rust: use the bytes-cast crate to parse persistent nodemaps

This crate casts pointers to custom structs, with compile-time safety checks,
for easy and efficient binary data parsing.

See https://crates.io/crates/bytes-cast and
https://docs.rs/bytes-cast/0.1.0/bytes_cast/

Differential Revision: https://phab.mercurial-scm.org/D9788

rust/Cargo.lock

@@ -1,1012 +1,1033 b''
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rust/hg-core/Cargo.toml

@@ -1,48 +1,49 b''
 [package]
 name = "hg-core"
 version = "0.1.0"
 authors = ["Georges Racinet <[e-mail]>"]
 description = "Mercurial pure Rust core library, with no assumption on Python bindings (FFI)"
 edition = "2018"
 
 [lib]
 name = "hg"
 
 [dependencies]
+bytes-cast = "0.1"
 byteorder = "1.3.4"
 hex = "0.4.2"
 im-rc = "15.0.*"
 lazy_static = "1.4.0"
 memchr = "2.3.3"
 rand = "0.7.3"
 rand_pcg = "0.2.1"
 rand_distr = "0.2.2"
 rayon = "1.3.0"
 regex = "1.3.9"
 twox-hash = "1.5.0"
 same-file = "1.0.6"
 crossbeam-channel = "0.4"
 micro-timer = "0.3.0"
 log = "0.4.8"
 memmap = "0.7.0"
 zstd = "0.5.3"
 rust-crypto = "0.2.36"
 format-bytes = "0.1.2"
 
 # We don't use the `miniz-oxide` backend to not change rhg benchmarks and until
 # we have a clearer view of which backend is the fastest.
 [dependencies.flate2]
 version = "1.0.16"
 features = ["zlib"]
 default-features = false
 
 [dev-dependencies]
 clap = "*"
 pretty_assertions = "0.6.1"
 tempfile = "3.1.0"
 
 [features]
 # Use a (still unoptimized) tree for the dirstate instead of the current flat
 # dirstate. This is not yet recommended for performance reasons. A future
 # version might make it the default, or make it a runtime option.
 dirstate-tree = []

rust/hg-core/src/revlog/nodemap.rs

@@ -1,1118 +1,1101 b''
 // Copyright 2018-2020 Georges Racinet <[e-mail]>
 //           and Mercurial contributors
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 //! Indexing facilities for fast retrieval of `Revision` from `Node`
 //!
 //! This provides a variation on the 16-ary radix tree that is
 //! provided as "nodetree" in revlog.c, ready for append-only persistence
 //! on disk.
 //!
 //! Following existing implicit conventions, the "nodemap" terminology
 //! is used in a more abstract context.
 
 use super::{
     node::NULL_NODE, Node, NodeError, NodePrefix, NodePrefixRef, Revision,
     RevlogIndex, NULL_REVISION,
 };
 
+use bytes_cast::{unaligned, BytesCast};
 use std::cmp::max;
 use std::fmt;
-use std::mem;
+use std::mem::{self, align_of, size_of};
 use std::ops::Deref;
 use std::ops::Index;
-use std::slice;
 
 #[derive(Debug, PartialEq)]
 pub enum NodeMapError {
     MultipleResults,
     InvalidNodePrefix(NodeError),
     /// A `Revision` stored in the nodemap could not be found in the index
     RevisionNotInIndex(Revision),
 }
 
 impl From<NodeError> for NodeMapError {
     fn from(err: NodeError) -> Self {
         NodeMapError::InvalidNodePrefix(err)
     }
 }
 
 /// Mapping system from Mercurial nodes to revision numbers.
 ///
 /// ## `RevlogIndex` and `NodeMap`
 ///
 /// One way to think about their relationship is that
 /// the `NodeMap` is a prefix-oriented reverse index of the `Node` information
 /// carried by a [`RevlogIndex`].
 ///
 /// Many of the methods in this trait take a `RevlogIndex` argument
 /// which is used for validation of their results. This index must naturally
 /// be the one the `NodeMap` is about, and it must be consistent.
 ///
 /// Notably, the `NodeMap` must not store
 /// information about more `Revision` values than there are in the index.
 /// In these methods, an encountered `Revision` is not in the index, a
 /// [`RevisionNotInIndex`] error is returned.
 ///
 /// In insert operations, the rule is thus that the `NodeMap` must always
 /// be updated after the `RevlogIndex`
 /// be updated first, and the `NodeMap` second.
 ///
 /// [`RevisionNotInIndex`]: enum.NodeMapError.html#variant.RevisionNotInIndex
 /// [`RevlogIndex`]: ../trait.RevlogIndex.html
 pub trait NodeMap {
     /// Find the unique `Revision` having the given `Node`
     ///
     /// If no Revision matches the given `Node`, `Ok(None)` is returned.
     fn find_node(
         &self,
         index: &impl RevlogIndex,
         node: &Node,
     ) -> Result<Option<Revision>, NodeMapError> {
         self.find_bin(index, node.into())
     }
 
     /// Find the unique Revision whose `Node` starts with a given binary prefix
     ///
     /// If no Revision matches the given prefix, `Ok(None)` is returned.
     ///
     /// If several Revisions match the given prefix, a [`MultipleResults`]
     /// error is returned.
     fn find_bin<'a>(
         &self,
         idx: &impl RevlogIndex,
         prefix: NodePrefixRef<'a>,
     ) -> Result<Option<Revision>, NodeMapError>;
 
     /// Find the unique Revision whose `Node` hexadecimal string representation
     /// starts with a given prefix
     ///
     /// If no Revision matches the given prefix, `Ok(None)` is returned.
     ///
     /// If several Revisions match the given prefix, a [`MultipleResults`]
     /// error is returned.
     fn find_hex(
         &self,
         idx: &impl RevlogIndex,
         prefix: &str,
     ) -> Result<Option<Revision>, NodeMapError> {
         self.find_bin(idx, NodePrefix::from_hex(prefix)?.borrow())
     }
 
     /// Give the size of the shortest node prefix that determines
     /// the revision uniquely.
     ///
     /// From a binary node prefix, if it is matched in the node map, this
     /// returns the number of hexadecimal digits that would had sufficed
     /// to find the revision uniquely.
     ///
     /// Returns `None` if no `Revision` could be found for the prefix.
     ///
     /// If several Revisions match the given prefix, a [`MultipleResults`]
     /// error is returned.
     fn unique_prefix_len_bin<'a>(
         &self,
         idx: &impl RevlogIndex,
         node_prefix: NodePrefixRef<'a>,
     ) -> Result<Option<usize>, NodeMapError>;
 
     /// Same as `unique_prefix_len_bin`, with the hexadecimal representation
     /// of the prefix as input.
     fn unique_prefix_len_hex(
         &self,
         idx: &impl RevlogIndex,
         prefix: &str,
     ) -> Result<Option<usize>, NodeMapError> {
         self.unique_prefix_len_bin(idx, NodePrefix::from_hex(prefix)?.borrow())
     }
 
