dirstate-v2: shrink on-disk path lengths to 16-bits

Differential Revision: https://phab.mercurial-scm.org/D11091

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

@@ -1,656 +1,678
 //! The "version 2" disk representation of the dirstate
 //!
 //! # File format
 //!
 //! In dirstate-v2 format, the `.hg/dirstate` file is a "docket that starts
 //! with a fixed-sized header whose layout is defined by the `DocketHeader`
 //! struct, followed by the data file identifier.
 //!
 //! A separate `.hg/dirstate.{uuid}.d` file contains most of the data. That
 //! file may be longer than the size given in the docket, but not shorter. Only
 //! the start of the data file up to the given size is considered. The
 //! fixed-size "root" of the dirstate tree whose layout is defined by the
 //! `Root` struct is found at the end of that slice of data.
 //!
 //! Its `root_nodes` 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 crate::EntryState;
-use bytes_cast::unaligned::{I32Be, I64Be, U32Be};
+use bytes_cast::unaligned::{I32Be, I64Be, U16Be, U32Be};
 use bytes_cast::BytesCast;
 use format_bytes::format_bytes;
 use std::borrow::Cow;
 use std::convert::{TryFrom, TryInto};
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 /// 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";
 
 /// Keep space for 256-bit hashes
 const STORED_NODE_ID_BYTES: usize = 32;
 
 /// … even though only 160 bits are used for now, with SHA-1
 const USED_NODE_ID_BYTES: usize = 20;
 
 pub(super) const IGNORE_PATTERNS_HASH_LEN: usize = 20;
 pub(super) type IgnorePatternsHash = [u8; IGNORE_PATTERNS_HASH_LEN];
 
 // Must match `HEADER` in `mercurial/dirstateutils/docket.py`
 #[derive(BytesCast)]
 #[repr(C)]
 struct DocketHeader {
     marker: [u8; V2_FORMAT_MARKER.len()],
     parent_1: [u8; STORED_NODE_ID_BYTES],
     parent_2: [u8; STORED_NODE_ID_BYTES],
+
+    /// Counted in bytes
     data_size: Size,
+
     uuid_size: u8,
 }
 
 pub struct Docket<'on_disk> {
     header: &'on_disk DocketHeader,
     uuid: &'on_disk [u8],
 }
 
 #[derive(BytesCast)]
 #[repr(C)]
 struct Root {
     root_nodes: ChildNodes,
     nodes_with_entry_count: Size,
     nodes_with_copy_source_count: Size,
 
     /// If non-zero, a hash of ignore files that were used for some previous
     /// run of the `status` algorithm.
     ///
     /// We define:
     ///
     /// * "Root" ignore files are `.hgignore` at the root of the repository if
     ///   it exists, and files from `ui.ignore.*` config. This set of files is
     ///   then sorted by the string representation of their path.
     /// * The "expanded contents" of an ignore files is the byte string made
     ///   by concatenating its contents with the "expanded contents" of other
     ///   files included with `include:` or `subinclude:` files, in inclusion
     ///   order. This definition is recursive, as included files can
     ///   themselves include more files.
     ///
     /// This hash is defined as the SHA-1 of the concatenation (in sorted
     /// order) of the "expanded contents" of each "root" ignore file.
     /// (Note that computing this does not require actually concatenating byte
     /// strings into contiguous memory, instead SHA-1 hashing can be done
     /// incrementally.)
     ignore_patterns_hash: IgnorePatternsHash,
 }
 
 #[derive(BytesCast)]
 #[repr(C)]
 pub(super) struct Node {
     full_path: PathSlice,
 
     /// In bytes from `self.full_path.start`
-    base_name_start: Size,
+    base_name_start: PathSize,
 
     copy_source: OptPathSlice,
     children: ChildNodes,
     pub(super) descendants_with_entry_count: Size,
     pub(super) tracked_descendants_count: Size,
 
