.is_empty()
             {
                 nodes.make_mut().remove(first_path_component);
             }
             Some(dropped)
         }
 
         if let Some(dropped) = recur(&mut self.root, filename) {
             if dropped.had_entry {
                 self.nodes_with_entry_count -= 1
             }
             if dropped.had_copy_source {
                 self.nodes_with_copy_source_count -= 1
             }
             Ok(dropped.had_entry)
         } else {
             debug_assert!(!old_state.is_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() {
                     entry.clear_ambiguous_mtime(now);
                 }
             }
         }
     }
 
     fn non_normal_entries_contains(&mut self, key: &HgPath) -> bool {
         self.get_node(key)
             .and_then(|node| node.entry())
             .map_or(false, |entry| entry.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 = &HgPath> + '_> {
         Box::new(self.iter_nodes().filter_map(|node| {
             node.entry()
                 .filter(|entry| {
                     entry.is_non_normal() || entry.is_from_other_parent()
                 })
                 .map(|_| node.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 = &HgPath> + Send + '_> {
         self.iter_non_normal_paths_panic()
     }
 
     fn iter_non_normal_paths_panic(
         &self,
     ) -> Box<dyn Iterator<Item = &HgPath> + Send + '_> {
         Box::new(self.iter_nodes().filter_map(|node| {
             node.entry()
                 .filter(|entry| entry.is_non_normal())
                 .map(|_| node.full_path())
         }))
     }
 
     fn iter_other_parent_paths(
         &mut self,
     ) -> Box<dyn Iterator<Item = &HgPath> + Send + '_> {
         Box::new(self.iter_nodes().filter_map(|node| {
             node.entry()
                 .filter(|entry| entry.is_from_other_parent())
                 .map(|_| node.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.has_entry() && 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.has_entry())
         } else {
             Ok(false)
         }
     }
 
     #[timed]
     fn pack_v1(
         &mut self,
         parents: DirstateParents,
         now: Timestamp,
     ) -> Result<Vec<u8>, DirstateError> {
         let now: i32 = now.0.try_into().expect("time overflow");
         let mut ambiguous_mtimes = Vec::new();
         // Optizimation (to be measured?): pre-compute size to avoid `Vec`
         // reallocations
         let mut size = parents.as_bytes().len();
         for node in self.iter_nodes() {
             if let Some(entry) = node.entry() {
                 size +=
                     packed_entry_size(node.full_path(), node.copy_source());
                 if entry.mtime_is_ambiguous(now) {
                     ambiguous_mtimes.push(node.full_path_cow())
                 }
             }
         }
         self.clear_known_ambiguous_mtimes(&ambiguous_mtimes);
 
         let mut packed = Vec::with_capacity(size);
         packed.extend(parents.as_bytes());
 
         for node in self.iter_nodes() {
             if let Some(entry) = node.entry() {
                 pack_entry(
                     node.full_path(),
                     &entry,
                     node.copy_source(),
                     &mut packed,
                 );
             }
         }
         Ok(packed)
     }
 
     #[timed]
     fn pack_v2(
         &mut self,
         parents: DirstateParents,
         now: Timestamp,
     ) -> Result<Vec<u8>, DirstateError> {
         // TODO: how do we want to handle this in 2038?
         let now: i32 = now.0.try_into().expect("time overflow");
         let mut paths = Vec::new();
         for node in self.iter_nodes() {
             if let Some(entry) = node.entry() {
                 if entry.mtime_is_ambiguous(now) {
                     paths.push(node.full_path_cow())
                 }
             }
         }
         // Borrow of `self` ends here since we collect cloned paths
 
         self.clear_known_ambiguous_mtimes(&paths);
 
         on_disk::write(self, parents)
     }
 
     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 as usize
     }
 
     fn copy_map_iter(&self) -> CopyMapIter<'_> {
         Box::new(self.iter_nodes().filter_map(|node| {
             node.copy_source()
                 .map(|copy_source| (node.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<&HgPath> {
         self.get_node(key)?.copy_source()
     }
 
     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().map(Cow::into_owned)
         })
     }
 
     fn copy_map_insert(
         &mut self,
         key: HgPathBuf,
         value: HgPathBuf,
     ) -> Option<HgPathBuf> {
         let node = Self::get_or_insert_node(
             &mut self.root,
             &key,
             WithBasename::to_cow_owned,
             |_ancestor| {},
         );
         if node.copy_source.is_none() {
             self.nodes_with_copy_source_count += 1
         }
         node.copy_source.replace(value.into()).map(Cow::into_owned)
     }
 
     fn len(&self) -> usize {
         self.nodes_with_entry_count as usize
     }
 
     fn contains_key(&self, key: &HgPath) -> bool {
         self.get(key).is_some()
     }
 
     fn get(&self, key: &HgPath) -> Option<DirstateEntry> {
         self.get_node(key)?.entry()
     }
 
     fn iter(&self) -> StateMapIter<'_> {
         Box::new(self.iter_nodes().filter_map(|node| {
             node.entry().map(|entry| (node.full_path(), entry))
         }))
     }
 }