     /// Same as `unique_prefix_len_bin`, with a full `Node` as input
     fn unique_prefix_len_node(
         &self,
         idx: &impl RevlogIndex,
         node: &Node,
     ) -> Result<Option<usize>, NodeMapError> {
         self.unique_prefix_len_bin(idx, node.into())
     }
 }
 
 pub trait MutableNodeMap: NodeMap {
     fn insert<I: RevlogIndex>(
         &mut self,
         index: &I,
         node: &Node,
         rev: Revision,
     ) -> Result<(), NodeMapError>;
 }
 
 /// Low level NodeTree [`Blocks`] elements
 ///
 /// These are exactly as for instance on persistent storage.
-type RawElement = i32;
+type RawElement = unaligned::I32Be;
 
 /// High level representation of values in NodeTree
 /// [`Blocks`](struct.Block.html)
 ///
 /// This is the high level representation that most algorithms should
 /// use.
 #[derive(Clone, Debug, Eq, PartialEq)]
 enum Element {
     Rev(Revision),
     Block(usize),
     None,
 }
 
 impl From<RawElement> for Element {
     /// Conversion from low level representation, after endianness conversion.
     ///
     /// See [`Block`](struct.Block.html) for explanation about the encoding.
     fn from(raw: RawElement) -> Element {
-        if raw >= 0 {
-            Element::Block(raw as usize)
-        } else if raw == -1 {
+        let int = raw.get();
+        if int >= 0 {
+            Element::Block(int as usize)
+        } else if int == -1 {
             Element::None
         } else {
-            Element::Rev(-raw - 2)
+            Element::Rev(-int - 2)
         }
     }
 }
 
 impl From<Element> for RawElement {
     fn from(element: Element) -> RawElement {
-        match element {
+        RawElement::from(match element {
             Element::None => 0,
-            Element::Block(i) => i as RawElement,
+            Element::Block(i) => i as i32,
             Element::Rev(rev) => -rev - 2,
-        }
+        })
     }
 }
 
 /// A logical block of the `NodeTree`, packed with a fixed size.
 ///
 /// These are always used in container types implementing `Index<Block>`,
 /// such as `&Block`
 ///
 /// As an array of integers, its ith element encodes that the
 /// ith potential edge from the block, representing the ith hexadecimal digit
 /// (nybble) `i` is either:
 ///
 /// - absent (value -1)
 /// - another `Block` in the same indexable container (value ≥ 0)
 ///  - a `Revision` leaf (value ≤ -2)
 ///
 /// Endianness has to be fixed for consistency on shared storage across
 /// different architectures.
 ///
 /// A key difference with the C `nodetree` is that we need to be
 /// able to represent the [`Block`] at index 0, hence -1 is the empty marker
 /// rather than 0 and the `Revision` range upper limit of -2 instead of -1.
 ///
 /// Another related difference is that `NULL_REVISION` (-1) is not
 /// represented at all, because we want an immutable empty nodetree
 /// to be valid.
 
-#[derive(Copy, Clone)]
-pub struct Block([u8; BLOCK_SIZE]);
+const ELEMENTS_PER_BLOCK: usize = 16; // number of different values in a nybble
 
-/// Not derivable for arrays of length >32 until const generics are stable
-impl PartialEq for Block {
-    fn eq(&self, other: &Self) -> bool {
-        self.0[..] == other.0[..]
-    }
-}
-
-pub const BLOCK_SIZE: usize = 64;
+#[derive(Copy, Clone, BytesCast, PartialEq)]
+#[repr(transparent)]
+pub struct Block([RawElement; ELEMENTS_PER_BLOCK]);
 
 impl Block {
     fn new() -> Self {
-        // -1 in 2's complement to create an absent node
-        let byte: u8 = 255;
-        Block([byte; BLOCK_SIZE])
+        let absent_node = RawElement::from(-1);
+        Block([absent_node; ELEMENTS_PER_BLOCK])
     }
 
     fn get(&self, nybble: u8) -> Element {
-        let index = nybble as usize * mem::size_of::<RawElement>();
-        Element::from(RawElement::from_be_bytes([
-            self.0[index],
-            self.0[index + 1],
-            self.0[index + 2],
-            self.0[index + 3],
-        ]))
+        self.0[nybble as usize].into()
     }
 
     fn set(&mut self, nybble: u8, element: Element) {
-        let values = RawElement::to_be_bytes(element.into());
-        let index = nybble as usize * mem::size_of::<RawElement>();
-        self.0[index] = values[0];
-        self.0[index + 1] = values[1];
-        self.0[index + 2] = values[2];
-        self.0[index + 3] = values[3];
+        self.0[nybble as usize] = element.into()
     }
 }
 
 impl fmt::Debug for Block {
     /// sparse representation for testing and debugging purposes
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_map()
             .entries((0..16).filter_map(|i| match self.get(i) {
                 Element::None => None,
                 element => Some((i, element)),
             }))
             .finish()
     }
 }
 
 /// A mutable 16-radix tree with the root block logically at the end
 ///
 /// Because of the append only nature of our node trees, we need to
 /// keep the original untouched and store new blocks separately.
 ///
 /// The mutable root `Block` is kept apart so that we don't have to rebump
 /// it on each insertion.
 pub struct NodeTree {
     readonly: Box<dyn Deref<Target = [Block]> + Send>,
     growable: Vec<Block>,
     root: Block,
     masked_inner_blocks: usize,
 }
 
 impl Index<usize> for NodeTree {
     type Output = Block;
 
     fn index(&self, i: usize) -> &Block {
         let ro_len = self.readonly.len();
         if i < ro_len {
             &self.readonly[i]
         } else if i == ro_len + self.growable.len() {
             &self.root
         } else {
             &self.growable[i - ro_len]
         }
     }
 }
 
 /// Return `None` unless the `Node` for `rev` has given prefix in `index`.
 fn has_prefix_or_none(
     idx: &impl RevlogIndex,
     prefix: NodePrefixRef,
     rev: Revision,
 ) -> Result<Option<Revision>, NodeMapError> {
     idx.node(rev)
         .ok_or_else(|| NodeMapError::RevisionNotInIndex(rev))
         .map(|node| {
             if prefix.is_prefix_of(node) {
                 Some(rev)
             } else {
                 None
             }
         })
 }
 
 /// validate that the candidate's node starts indeed with given prefix,
 /// and treat ambiguities related to `NULL_REVISION`.
 ///
 /// From the data in the NodeTree, one can only conclude that some
 /// revision is the only one for a *subprefix* of the one being looked up.
 fn validate_candidate(
     idx: &impl RevlogIndex,
     prefix: NodePrefixRef,
     candidate: (Option<Revision>, usize),
 ) -> Result<(Option<Revision>, usize), NodeMapError> {
     let (rev, steps) = candidate;
     if let Some(nz_nybble) = prefix.first_different_nybble(&NULL_NODE) {
         rev.map_or(Ok((None, steps)), |r| {
             has_prefix_or_none(idx, prefix, r)
                 .map(|opt| (opt, max(steps, nz_nybble + 1)))
         })
     } else {
         // the prefix is only made of zeros; NULL_REVISION always matches it
         // and any other *valid* result is an ambiguity
         match rev {
             None => Ok((Some(NULL_REVISION), steps + 1)),
             Some(r) => match has_prefix_or_none(idx, prefix, r)? {
                 None => Ok((Some(NULL_REVISION), steps + 1)),
                 _ => Err(NodeMapError::MultipleResults),
             },
         }
     }
 }
 
 impl NodeTree {
     /// Initiate a NodeTree from an immutable slice-like of `Block`
     ///
     /// We keep `readonly` and clone its root block if it isn't empty.
     fn new(readonly: Box<dyn Deref<Target = [Block]> + Send>) -> Self {
         let root = readonly.last().cloned().unwrap_or_else(Block::new);
         NodeTree {
             readonly,
             growable: Vec::new(),
             root,
             masked_inner_blocks: 0,
         }
     }
 