     /// Depending on the value of `state`:
     ///
     /// * A null byte: `data` is not used.
     ///
     /// * A `n`, `a`, `r`, or `m` ASCII byte: `state` and `data` together
     ///   represent a dirstate entry like in the v1 format.
     ///
     /// * A `d` ASCII byte: the bytes of `data` should instead be interpreted
     ///   as the `Timestamp` for the mtime of a cached directory.
     ///
     ///   The presence of this state means that at some point, this path in
     ///   the working directory was observed:
     ///
     ///   - To be a directory
     ///   - With the modification time as given by `Timestamp`
     ///   - That timestamp was already strictly in the past when observed,
     ///     meaning that later changes cannot happen in the same clock tick
     ///     and must cause a different modification time (unless the system
     ///     clock jumps back and we get unlucky, which is not impossible but
     ///     but deemed unlikely enough).
     ///   - All direct children of this directory (as returned by
     ///     `std::fs::read_dir`) either have a corresponding dirstate node, or
     ///     are ignored by ignore patterns whose hash is in
     ///     `Root::ignore_patterns_hash`.
     ///
     ///   This means that if `std::fs::symlink_metadata` later reports the
     ///   same modification time and ignored patterns haven’t changed, a run
     ///   of status that is not listing ignored   files can skip calling
     ///   `std::fs::read_dir` again for this directory,   iterate child
     ///   dirstate nodes instead.
     state: u8,
     data: Entry,
 }
 
 #[derive(BytesCast, Copy, Clone)]
 #[repr(C)]
 struct Entry {
     mode: I32Be,
     mtime: I32Be,
     size: I32Be,
 }
 
 /// Duration since the Unix epoch
 #[derive(BytesCast, Copy, Clone, PartialEq)]
 #[repr(C)]
 pub(super) struct Timestamp {
     seconds: I64Be,
 
     /// In `0 .. 1_000_000_000`.
     ///
     /// This timestamp is later or earlier than `(seconds, 0)` by this many
     /// nanoseconds, if `seconds` is non-negative or negative, respectively.
     nanoseconds: U32Be,
 }
 
 /// Counted in bytes from the start of the file
 ///
 /// NOTE: not supporting `.hg/dirstate` files larger than 4 GiB.
 type Offset = U32Be;
 
 /// Counted in number of items
 ///
-/// NOTE: not supporting directories with more than 4 billion direct children,
-/// or filenames more than 4 GiB.
+/// NOTE: we choose not to support counting more than 4 billion nodes anywhere.
 type Size = U32Be;
 
-/// Location of consecutive, fixed-size items.
+/// Counted in bytes
 ///
-/// 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,
-}
+/// NOTE: we choose not to support file names/paths longer than 64 KiB.
+type PathSize = U16Be;
 
 /// 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;
+#[derive(BytesCast, Copy, Clone)]
+#[repr(C)]
+struct ChildNodes {
+    start: Offset,
+    len: Size,
+}
 
 /// A `HgPath` of `len` bytes
-type PathSlice = Slice;
+#[derive(BytesCast, Copy, Clone)]
+#[repr(C)]
+struct PathSlice {
+    start: Offset,
+    len: PathSize,
+}
 
 /// Either nothing if `start == 0`, or a `HgPath` of `len` bytes
-type OptPathSlice = Slice;
+type OptPathSlice = PathSlice;
 
 /// Make sure that size-affecting changes are made knowingly
 fn _static_assert_size_of() {
     let _ = std::mem::transmute::<DocketHeader, [u8; 81]>;
     let _ = std::mem::transmute::<Root, [u8; 36]>;
-    let _ = std::mem::transmute::<Node, [u8; 49]>;
+    let _ = std::mem::transmute::<Node, [u8; 43]>;
 }
 