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

@@ -1,331 +1,357 b''
 //! The "version 2" disk representation of the dirstate
 //!
 //! # File format
 //!
 //! The file starts with a fixed-sized header, whose layout is defined by the
 //! `Header` struct. Its `root` field contains the slice (offset and length) to
 //! the nodes representing the files and directories at the root of the
 //! repository. Each node is also fixed-size, defined by the `Node` struct.
 //! Nodes in turn contain slices to variable-size paths, and to their own child
 //! nodes (if any) for nested files and directories.
 
 use crate::dirstate_tree::dirstate_map::{self, DirstateMap, NodeRef};
 use crate::dirstate_tree::path_with_basename::WithBasename;
 use crate::errors::HgError;
 use crate::utils::hg_path::HgPath;
 use crate::DirstateEntry;
 use crate::DirstateError;
 use crate::DirstateParents;
 use bytes_cast::unaligned::{I32Be, U32Be, U64Be};
 use bytes_cast::BytesCast;
 use std::borrow::Cow;
 use std::convert::{TryFrom, TryInto};
 
 /// Added at the start of `.hg/dirstate` when the "v2" format is used.
 /// This a redundant sanity check more than an actual "magic number" since
 /// `.hg/requires` already governs which format should be used.
 pub const V2_FORMAT_MARKER: &[u8; 12] = b"dirstate-v2\n";
 
 #[derive(BytesCast)]
 #[repr(C)]
 struct Header {
     marker: [u8; V2_FORMAT_MARKER.len()],
 
     /// `dirstatemap.parents()` in `mercurial/dirstate.py` relies on this
     /// `parents` field being at this offset, immediately after `marker`.
     parents: DirstateParents,
 
     root: ChildNodes,
     nodes_with_entry_count: Size,
     nodes_with_copy_source_count: Size,
 }
 
 #[derive(BytesCast)]
 #[repr(C)]
 struct Node {
     full_path: PathSlice,
 
     /// In bytes from `self.full_path.start`
     base_name_start: Size,
 
     copy_source: OptPathSlice,
     entry: OptEntry,
     children: ChildNodes,
     tracked_descendants_count: Size,
 }
 
 /// Either nothing if `state == b'\0'`, or a dirstate entry like in the v1
 /// format
 #[derive(BytesCast)]
 #[repr(C)]
 struct OptEntry {
     state: u8,
     mode: I32Be,
     mtime: I32Be,
     size: I32Be,
 }
 
 /// Counted in bytes from the start of the file
 ///
 /// NOTE: If we decide to never support `.hg/dirstate` files larger than 4 GiB
 /// we could save space by using `U32Be` instead.
 type Offset = U64Be;
 
 /// Counted in number of items
 ///
 /// NOTE: not supporting directories with more than 4 billion direct children,
 /// or filenames more than 4 GiB.
 type Size = U32Be;
 
 /// Location of consecutive, fixed-size items.
 ///
 /// An item can be a single byte for paths, or a struct with
 /// `derive(BytesCast)`.
 #[derive(BytesCast, Copy, Clone)]
 #[repr(C)]
 struct Slice {
     start: Offset,
     len: Size,
 }
 
 /// A contiguous sequence of `len` times `Node`, representing the child nodes
 /// of either some other node or of the repository root.
 ///
 /// Always sorted by ascending `full_path`, to allow binary search.
 /// Since nodes with the same parent nodes also have the same parent path,
 /// only the `base_name`s need to be compared during binary search.
 type ChildNodes = Slice;
 
 /// A `HgPath` of `len` bytes
 type PathSlice = Slice;
 
 /// Either nothing if `start == 0`, or a `HgPath` of `len` bytes
 type OptPathSlice = Slice;
 
 /// Make sure that size-affecting changes are made knowingly
 fn _static_assert_size_of() {
     let _ = std::mem::transmute::<Header, [u8; 72]>;
     let _ = std::mem::transmute::<Node, [u8; 57]>;
 }
 