     /// Create from an opaque bunch of bytes
     ///
     /// The created `NodeTreeBytes` from `buffer`,
     /// of which exactly `amount` bytes are used.
     ///
     /// - `buffer` could be derived from `PyBuffer` and `Mmap` objects.
     /// - `offset` allows for the final file format to include fixed data
     ///   (generation number, behavioural flags)
     /// - `amount` is expressed in bytes, and is not automatically derived from
     ///   `bytes`, so that a caller that manages them atomically can perform
     ///   temporary disk serializations and still rollback easily if needed.
     ///   First use-case for this would be to support Mercurial shell hooks.
     ///
     /// panics if `buffer` is smaller than `amount`
     pub fn load_bytes(
         bytes: Box<dyn Deref<Target = [u8]> + Send>,
         amount: usize,
     ) -> Self {
         NodeTree::new(Box::new(NodeTreeBytes::new(bytes, amount)))
     }
 
     /// Retrieve added `Block` and the original immutable data
     pub fn into_readonly_and_added(
         self,
     ) -> (Box<dyn Deref<Target = [Block]> + Send>, Vec<Block>) {
         let mut vec = self.growable;
         let readonly = self.readonly;
         if readonly.last() != Some(&self.root) {
             vec.push(self.root);
         }
         (readonly, vec)
     }
 
     /// Retrieve added `Blocks` as bytes, ready to be written to persistent
     /// storage
     pub fn into_readonly_and_added_bytes(
         self,
     ) -> (Box<dyn Deref<Target = [Block]> + Send>, Vec<u8>) {
         let (readonly, vec) = self.into_readonly_and_added();
         // Prevent running `v`'s destructor so we are in complete control
         // of the allocation.
         let vec = mem::ManuallyDrop::new(vec);
 
         // Transmute the `Vec<Block>` to a `Vec<u8>`. Blocks are contiguous
         // bytes, so this is perfectly safe.
         let bytes = unsafe {
-            // Assert that `Block` hasn't been changed and has no padding
-            let _: [u8; 4 * BLOCK_SIZE] =
-                std::mem::transmute([Block::new(); 4]);
+            // Check for compatible allocation layout.
+            // (Optimized away by constant-folding + dead code elimination.)
+            assert_eq!(size_of::<Block>(), 64);
+            assert_eq!(align_of::<Block>(), 1);
 
             // /!\ Any use of `vec` after this is use-after-free.
             // TODO: use `into_raw_parts` once stabilized
             Vec::from_raw_parts(
                 vec.as_ptr() as *mut u8,
-                vec.len() * BLOCK_SIZE,
-                vec.capacity() * BLOCK_SIZE,
+                vec.len() * size_of::<Block>(),
+                vec.capacity() * size_of::<Block>(),
             )
         };
         (readonly, bytes)
     }
 
     /// Total number of blocks
     fn len(&self) -> usize {
         self.readonly.len() + self.growable.len() + 1
     }
 
     /// Implemented for completeness
     ///
     /// A `NodeTree` always has at least the mutable root block.
     #[allow(dead_code)]
     fn is_empty(&self) -> bool {
         false
     }
 
     /// Main working method for `NodeTree` searches
     ///
     /// The first returned value is the result of analysing `NodeTree` data
     /// *alone*: whereas `None` guarantees that the given prefix is absent
     /// from the `NodeTree` data (but still could match `NULL_NODE`), with
     /// `Some(rev)`, it is to be understood that `rev` is the unique `Revision`
     /// that could match the prefix. Actually, all that can be inferred from
     /// the `NodeTree` data is that `rev` is the revision with the longest
     /// common node prefix with the given prefix.
     ///
     /// The second returned value is the size of the smallest subprefix
     /// of `prefix` that would give the same result, i.e. not the
     /// `MultipleResults` error variant (again, using only the data of the
     /// `NodeTree`).
     fn lookup(
         &self,
         prefix: NodePrefixRef,
     ) -> Result<(Option<Revision>, usize), NodeMapError> {
         for (i, visit_item) in self.visit(prefix).enumerate() {
             if let Some(opt) = visit_item.final_revision() {
                 return Ok((opt, i + 1));
             }
         }
         Err(NodeMapError::MultipleResults)
     }
 
     fn visit<'n, 'p>(
         &'n self,
         prefix: NodePrefixRef<'p>,
     ) -> NodeTreeVisitor<'n, 'p> {
         NodeTreeVisitor {
             nt: self,
             prefix,
             visit: self.len() - 1,
             nybble_idx: 0,
             done: false,
         }
     }
     /// Return a mutable reference for `Block` at index `idx`.
     ///
     /// If `idx` lies in the immutable area, then the reference is to
     /// a newly appended copy.
     ///
     /// Returns (new_idx, glen, mut_ref) where
     ///
     /// - `new_idx` is the index of the mutable `Block`
     /// - `mut_ref` is a mutable reference to the mutable Block.
     /// - `glen` is the new length of `self.growable`
     ///
     /// Note: the caller wouldn't be allowed to query `self.growable.len()`
     /// itself because of the mutable borrow taken with the returned `Block`
     fn mutable_block(&mut self, idx: usize) -> (usize, &mut Block, usize) {
         let ro_blocks = &self.readonly;
         let ro_len = ro_blocks.len();
         let glen = self.growable.len();
         if idx < ro_len {
             self.masked_inner_blocks += 1;
             self.growable.push(ro_blocks[idx]);
             (glen + ro_len, &mut self.growable[glen], glen + 1)
         } else if glen + ro_len == idx {
             (idx, &mut self.root, glen)
         } else {
             (idx, &mut self.growable[idx - ro_len], glen)
         }
     }
 
     /// Main insertion method
     ///
     /// This will dive in the node tree to find the deepest `Block` for
     /// `node`, split it as much as needed and record `node` in there.
     /// The method then backtracks, updating references in all the visited
     /// blocks from the root.
     ///
     /// All the mutated `Block` are copied first to the growable part if
     /// needed. That happens for those in the immutable part except the root.
     pub fn insert<I: RevlogIndex>(
         &mut self,
         index: &I,
         node: &Node,
         rev: Revision,
     ) -> Result<(), NodeMapError> {
         let ro_len = &self.readonly.len();
 
         let mut visit_steps: Vec<_> = self.visit(node.into()).collect();
         let read_nybbles = visit_steps.len();
         // visit_steps cannot be empty, since we always visit the root block
         let deepest = visit_steps.pop().unwrap();
 
         let (mut block_idx, mut block, mut glen) =
             self.mutable_block(deepest.block_idx);
 
         if let Element::Rev(old_rev) = deepest.element {
             let old_node = index
                 .node(old_rev)
                 .ok_or_else(|| NodeMapError::RevisionNotInIndex(old_rev))?;
             if old_node == node {
                 return Ok(()); // avoid creating lots of useless blocks
             }
 