 /// Unexpected file format found in `.hg/dirstate` with the "v2" format.
 ///
 /// This should only happen if Mercurial is buggy or a repository is corrupted.
 #[derive(Debug)]
 pub 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()
     }
 }
 
 impl<'on_disk> Docket<'on_disk> {
     pub fn parents(&self) -> DirstateParents {
         use crate::Node;
         let p1 = Node::try_from(&self.header.parent_1[..USED_NODE_ID_BYTES])
             .unwrap()
             .clone();
         let p2 = Node::try_from(&self.header.parent_2[..USED_NODE_ID_BYTES])
             .unwrap()
             .clone();
         DirstateParents { p1, p2 }
     }
 
     pub fn data_size(&self) -> usize {
         // This `unwrap` could only panic on a 16-bit CPU
         self.header.data_size.get().try_into().unwrap()
     }
 
     pub fn data_filename(&self) -> String {
         String::from_utf8(format_bytes!(b"dirstate.{}.d", self.uuid)).unwrap()
     }
 }
 
 pub fn read_docket(
     on_disk: &[u8],
 ) -> Result<Docket<'_>, DirstateV2ParseError> {
     let (header, uuid) =
         DocketHeader::from_bytes(on_disk).map_err(|_| DirstateV2ParseError)?;
     let uuid_size = header.uuid_size as usize;
     if header.marker == *V2_FORMAT_MARKER && uuid.len() == uuid_size {
         Ok(Docket { header, uuid })
     } else {
         Err(DirstateV2ParseError)
     }
 }
 
 fn read_root<'on_disk>(
     on_disk: &'on_disk [u8],
 ) -> Result<&'on_disk Root, DirstateV2ParseError> {
     // Find the `Root` at the end of the given slice
     let root_offset = on_disk
         .len()
         .checked_sub(std::mem::size_of::<Root>())
         // A non-empty slice too short is an error
         .ok_or(DirstateV2ParseError)?;
     let (root, _) = Root::from_bytes(&on_disk[root_offset..])
         .map_err(|_| DirstateV2ParseError)?;
     Ok(root)
 }
 
 pub(super) fn read<'on_disk>(
     on_disk: &'on_disk [u8],
 ) -> Result<DirstateMap<'on_disk>, DirstateV2ParseError> {
     if on_disk.is_empty() {
         return Ok(DirstateMap::empty(on_disk));
     }
     let root = read_root(on_disk)?;
     let dirstate_map = DirstateMap {
         on_disk,
-        root: dirstate_map::ChildNodes::OnDisk(read_slice::<Node>(
+        root: dirstate_map::ChildNodes::OnDisk(read_nodes(
             on_disk,
             root.root_nodes,
         )?),
         nodes_with_entry_count: root.nodes_with_entry_count.get(),
         nodes_with_copy_source_count: root.nodes_with_copy_source_count.get(),
         ignore_patterns_hash: root.ignore_patterns_hash,
     };
     Ok(dirstate_map)
 }
 
 impl Node {
     pub(super) fn full_path<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
     ) -> Result<&'on_disk HgPath, DirstateV2ParseError> {
         read_hg_path(on_disk, self.full_path)
     }
 
     pub(super) fn base_name_start<'on_disk>(
         &self,
     ) -> Result<usize, DirstateV2ParseError> {
         let start = self.base_name_start.get();
         if start < self.full_path.len.get() {
             let start = usize::try_from(start)
                 // u32 -> usize, could only panic on a 16-bit CPU
                 .expect("dirstate-v2 base_name_start out of bounds");
             Ok(start)
         } else {
             Err(DirstateV2ParseError)
         }
     }
 
     pub(super) fn base_name<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
     ) -> Result<&'on_disk HgPath, DirstateV2ParseError> {
         let full_path = self.full_path(on_disk)?;
         let base_name_start = self.base_name_start()?;
         Ok(HgPath::new(&full_path.as_bytes()[base_name_start..]))
     }
 