+/// Unexpected file format found in `.hg/dirstate` with the "v2" format.
+pub(crate) struct DirstateV2ParseError;
+
+impl From<DirstateV2ParseError> for HgError {
+    fn from(_: DirstateV2ParseError) -> Self {
+        HgError::corrupted("dirstate-v2 parse error")
+    }
+}
+
+impl From<DirstateV2ParseError> for crate::DirstateError {
+    fn from(error: DirstateV2ParseError) -> Self {
+        HgError::from(error).into()
+    }
+}
+
 pub(super) fn read<'on_disk>(
     on_disk: &'on_disk [u8],
-) -> Result<(DirstateMap<'on_disk>, Option<DirstateParents>), DirstateError> {
+) -> Result<
+    (DirstateMap<'on_disk>, Option<DirstateParents>),
+    DirstateV2ParseError,
+> {
     if on_disk.is_empty() {
         return Ok((DirstateMap::empty(on_disk), None));
     }
-    let (header, _) = Header::from_bytes(on_disk)
-        .map_err(|_| HgError::corrupted("truncated dirstate-v2"))?;
+    let (header, _) =
+        Header::from_bytes(on_disk).map_err(|_| DirstateV2ParseError)?;
     let Header {
         marker,
         parents,
         root,
         nodes_with_entry_count,
         nodes_with_copy_source_count,
     } = header;
     if marker != V2_FORMAT_MARKER {
-        return Err(HgError::corrupted("missing dirstated-v2 marker").into());
+        return Err(DirstateV2ParseError);
     }
     let dirstate_map = DirstateMap {
         on_disk,
         root: read_nodes(on_disk, *root)?,
         nodes_with_entry_count: nodes_with_entry_count.get(),
         nodes_with_copy_source_count: nodes_with_copy_source_count.get(),
     };
     let parents = Some(parents.clone());
     Ok((dirstate_map, parents))
 }
 
 impl Node {
     pub(super) fn path<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
-    ) -> Result<dirstate_map::NodeKey<'on_disk>, HgError> {
+    ) -> Result<dirstate_map::NodeKey<'on_disk>, DirstateV2ParseError> {
         let full_path = read_hg_path(on_disk, self.full_path)?;
         let base_name_start = usize::try_from(self.base_name_start.get())
             // u32 -> usize, could only panic on a 16-bit CPU
             .expect("dirstate-v2 base_name_start out of bounds");
         if base_name_start < full_path.len() {
             Ok(WithBasename::from_raw_parts(full_path, base_name_start))
         } else {
-            Err(HgError::corrupted(
-                "dirstate-v2 base_name_start out of bounds",
-            ))
+            Err(DirstateV2ParseError)
         }
     }
 
     pub(super) fn copy_source<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
-    ) -> Result<Option<Cow<'on_disk, HgPath>>, HgError> {
+    ) -> Result<Option<Cow<'on_disk, HgPath>>, DirstateV2ParseError> {
         Ok(if self.copy_source.start.get() != 0 {
             Some(read_hg_path(on_disk, self.copy_source)?)
         } else {
             None
         })
     }
 
-    pub(super) fn entry(&self) -> Result<Option<DirstateEntry>, HgError> {
+    pub(super) fn entry(
+        &self,
+    ) -> Result<Option<DirstateEntry>, DirstateV2ParseError> {
         Ok(if self.entry.state != b'\0' {
             Some(DirstateEntry {
-                state: self.entry.state.try_into()?,
+                state: self
+                    .entry
+                    .state
+                    .try_into()
+                    .map_err(|_| DirstateV2ParseError)?,
                 mode: self.entry.mode.get(),
                 mtime: self.entry.mtime.get(),
                 size: self.entry.size.get(),
             })
         } else {
             None
         })
     }
 
     pub(super) fn to_in_memory_node<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
-    ) -> Result<dirstate_map::Node<'on_disk>, HgError> {
+    ) -> Result<dirstate_map::Node<'on_disk>, DirstateV2ParseError> {
         Ok(dirstate_map::Node {
             children: read_nodes(on_disk, self.children)?,
             copy_source: self.copy_source(on_disk)?,
             entry: self.entry()?,
             tracked_descendants_count: self.tracked_descendants_count.get(),
         })
     }
 }
 
 fn read_nodes(
     on_disk: &[u8],
     slice: ChildNodes,
-) -> Result<dirstate_map::ChildNodes, HgError> {
+) -> Result<dirstate_map::ChildNodes, DirstateV2ParseError> {
     read_slice::<Node>(on_disk, slice)?
         .iter()
         .map(|node| {
             Ok((node.path(on_disk)?, node.to_in_memory_node(on_disk)?))
         })
         .collect::<Result<_, _>>()
         .map(dirstate_map::ChildNodes::InMemory)
 }
 
-fn read_hg_path(on_disk: &[u8], slice: Slice) -> Result<Cow<HgPath>, HgError> {
+fn read_hg_path(
+    on_disk: &[u8],
+    slice: Slice,
+) -> Result<Cow<HgPath>, DirstateV2ParseError> {
     let bytes = read_slice::<u8>(on_disk, slice)?;
     Ok(Cow::Borrowed(HgPath::new(bytes)))
 }
 