             // Looping over the tail of nybbles in both nodes, creating
             // new blocks until we find the difference
             let mut new_block_idx = ro_len + glen;
             let mut nybble = deepest.nybble;
             for nybble_pos in read_nybbles..node.nybbles_len() {
                 block.set(nybble, Element::Block(new_block_idx));
 
                 let new_nybble = node.get_nybble(nybble_pos);
                 let old_nybble = old_node.get_nybble(nybble_pos);
 
                 if old_nybble == new_nybble {
                     self.growable.push(Block::new());
                     block = &mut self.growable[glen];
                     glen += 1;
                     new_block_idx += 1;
                     nybble = new_nybble;
                 } else {
                     let mut new_block = Block::new();
                     new_block.set(old_nybble, Element::Rev(old_rev));
                     new_block.set(new_nybble, Element::Rev(rev));
                     self.growable.push(new_block);
                     break;
                 }
             }
         } else {
             // Free slot in the deepest block: no splitting has to be done
             block.set(deepest.nybble, Element::Rev(rev));
         }
 
         // Backtrack over visit steps to update references
         while let Some(visited) = visit_steps.pop() {
             let to_write = Element::Block(block_idx);
             if visit_steps.is_empty() {
                 self.root.set(visited.nybble, to_write);
                 break;
             }
             let (new_idx, block, _) = self.mutable_block(visited.block_idx);
             if block.get(visited.nybble) == to_write {
                 break;
             }
             block.set(visited.nybble, to_write);
             block_idx = new_idx;
         }
         Ok(())
     }
 
     /// Make the whole `NodeTree` logically empty, without touching the
     /// immutable part.
     pub fn invalidate_all(&mut self) {
         self.root = Block::new();
         self.growable = Vec::new();
         self.masked_inner_blocks = self.readonly.len();
     }
 
     /// Return the number of blocks in the readonly part that are currently
     /// masked in the mutable part.
     ///
     /// The `NodeTree` structure has no efficient way to know how many blocks
     /// are already unreachable in the readonly part.
     ///
     /// After a call to `invalidate_all()`, the returned number can be actually
     /// bigger than the whole readonly part, a conventional way to mean that
     /// all the readonly blocks have been masked. This is what is really
     /// useful to the caller and does not require to know how many were
     /// actually unreachable to begin with.
     pub fn masked_readonly_blocks(&self) -> usize {
         if let Some(readonly_root) = self.readonly.last() {
             if readonly_root == &self.root {
                 return 0;
             }
         } else {
             return 0;
         }
         self.masked_inner_blocks + 1
     }
 }
 
 pub struct NodeTreeBytes {
     buffer: Box<dyn Deref<Target = [u8]> + Send>,
     len_in_blocks: usize,
 }
 
 impl NodeTreeBytes {
     fn new(
         buffer: Box<dyn Deref<Target = [u8]> + Send>,
         amount: usize,
     ) -> Self {
         assert!(buffer.len() >= amount);
-        let len_in_blocks = amount / BLOCK_SIZE;
+        let len_in_blocks = amount / size_of::<Block>();
         NodeTreeBytes {
             buffer,
             len_in_blocks,
         }
     }
 }
 
 impl Deref for NodeTreeBytes {
     type Target = [Block];
 
     fn deref(&self) -> &[Block] {
-        unsafe {
-            slice::from_raw_parts(
-                (&self.buffer).as_ptr() as *const Block,
-                self.len_in_blocks,
-            )
-        }
+        Block::slice_from_bytes(&self.buffer, self.len_in_blocks)
+            // `NodeTreeBytes::new` already asserted that `self.buffer` is
+            // large enough.
+            .unwrap()
+            .0
     }
 }
 
 struct NodeTreeVisitor<'n, 'p> {
     nt: &'n NodeTree,
     prefix: NodePrefixRef<'p>,
     visit: usize,
     nybble_idx: usize,
     done: bool,
 }
 
 #[derive(Debug, PartialEq, Clone)]
 struct NodeTreeVisitItem {
     block_idx: usize,
     nybble: u8,
     element: Element,
 }
 
 impl<'n, 'p> Iterator for NodeTreeVisitor<'n, 'p> {
     type Item = NodeTreeVisitItem;
 
     fn next(&mut self) -> Option<Self::Item> {
         if self.done || self.nybble_idx >= self.prefix.len() {
             return None;
         }
 
         let nybble = self.prefix.get_nybble(self.nybble_idx);
         self.nybble_idx += 1;
 
         let visit = self.visit;
         let element = self.nt[visit].get(nybble);
         if let Element::Block(idx) = element {
             self.visit = idx;
         } else {
             self.done = true;
         }
 
         Some(NodeTreeVisitItem {
             block_idx: visit,
             nybble,
             element,
         })
     }
 }
 
 impl NodeTreeVisitItem {
     // Return `Some(opt)` if this item is final, with `opt` being the
     // `Revision` that it may represent.
     //
     // If the item is not terminal, return `None`
     fn final_revision(&self) -> Option<Option<Revision>> {
         match self.element {
             Element::Block(_) => None,
             Element::Rev(r) => Some(Some(r)),
             Element::None => Some(None),
         }
     }
 }
 
 impl From<Vec<Block>> for NodeTree {
     fn from(vec: Vec<Block>) -> Self {
         Self::new(Box::new(vec))
     }
 }
 
 impl fmt::Debug for NodeTree {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let readonly: &[Block] = &*self.readonly;
         write!(
             f,
             "readonly: {:?}, growable: {:?}, root: {:?}",
             readonly, self.growable, self.root
         )
     }
 }
 
 impl Default for NodeTree {
     /// Create a fully mutable empty NodeTree
     fn default() -> Self {
         NodeTree::new(Box::new(Vec::new()))
     }
 }
 
 impl NodeMap for NodeTree {
     fn find_bin<'a>(
         &self,
         idx: &impl RevlogIndex,
         prefix: NodePrefixRef<'a>,
     ) -> Result<Option<Revision>, NodeMapError> {
         validate_candidate(idx, prefix.clone(), self.lookup(prefix)?)
             .map(|(opt, _shortest)| opt)
     }
 
     fn unique_prefix_len_bin<'a>(
         &self,
         idx: &impl RevlogIndex,
         prefix: NodePrefixRef<'a>,
     ) -> Result<Option<usize>, NodeMapError> {
         validate_candidate(idx, prefix.clone(), self.lookup(prefix)?)
             .map(|(opt, shortest)| opt.map(|_rev| shortest))
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::NodeMapError::*;
     use super::*;
     use crate::revlog::node::{hex_pad_right, Node};
     use std::collections::HashMap;
 