     pub(super) fn path<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
     ) -> Result<dirstate_map::NodeKey<'on_disk>, DirstateV2ParseError> {
         Ok(WithBasename::from_raw_parts(
             Cow::Borrowed(self.full_path(on_disk)?),
             self.base_name_start()?,
         ))
     }
 
     pub(super) fn has_copy_source<'on_disk>(&self) -> bool {
         self.copy_source.start.get() != 0
     }
 
     pub(super) fn copy_source<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
     ) -> Result<Option<&'on_disk HgPath>, DirstateV2ParseError> {
         Ok(if self.has_copy_source() {
             Some(read_hg_path(on_disk, self.copy_source)?)
         } else {
             None
         })
     }
 
     pub(super) fn node_data(
         &self,
     ) -> Result<dirstate_map::NodeData, DirstateV2ParseError> {
         let entry = |state| {
             dirstate_map::NodeData::Entry(self.entry_with_given_state(state))
         };
 
         match self.state {
             b'\0' => Ok(dirstate_map::NodeData::None),
             b'd' => Ok(dirstate_map::NodeData::CachedDirectory {
                 mtime: *self.data.as_timestamp(),
             }),
             b'n' => Ok(entry(EntryState::Normal)),
             b'a' => Ok(entry(EntryState::Added)),
             b'r' => Ok(entry(EntryState::Removed)),
             b'm' => Ok(entry(EntryState::Merged)),
             _ => Err(DirstateV2ParseError),
         }
     }
 
     pub(super) fn cached_directory_mtime(&self) -> Option<&Timestamp> {
         if self.state == b'd' {
             Some(self.data.as_timestamp())
         } else {
             None
         }
     }
 
     pub(super) fn state(
         &self,
     ) -> Result<Option<EntryState>, DirstateV2ParseError> {
         match self.state {
             b'\0' | b'd' => Ok(None),
             b'n' => Ok(Some(EntryState::Normal)),
             b'a' => Ok(Some(EntryState::Added)),
             b'r' => Ok(Some(EntryState::Removed)),
             b'm' => Ok(Some(EntryState::Merged)),
             _ => Err(DirstateV2ParseError),
         }
     }
 
     fn entry_with_given_state(&self, state: EntryState) -> DirstateEntry {
         DirstateEntry {
             state,
             mode: self.data.mode.get(),
             mtime: self.data.mtime.get(),
             size: self.data.size.get(),
         }
     }
 
     pub(super) fn entry(
         &self,
     ) -> Result<Option<DirstateEntry>, DirstateV2ParseError> {
         Ok(self
             .state()?
             .map(|state| self.entry_with_given_state(state)))
     }
 
     pub(super) fn children<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
     ) -> Result<&'on_disk [Node], DirstateV2ParseError> {
-        read_slice::<Node>(on_disk, self.children)
+        read_nodes(on_disk, self.children)
     }
 
     pub(super) fn to_in_memory_node<'on_disk>(
         &self,
         on_disk: &'on_disk [u8],
     ) -> Result<dirstate_map::Node<'on_disk>, DirstateV2ParseError> {
         Ok(dirstate_map::Node {
             children: dirstate_map::ChildNodes::OnDisk(
                 self.children(on_disk)?,
             ),
             copy_source: self.copy_source(on_disk)?.map(Cow::Borrowed),
             data: self.node_data()?,
             descendants_with_entry_count: self
                 .descendants_with_entry_count
                 .get(),
             tracked_descendants_count: self.tracked_descendants_count.get(),
         })
     }
 }
 
 impl Entry {
     fn from_timestamp(timestamp: Timestamp) -> Self {
         // Safety: both types implement the `ByteCast` trait, so we could
         // safely use `as_bytes` and `from_bytes` to do this conversion. Using
         // `transmute` instead makes the compiler check that the two types
         // have the same size, which eliminates the error case of
         // `from_bytes`.
         unsafe { std::mem::transmute::<Timestamp, Entry>(timestamp) }
     }
 