-fn read_slice<T>(on_disk: &[u8], slice: Slice) -> Result<&[T], HgError>
+fn read_slice<T>(
+    on_disk: &[u8],
+    slice: Slice,
+) -> Result<&[T], DirstateV2ParseError>
 where
     T: BytesCast,
 {
     // Either `usize::MAX` would result in "out of bounds" error since a single
     // `&[u8]` cannot occupy the entire addess space.
     let start = usize::try_from(slice.start.get()).unwrap_or(std::usize::MAX);
     let len = usize::try_from(slice.len.get()).unwrap_or(std::usize::MAX);
     on_disk
         .get(start..)
         .and_then(|bytes| T::slice_from_bytes(bytes, len).ok())
         .map(|(slice, _rest)| slice)
-        .ok_or_else(|| {
-            HgError::corrupted("dirstate v2 slice is out of bounds")
-        })
+        .ok_or_else(|| DirstateV2ParseError)
 }
 
 pub(super) fn write(
     dirstate_map: &mut DirstateMap,
     parents: DirstateParents,
 ) -> Result<Vec<u8>, DirstateError> {
     let header_len = std::mem::size_of::<Header>();
 
     // This ignores the space for paths, and for nodes without an entry.
     // TODO: better estimate? Skip the `Vec` and write to a file directly?
     let size_guess = header_len
         + std::mem::size_of::<Node>()
             * dirstate_map.nodes_with_entry_count as usize;
     let mut out = Vec::with_capacity(size_guess);
 
     // Keep space for the header. We’ll fill it out at the end when we know the
     // actual offset for the root nodes.
     out.resize(header_len, 0_u8);
 
     let root = write_nodes(dirstate_map.root.as_ref(), &mut out)?;
 
     let header = Header {
         marker: *V2_FORMAT_MARKER,
         parents: parents,
         root,
         nodes_with_entry_count: dirstate_map.nodes_with_entry_count.into(),
         nodes_with_copy_source_count: dirstate_map
             .nodes_with_copy_source_count
             .into(),
     };
     out[..header_len].copy_from_slice(header.as_bytes());
     Ok(out)
 }
 
 fn write_nodes(
     nodes: dirstate_map::ChildNodesRef,
     out: &mut Vec<u8>,
 ) -> Result<ChildNodes, DirstateError> {
     // `dirstate_map::ChildNodes` is a `HashMap` with undefined iteration
     // order. Sort to enable binary search in the written file.
     let nodes = nodes.sorted();
 
     // First accumulate serialized nodes in a `Vec`
     let mut on_disk_nodes = Vec::with_capacity(nodes.len());
     for node in nodes {
         let children = write_nodes(node.children(), out)?;
         let full_path = write_slice::<u8>(node.full_path().as_bytes(), out);
         let copy_source = if let Some(source) = node.copy_source() {
             write_slice::<u8>(source.as_bytes(), out)
         } else {
             Slice {
                 start: 0.into(),
                 len: 0.into(),
             }
         };
         on_disk_nodes.push(match node {
             NodeRef::InMemory(path, node) => Node {
                 children,
                 copy_source,
                 full_path,
                 base_name_start: u32::try_from(path.base_name_start())
                     // Could only panic for paths over 4 GiB
                     .expect("dirstate-v2 offset overflow")
                     .into(),
                 tracked_descendants_count: node
                     .tracked_descendants_count
                     .into(),
                 entry: if let Some(entry) = &node.entry {
                     OptEntry {
                         state: entry.state.into(),
                         mode: entry.mode.into(),
                         mtime: entry.mtime.into(),
                         size: entry.size.into(),
                     }
                 } else {
                     OptEntry {
                         state: b'\0',
                         mode: 0.into(),
                         mtime: 0.into(),
                         size: 0.into(),
                     }
                 },
             },
         })
     }
     // … so we can write them contiguously
     Ok(write_slice::<Node>(&on_disk_nodes, out))
 }
 
 fn write_slice<T>(slice: &[T], out: &mut Vec<u8>) -> Slice
 where
     T: BytesCast,
 {
     let start = u64::try_from(out.len())
         // Could only panic on a 128-bit CPU with a dirstate over 16 EiB
         .expect("dirstate-v2 offset overflow")
         .into();
     let len = u32::try_from(slice.len())
         // Could only panic for paths over 4 GiB or nodes with over 4 billions
         // child nodes
         .expect("dirstate-v2 offset overflow")
         .into();
     out.extend(slice.as_bytes());
     Slice { start, len }
 }