     /// Creates a `Block` using a syntax close to the `Debug` output
     macro_rules! block {
         {$($nybble:tt : $variant:ident($val:tt)),*} => (
             {
                 let mut block = Block::new();
                 $(block.set($nybble, Element::$variant($val)));*;
                 block
             }
         )
     }
 
     #[test]
     fn test_block_debug() {
         let mut block = Block::new();
         block.set(1, Element::Rev(3));
         block.set(10, Element::Block(0));
         assert_eq!(format!("{:?}", block), "{1: Rev(3), 10: Block(0)}");
     }
 
     #[test]
     fn test_block_macro() {
         let block = block! {5: Block(2)};
         assert_eq!(format!("{:?}", block), "{5: Block(2)}");
 
         let block = block! {13: Rev(15), 5: Block(2)};
         assert_eq!(format!("{:?}", block), "{5: Block(2), 13: Rev(15)}");
     }
 
     #[test]
     fn test_raw_block() {
         let mut raw = [255u8; 64];
 
         let mut counter = 0;
-        for val in [0, 15, -2, -1, -3].iter() {
-            for byte in RawElement::to_be_bytes(*val).iter() {
+        for val in [0_i32, 15, -2, -1, -3].iter() {
+            for byte in val.to_be_bytes().iter() {
                 raw[counter] = *byte;
                 counter += 1;
             }
         }
-        let block = Block(raw);
+        let (block, _) = Block::from_bytes(&raw).unwrap();
         assert_eq!(block.get(0), Element::Block(0));
         assert_eq!(block.get(1), Element::Block(15));
         assert_eq!(block.get(3), Element::None);
         assert_eq!(block.get(2), Element::Rev(0));
         assert_eq!(block.get(4), Element::Rev(1));
     }
 
     type TestIndex = HashMap<Revision, Node>;
 
     impl RevlogIndex for TestIndex {
         fn node(&self, rev: Revision) -> Option<&Node> {
             self.get(&rev)
         }
 
         fn len(&self) -> usize {
             self.len()
         }
     }
 
     /// Pad hexadecimal Node prefix with zeros on the right
     ///
     /// This avoids having to repeatedly write very long hexadecimal
     /// strings for test data, and brings actual hash size independency.
     #[cfg(test)]
     fn pad_node(hex: &str) -> Node {
         Node::from_hex(&hex_pad_right(hex)).unwrap()
     }
 
     /// Pad hexadecimal Node prefix with zeros on the right, then insert
     fn pad_insert(idx: &mut TestIndex, rev: Revision, hex: &str) {
         idx.insert(rev, pad_node(hex));
     }
 
     fn sample_nodetree() -> NodeTree {
         NodeTree::from(vec![
             block![0: Rev(9)],
             block![0: Rev(0), 1: Rev(9)],
             block![0: Block(1), 1:Rev(1)],
         ])
     }
 
     #[test]
     fn test_nt_debug() {
         let nt = sample_nodetree();
         assert_eq!(
             format!("{:?}", nt),
             "readonly: \
              [{0: Rev(9)}, {0: Rev(0), 1: Rev(9)}, {0: Block(1), 1: Rev(1)}], \
              growable: [], \
              root: {0: Block(1), 1: Rev(1)}",
         );
     }
 
     #[test]
     fn test_immutable_find_simplest() -> Result<(), NodeMapError> {
         let mut idx: TestIndex = HashMap::new();
         pad_insert(&mut idx, 1, "1234deadcafe");
 
         let nt = NodeTree::from(vec![block! {1: Rev(1)}]);
         assert_eq!(nt.find_hex(&idx, "1")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "12")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "1234de")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "1a")?, None);
         assert_eq!(nt.find_hex(&idx, "ab")?, None);
 
         // and with full binary Nodes
         assert_eq!(nt.find_node(&idx, idx.get(&1).unwrap())?, Some(1));
         let unknown = Node::from_hex(&hex_pad_right("3d")).unwrap();
         assert_eq!(nt.find_node(&idx, &unknown)?, None);
         Ok(())
     }
 
     #[test]
     fn test_immutable_find_one_jump() {
         let mut idx = TestIndex::new();
         pad_insert(&mut idx, 9, "012");
         pad_insert(&mut idx, 0, "00a");
 
         let nt = sample_nodetree();
 
         assert_eq!(nt.find_hex(&idx, "0"), Err(MultipleResults));
         assert_eq!(nt.find_hex(&idx, "01"), Ok(Some(9)));
         assert_eq!(nt.find_hex(&idx, "00"), Err(MultipleResults));
         assert_eq!(nt.find_hex(&idx, "00a"), Ok(Some(0)));
         assert_eq!(nt.unique_prefix_len_hex(&idx, "00a"), Ok(Some(3)));
         assert_eq!(nt.find_hex(&idx, "000"), Ok(Some(NULL_REVISION)));
     }
 
     #[test]
     fn test_mutated_find() -> Result<(), NodeMapError> {
         let mut idx = TestIndex::new();
         pad_insert(&mut idx, 9, "012");
         pad_insert(&mut idx, 0, "00a");
         pad_insert(&mut idx, 2, "cafe");
         pad_insert(&mut idx, 3, "15");
         pad_insert(&mut idx, 1, "10");
 
         let nt = NodeTree {
             readonly: sample_nodetree().readonly,
             growable: vec![block![0: Rev(1), 5: Rev(3)]],
             root: block![0: Block(1), 1:Block(3), 12: Rev(2)],
             masked_inner_blocks: 1,
         };
         assert_eq!(nt.find_hex(&idx, "10")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "c")?, Some(2));
         assert_eq!(nt.unique_prefix_len_hex(&idx, "c")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "00"), Err(MultipleResults));
         assert_eq!(nt.find_hex(&idx, "000")?, Some(NULL_REVISION));
         assert_eq!(nt.unique_prefix_len_hex(&idx, "000")?, Some(3));
         assert_eq!(nt.find_hex(&idx, "01")?, Some(9));
         assert_eq!(nt.masked_readonly_blocks(), 2);
         Ok(())
     }
 
     struct TestNtIndex {
         index: TestIndex,
         nt: NodeTree,
     }
 
     impl TestNtIndex {
         fn new() -> Self {
             TestNtIndex {
                 index: HashMap::new(),
                 nt: NodeTree::default(),
             }
         }
 
         fn insert(
             &mut self,
             rev: Revision,
             hex: &str,
         ) -> Result<(), NodeMapError> {
             let node = pad_node(hex);
             self.index.insert(rev, node.clone());
             self.nt.insert(&self.index, &node, rev)?;
             Ok(())
         }
 
         fn find_hex(
             &self,
             prefix: &str,
         ) -> Result<Option<Revision>, NodeMapError> {
             self.nt.find_hex(&self.index, prefix)
         }
 
         fn unique_prefix_len_hex(
             &self,
             prefix: &str,
         ) -> Result<Option<usize>, NodeMapError> {
             self.nt.unique_prefix_len_hex(&self.index, prefix)
         }
 