     fn as_timestamp(&self) -> &Timestamp {
         // Safety: same as above in `from_timestamp`
         unsafe { &*(self as *const Entry as *const Timestamp) }
     }
 }
 
 impl Timestamp {
     pub fn seconds(&self) -> i64 {
         self.seconds.get()
     }
 }
 
 impl From<SystemTime> for Timestamp {
     fn from(system_time: SystemTime) -> Self {
         let (secs, nanos) = match system_time.duration_since(UNIX_EPOCH) {
             Ok(duration) => {
                 (duration.as_secs() as i64, duration.subsec_nanos())
             }
             Err(error) => {
                 let negative = error.duration();
                 (-(negative.as_secs() as i64), negative.subsec_nanos())
             }
         };
         Timestamp {
             seconds: secs.into(),
             nanoseconds: nanos.into(),
         }
     }
 }
 
 impl From<&'_ Timestamp> for SystemTime {
     fn from(timestamp: &'_ Timestamp) -> Self {
         let secs = timestamp.seconds.get();
         let nanos = timestamp.nanoseconds.get();
         if secs >= 0 {
             UNIX_EPOCH + Duration::new(secs as u64, nanos)
         } else {
             UNIX_EPOCH - Duration::new((-secs) as u64, nanos)
         }
     }
 }
 
 fn read_hg_path(
     on_disk: &[u8],
-    slice: Slice,
+    slice: PathSlice,
 ) -> Result<&HgPath, DirstateV2ParseError> {
-    let bytes = read_slice::<u8>(on_disk, slice)?;
-    Ok(HgPath::new(bytes))
+    read_slice(on_disk, slice.start, slice.len.get()).map(HgPath::new)
 }
 
-fn read_slice<T>(
+fn read_nodes(
     on_disk: &[u8],
-    slice: Slice,
+    slice: ChildNodes,
+) -> Result<&[Node], DirstateV2ParseError> {
+    read_slice(on_disk, slice.start, slice.len.get())
+}
+
+fn read_slice<T, Len>(
+    on_disk: &[u8],
+    start: Offset,
+    len: Len,
 ) -> Result<&[T], DirstateV2ParseError>
 where
     T: BytesCast,
+    Len: TryInto<usize>,
 {
     // 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);
+    let start = start.get().try_into().unwrap_or(std::usize::MAX);
+    let len = len.try_into().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(|| DirstateV2ParseError)
 }
 
 pub(crate) fn for_each_tracked_path<'on_disk>(
     on_disk: &'on_disk [u8],
     mut f: impl FnMut(&'on_disk HgPath),
 ) -> Result<(), DirstateV2ParseError> {
     let root = read_root(on_disk)?;
     fn recur<'on_disk>(
         on_disk: &'on_disk [u8],
-        nodes: Slice,
+        nodes: ChildNodes,
         f: &mut impl FnMut(&'on_disk HgPath),
     ) -> Result<(), DirstateV2ParseError> {
-        for node in read_slice::<Node>(on_disk, nodes)? {
+        for node in read_nodes(on_disk, nodes)? {
             if let Some(state) = node.state()? {
                 if state.is_tracked() {
                     f(node.full_path(on_disk)?)
                 }
             }
             recur(on_disk, node.children, f)?
         }
         Ok(())
     }
     recur(on_disk, root.root_nodes, &mut f)
 }
 
 pub(super) fn write(
     dirstate_map: &mut DirstateMap,
 ) -> Result<Vec<u8>, DirstateError> {
     let root_len = std::mem::size_of::<Root>();
 