         /// Drain `added` and restart a new one
         fn commit(self) -> Self {
             let mut as_vec: Vec<Block> =
                 self.nt.readonly.iter().map(|block| block.clone()).collect();
             as_vec.extend(self.nt.growable);
             as_vec.push(self.nt.root);
 
             Self {
                 index: self.index,
                 nt: NodeTree::from(as_vec).into(),
             }
         }
     }
 
     #[test]
     fn test_insert_full_mutable() -> Result<(), NodeMapError> {
         let mut idx = TestNtIndex::new();
         idx.insert(0, "1234")?;
         assert_eq!(idx.find_hex("1")?, Some(0));
         assert_eq!(idx.find_hex("12")?, Some(0));
 
         // let's trigger a simple split
         idx.insert(1, "1a34")?;
         assert_eq!(idx.nt.growable.len(), 1);
         assert_eq!(idx.find_hex("12")?, Some(0));
         assert_eq!(idx.find_hex("1a")?, Some(1));
 
         // reinserting is a no_op
         idx.insert(1, "1a34")?;
         assert_eq!(idx.nt.growable.len(), 1);
         assert_eq!(idx.find_hex("12")?, Some(0));
         assert_eq!(idx.find_hex("1a")?, Some(1));
 
         idx.insert(2, "1a01")?;
         assert_eq!(idx.nt.growable.len(), 2);
         assert_eq!(idx.find_hex("1a"), Err(NodeMapError::MultipleResults));
         assert_eq!(idx.find_hex("12")?, Some(0));
         assert_eq!(idx.find_hex("1a3")?, Some(1));
         assert_eq!(idx.find_hex("1a0")?, Some(2));
         assert_eq!(idx.find_hex("1a12")?, None);
 
         // now let's make it split and create more than one additional block
         idx.insert(3, "1a345")?;
         assert_eq!(idx.nt.growable.len(), 4);
         assert_eq!(idx.find_hex("1a340")?, Some(1));
         assert_eq!(idx.find_hex("1a345")?, Some(3));
         assert_eq!(idx.find_hex("1a341")?, None);
 
         // there's no readonly block to mask
         assert_eq!(idx.nt.masked_readonly_blocks(), 0);
         Ok(())
     }
 
     #[test]
     fn test_unique_prefix_len_zero_prefix() {
         let mut idx = TestNtIndex::new();
         idx.insert(0, "00000abcd").unwrap();
 
         assert_eq!(idx.find_hex("000"), Err(NodeMapError::MultipleResults));
         // in the nodetree proper, this will be found at the first nybble
         // yet the correct answer for unique_prefix_len is not 1, nor 1+1,
         // but the first difference with `NULL_NODE`
         assert_eq!(idx.unique_prefix_len_hex("00000a"), Ok(Some(6)));
         assert_eq!(idx.unique_prefix_len_hex("00000ab"), Ok(Some(6)));
 
         // same with odd result
         idx.insert(1, "00123").unwrap();
         assert_eq!(idx.unique_prefix_len_hex("001"), Ok(Some(3)));
         assert_eq!(idx.unique_prefix_len_hex("0012"), Ok(Some(3)));
 
         // these are unchanged of course
         assert_eq!(idx.unique_prefix_len_hex("00000a"), Ok(Some(6)));
         assert_eq!(idx.unique_prefix_len_hex("00000ab"), Ok(Some(6)));
     }
 
     #[test]
     fn test_insert_extreme_splitting() -> Result<(), NodeMapError> {
         // check that the splitting loop is long enough
         let mut nt_idx = TestNtIndex::new();
         let nt = &mut nt_idx.nt;
         let idx = &mut nt_idx.index;
 
         let node0_hex = hex_pad_right("444444");
         let mut node1_hex = hex_pad_right("444444").clone();
         node1_hex.pop();
         node1_hex.push('5');
         let node0 = Node::from_hex(&node0_hex).unwrap();
         let node1 = Node::from_hex(&node1_hex).unwrap();
 
         idx.insert(0, node0.clone());
         nt.insert(idx, &node0, 0)?;
         idx.insert(1, node1.clone());
         nt.insert(idx, &node1, 1)?;
 
         assert_eq!(nt.find_bin(idx, (&node0).into())?, Some(0));
         assert_eq!(nt.find_bin(idx, (&node1).into())?, Some(1));
         Ok(())
     }
 
     #[test]
     fn test_insert_partly_immutable() -> Result<(), NodeMapError> {
         let mut idx = TestNtIndex::new();
         idx.insert(0, "1234")?;
         idx.insert(1, "1235")?;
         idx.insert(2, "131")?;
         idx.insert(3, "cafe")?;
         let mut idx = idx.commit();
         assert_eq!(idx.find_hex("1234")?, Some(0));
         assert_eq!(idx.find_hex("1235")?, Some(1));
         assert_eq!(idx.find_hex("131")?, Some(2));
         assert_eq!(idx.find_hex("cafe")?, Some(3));
         // we did not add anything since init from readonly
         assert_eq!(idx.nt.masked_readonly_blocks(), 0);
 
         idx.insert(4, "123A")?;
         assert_eq!(idx.find_hex("1234")?, Some(0));
         assert_eq!(idx.find_hex("1235")?, Some(1));
         assert_eq!(idx.find_hex("131")?, Some(2));
         assert_eq!(idx.find_hex("cafe")?, Some(3));
         assert_eq!(idx.find_hex("123A")?, Some(4));
         // we masked blocks for all prefixes of "123", including the root
         assert_eq!(idx.nt.masked_readonly_blocks(), 4);
 
         eprintln!("{:?}", idx.nt);
         idx.insert(5, "c0")?;
         assert_eq!(idx.find_hex("cafe")?, Some(3));
         assert_eq!(idx.find_hex("c0")?, Some(5));
         assert_eq!(idx.find_hex("c1")?, None);
         assert_eq!(idx.find_hex("1234")?, Some(0));
         // inserting "c0" is just splitting the 'c' slot of the mutable root,
         // it doesn't mask anything
         assert_eq!(idx.nt.masked_readonly_blocks(), 4);
 
         Ok(())
     }
 
     #[test]
     fn test_invalidate_all() -> Result<(), NodeMapError> {
         let mut idx = TestNtIndex::new();
         idx.insert(0, "1234")?;
         idx.insert(1, "1235")?;
         idx.insert(2, "131")?;
         idx.insert(3, "cafe")?;
         let mut idx = idx.commit();
 
         idx.nt.invalidate_all();
 
         assert_eq!(idx.find_hex("1234")?, None);
         assert_eq!(idx.find_hex("1235")?, None);
         assert_eq!(idx.find_hex("131")?, None);
         assert_eq!(idx.find_hex("cafe")?, None);
         // all the readonly blocks have been masked, this is the
         // conventional expected response
         assert_eq!(idx.nt.masked_readonly_blocks(), idx.nt.readonly.len() + 1);
         Ok(())
     }
 