     // 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 = root_len
         + std::mem::size_of::<Node>()
             * dirstate_map.nodes_with_entry_count as usize;
     let mut out = Vec::with_capacity(size_guess);
 
     let root_nodes =
         write_nodes(dirstate_map, dirstate_map.root.as_ref(), &mut out)?;
 
     let root = Root {
         root_nodes,
         nodes_with_entry_count: dirstate_map.nodes_with_entry_count.into(),
         nodes_with_copy_source_count: dirstate_map
             .nodes_with_copy_source_count
             .into(),
         ignore_patterns_hash: dirstate_map.ignore_patterns_hash,
     };
     out.extend(root.as_bytes());
     Ok(out)
 }
 
 fn write_nodes(
     dirstate_map: &DirstateMap,
     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();
+    let nodes_len = nodes.len();
 
     // First accumulate serialized nodes in a `Vec`
-    let mut on_disk_nodes = Vec::with_capacity(nodes.len());
+    let mut on_disk_nodes = Vec::with_capacity(nodes_len);
     for node in nodes {
         let children = write_nodes(
             dirstate_map,
             node.children(dirstate_map.on_disk)?,
             out,
         )?;
         let full_path = node.full_path(dirstate_map.on_disk)?;
-        let full_path = write_slice::<u8>(full_path.as_bytes(), out);
+        let full_path = write_path(full_path.as_bytes(), out);
         let copy_source =
             if let Some(source) = node.copy_source(dirstate_map.on_disk)? {
-                write_slice::<u8>(source.as_bytes(), out)
+                write_path(source.as_bytes(), out)
             } else {
-                Slice {
+                PathSlice {
                     start: 0.into(),
                     len: 0.into(),
                 }
             };
         on_disk_nodes.push(match node {
             NodeRef::InMemory(path, node) => {
                 let (state, data) = match &node.data {
                     dirstate_map::NodeData::Entry(entry) => (
                         entry.state.into(),
                         Entry {
                             mode: entry.mode.into(),
                             mtime: entry.mtime.into(),
                             size: entry.size.into(),
                         },
                     ),
                     dirstate_map::NodeData::CachedDirectory { mtime } => {
                         (b'd', Entry::from_timestamp(*mtime))
                     }
                     dirstate_map::NodeData::None => (
                         b'\0',
                         Entry {
                             mode: 0.into(),
                             mtime: 0.into(),
                             size: 0.into(),
                         },
                     ),
                 };
                 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")
+                    base_name_start: u16::try_from(path.base_name_start())
+                        // Could only panic for paths over 64 KiB
+                        .expect("dirstate-v2 path length overflow")
                         .into(),
                     descendants_with_entry_count: node
                         .descendants_with_entry_count
                         .into(),
                     tracked_descendants_count: node
                         .tracked_descendants_count
                         .into(),
                     state,
                     data,
                 }
             }
             NodeRef::OnDisk(node) => Node {
                 children,
                 copy_source,
                 full_path,
                 ..*node
             },
         })
     }
-    // … so we can write them contiguously
-    Ok(write_slice::<Node>(&on_disk_nodes, out))
+    // … so we can write them contiguously, after writing everything else they
+    // refer to.
+    let start = current_offset(out);
+    let len = u32::try_from(nodes_len)
+        // Could only panic with over 4 billion nodes
+        .expect("dirstate-v2 path length overflow")
+        .into();
+    out.extend(on_disk_nodes.as_bytes());
+    Ok(ChildNodes { start, len })
 }
 
-fn write_slice<T>(slice: &[T], out: &mut Vec<u8>) -> Slice
-where
-    T: BytesCast,
-{
-    let start = u32::try_from(out.len())
+fn current_offset(out: &Vec<u8>) -> Offset {
+    u32::try_from(out.len())
         // Could only panic for a dirstate file larger than 4 GiB
         .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()
+}
+
+fn write_path(slice: &[u8], out: &mut Vec<u8>) -> PathSlice {
+    let start = current_offset(out);
+    let len = u16::try_from(slice.len())
+        // Could only panic for paths over 64 KiB
+        .expect("dirstate-v2 path length overflow")
         .into();
     out.extend(slice.as_bytes());
-    Slice { start, len }
+    PathSlice { start, len }
 }