     #[test]
     fn test_into_added_empty() {
         assert!(sample_nodetree().into_readonly_and_added().1.is_empty());
         assert!(sample_nodetree()
             .into_readonly_and_added_bytes()
             .1
             .is_empty());
     }
 
     #[test]
     fn test_into_added_bytes() -> Result<(), NodeMapError> {
         let mut idx = TestNtIndex::new();
         idx.insert(0, "1234")?;
         let mut idx = idx.commit();
         idx.insert(4, "cafe")?;
         let (_, bytes) = idx.nt.into_readonly_and_added_bytes();
 
         // only the root block has been changed
-        assert_eq!(bytes.len(), BLOCK_SIZE);
+        assert_eq!(bytes.len(), size_of::<Block>());
         // big endian for -2
         assert_eq!(&bytes[4..2 * 4], [255, 255, 255, 254]);
         // big endian for -6
         assert_eq!(&bytes[12 * 4..13 * 4], [255, 255, 255, 250]);
         Ok(())
     }
 }

rust/hg-core/src/revlog/nodemap_docket.rs

@@ -1,121 +1,105 b''
+use bytes_cast::{unaligned, BytesCast};
 use memmap::Mmap;
-use std::convert::TryInto;
 use std::path::{Path, PathBuf};
 
 use super::revlog::RevlogError;
 use crate::repo::Repo;
 use crate::utils::strip_suffix;
 
 const ONDISK_VERSION: u8 = 1;
 
 pub(super) struct NodeMapDocket {
     pub data_length: usize,
     // TODO: keep here more of the data from `parse()` when we need it
 }
 
+#[derive(BytesCast)]
+#[repr(C)]
+struct DocketHeader {
+    uid_size: u8,
+    _tip_rev: unaligned::U64Be,
+    data_length: unaligned::U64Be,
+    _data_unused: unaligned::U64Be,
+    tip_node_size: unaligned::U64Be,
+}
+
 impl NodeMapDocket {
     /// Return `Ok(None)` when the caller should proceed without a persistent
     /// nodemap:
     ///
     /// * This revlog does not have a `.n` docket file (it is not generated for
     ///   small revlogs), or
     /// * The docket has an unsupported version number (repositories created by
     ///   later hg, maybe that should be a requirement instead?), or
     /// * The docket file points to a missing (likely deleted) data file (this
     ///   can happen in a rare race condition).
     pub fn read_from_file(
         repo: &Repo,
         index_path: &Path,
     ) -> Result<Option<(Self, Mmap)>, RevlogError> {
         let docket_path = index_path.with_extension("n");
         let docket_bytes = match repo.store_vfs().read(&docket_path) {
             Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
                 return Ok(None)
             }
             Err(e) => return Err(RevlogError::IoError(e)),
             Ok(bytes) => bytes,
         };
 
-        let mut input = if let Some((&ONDISK_VERSION, rest)) =
+        let input = if let Some((&ONDISK_VERSION, rest)) =
             docket_bytes.split_first()
         {
             rest
         } else {
             return Ok(None);
         };
-        let input = &mut input;
 
-        let uid_size = read_u8(input)? as usize;
-        let _tip_rev = read_be_u64(input)?;
+        let (header, rest) = DocketHeader::from_bytes(input)?;
+        let uid_size = header.uid_size as usize;
         // TODO: do we care about overflow for 4 GB+ nodemap files on 32-bit
         // systems?
-        let data_length = read_be_u64(input)? as usize;
-        let _data_unused = read_be_u64(input)?;
-        let tip_node_size = read_be_u64(input)? as usize;
-        let uid = read_bytes(input, uid_size)?;
-        let _tip_node = read_bytes(input, tip_node_size)?;
-
+        let tip_node_size = header.tip_node_size.get() as usize;
+        let data_length = header.data_length.get() as usize;
+        let (uid, rest) = u8::slice_from_bytes(rest, uid_size)?;
+        let (_tip_node, _rest) = u8::slice_from_bytes(rest, tip_node_size)?;
         let uid =
             std::str::from_utf8(uid).map_err(|_| RevlogError::Corrupted)?;
         let docket = NodeMapDocket { data_length };
 
         let data_path = rawdata_path(&docket_path, uid);
         // TODO: use `std::fs::read` here when the `persistent-nodemap.mmap`
         // config is false?
         match repo.store_vfs().mmap_open(&data_path) {
             Ok(mmap) => {
                 if mmap.len() >= data_length {
                     Ok(Some((docket, mmap)))
                 } else {
                     Err(RevlogError::Corrupted)
                 }
             }
             Err(error) => {
                 if error.kind() == std::io::ErrorKind::NotFound {
                     Ok(None)
                 } else {
                     Err(RevlogError::IoError(error))
                 }
             }
         }
     }
 }
 
-fn read_bytes<'a>(
-    input: &mut &'a [u8],
-    count: usize,
-) -> Result<&'a [u8], RevlogError> {
-    if let Some(start) = input.get(..count) {
-        *input = &input[count..];
-        Ok(start)
-    } else {
-        Err(RevlogError::Corrupted)
-    }
-}
-
-fn read_u8<'a>(input: &mut &[u8]) -> Result<u8, RevlogError> {
-    Ok(read_bytes(input, 1)?[0])
-}
-
-fn read_be_u64<'a>(input: &mut &[u8]) -> Result<u64, RevlogError> {
-    let array = read_bytes(input, std::mem::size_of::<u64>())?
-        .try_into()
-        .unwrap();
-    Ok(u64::from_be_bytes(array))
-}
-
 fn rawdata_path(docket_path: &Path, uid: &str) -> PathBuf {
     let docket_name = docket_path
         .file_name()
         .expect("expected a base name")
         .to_str()
         .expect("expected an ASCII file name in the store");
     let prefix = strip_suffix(docket_name, ".n.a")
         .or_else(|| strip_suffix(docket_name, ".n"))
         .expect("expected docket path in .n or .n.a");
     let name = format!("{}-{}.nd", prefix, uid);
     docket_path
         .parent()
         .expect("expected a non-root path")
         .join(name)
 }

rust/hg-core/src/revlog/revlog.rs

@@ -1,376 +1,382 b''
 use std::borrow::Cow;
 use std::io::Read;
 use std::ops::Deref;
 use std::path::Path;
 
 use byteorder::{BigEndian, ByteOrder};
 use crypto::digest::Digest;
 use crypto::sha1::Sha1;
 use flate2::read::ZlibDecoder;
 use micro_timer::timed;
 use zstd;
 
 use super::index::Index;
 use super::node::{NodePrefixRef, NODE_BYTES_LENGTH, NULL_NODE};
 use super::nodemap;
 use super::nodemap::NodeMap;
 use super::nodemap_docket::NodeMapDocket;
 use super::patch;
 use crate::repo::Repo;
 use crate::revlog::Revision;
 
 pub enum RevlogError {
     IoError(std::io::Error),
     UnsuportedVersion(u16),
     InvalidRevision,
     /// Found more than one entry whose ID match the requested prefix
     AmbiguousPrefix,
     Corrupted,
     UnknowDataFormat(u8),
 }
 
+impl From<bytes_cast::FromBytesError> for RevlogError {
+    fn from(_: bytes_cast::FromBytesError) -> Self {
+        RevlogError::Corrupted
+    }
+}
+
 /// Read only implementation of revlog.
 pub struct Revlog {
     /// When index and data are not interleaved: bytes of the revlog index.
     /// When index and data are interleaved: bytes of the revlog index and
     /// data.
     index: Index,
     /// When index and data are not interleaved: bytes of the revlog data
     data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>>,
     /// When present on disk: the persistent nodemap for this revlog
     nodemap: Option<nodemap::NodeTree>,
 }
 
 impl Revlog {
     /// Open a revlog index file.
     ///
     /// It will also open the associated data file if index and data are not
     /// interleaved.
     #[timed]
     pub fn open(
         repo: &Repo,
         index_path: impl AsRef<Path>,
         data_path: Option<&Path>,
     ) -> Result<Self, RevlogError> {
         let index_path = index_path.as_ref();
         let index_mmap = repo
             .store_vfs()
             .mmap_open(&index_path)
             .map_err(RevlogError::IoError)?;
 
         let version = get_version(&index_mmap);
         if version != 1 {
             return Err(RevlogError::UnsuportedVersion(version));
         }
 
         let index = Index::new(Box::new(index_mmap))?;
 
         let default_data_path = index_path.with_extension("d");
 
         // type annotation required
         // won't recognize Mmap as Deref<Target = [u8]>
         let data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>> =
             if index.is_inline() {
                 None
             } else {
                 let data_path = data_path.unwrap_or(&default_data_path);
                 let data_mmap = repo
                     .store_vfs()
                     .mmap_open(data_path)
                     .map_err(RevlogError::IoError)?;
                 Some(Box::new(data_mmap))
             };
 
         let nodemap = NodeMapDocket::read_from_file(repo, index_path)?.map(
             |(docket, data)| {
                 nodemap::NodeTree::load_bytes(
                     Box::new(data),
                     docket.data_length,
                 )
             },
         );
 
         Ok(Revlog {
             index,
             data_bytes,
             nodemap,
         })
     }
 
     /// Return number of entries of the `Revlog`.
     pub fn len(&self) -> usize {
         self.index.len()
     }
 
     /// Returns `true` if the `Revlog` has zero `entries`.
     pub fn is_empty(&self) -> bool {
         self.index.is_empty()
     }
 
     /// Return the full data associated to a node.
     #[timed]
     pub fn get_node_rev(
         &self,
         node: NodePrefixRef,
     ) -> Result<Revision, RevlogError> {
         if let Some(nodemap) = &self.nodemap {
             return nodemap
                 .find_bin(&self.index, node)
                 // TODO: propagate details of this error:
                 .map_err(|_| RevlogError::Corrupted)?
                 .ok_or(RevlogError::InvalidRevision);
         }
 
         // Fallback to linear scan when a persistent nodemap is not present.
         // This happens when the persistent-nodemap experimental feature is not
         // enabled, or for small revlogs.
         //
         // TODO: consider building a non-persistent nodemap in memory to
         // optimize these cases.
         let mut found_by_prefix = None;
         for rev in (0..self.len() as Revision).rev() {
             let index_entry =
                 self.index.get_entry(rev).ok_or(RevlogError::Corrupted)?;
             if node == *index_entry.hash() {
                 return Ok(rev);
             }
             if node.is_prefix_of(index_entry.hash()) {
                 if found_by_prefix.is_some() {
                     return Err(RevlogError::AmbiguousPrefix);
                 }
                 found_by_prefix = Some(rev)
             }
         }
         found_by_prefix.ok_or(RevlogError::InvalidRevision)
     }
 
     /// Return the full data associated to a revision.
     ///
     /// All entries required to build the final data out of deltas will be
     /// retrieved as needed, and the deltas will be applied to the inital
     /// snapshot to rebuild the final data.
     #[timed]
     pub fn get_rev_data(&self, rev: Revision) -> Result<Vec<u8>, RevlogError> {
         // Todo return -> Cow
         let mut entry = self.get_entry(rev)?;
         let mut delta_chain = vec![];
         while let Some(base_rev) = entry.base_rev {
             delta_chain.push(entry);
             entry =
                 self.get_entry(base_rev).or(Err(RevlogError::Corrupted))?;
         }
 
         // TODO do not look twice in the index
         let index_entry = self
             .index
             .get_entry(rev)
             .ok_or(RevlogError::InvalidRevision)?;
 
         let data: Vec<u8> = if delta_chain.is_empty() {
             entry.data()?.into()
         } else {
             Revlog::build_data_from_deltas(entry, &delta_chain)?
         };
 
         if self.check_hash(
             index_entry.p1(),
             index_entry.p2(),
             index_entry.hash().as_bytes(),
             &data,
         ) {
             Ok(data)
         } else {
             Err(RevlogError::Corrupted)
         }
     }
 
     /// Check the hash of some given data against the recorded hash.
     pub fn check_hash(
         &self,
         p1: Revision,
         p2: Revision,
         expected: &[u8],
         data: &[u8],
     ) -> bool {
         let e1 = self.index.get_entry(p1);
         let h1 = match e1 {
             Some(ref entry) => entry.hash(),
             None => &NULL_NODE,
         };
         let e2 = self.index.get_entry(p2);
         let h2 = match e2 {
             Some(ref entry) => entry.hash(),
             None => &NULL_NODE,
         };
 
         hash(data, h1.as_bytes(), h2.as_bytes()).as_slice() == expected
     }
 
     /// Build the full data of a revision out its snapshot
     /// and its deltas.
     #[timed]
     fn build_data_from_deltas(
         snapshot: RevlogEntry,
         deltas: &[RevlogEntry],
     ) -> Result<Vec<u8>, RevlogError> {
         let snapshot = snapshot.data()?;
         let deltas = deltas
             .iter()
             .rev()
             .map(RevlogEntry::data)
             .collect::<Result<Vec<Cow<'_, [u8]>>, RevlogError>>()?;
         let patches: Vec<_> =
             deltas.iter().map(|d| patch::PatchList::new(d)).collect();
         let patch = patch::fold_patch_lists(&patches);
         Ok(patch.apply(&snapshot))
     }
 
     /// Return the revlog data.
     fn data(&self) -> &[u8] {
         match self.data_bytes {
             Some(ref data_bytes) => &data_bytes,
             None => panic!(
                 "forgot to load the data or trying to access inline data"
             ),
         }
     }
 
     /// Get an entry of the revlog.
     fn get_entry(&self, rev: Revision) -> Result<RevlogEntry, RevlogError> {
         let index_entry = self
             .index
             .get_entry(rev)
             .ok_or(RevlogError::InvalidRevision)?;
         let start = index_entry.offset();
         let end = start + index_entry.compressed_len();
         let data = if self.index.is_inline() {
             self.index.data(start, end)
         } else {
             &self.data()[start..end]
         };
         let entry = RevlogEntry {
             rev,
             bytes: data,
             compressed_len: index_entry.compressed_len(),
             uncompressed_len: index_entry.uncompressed_len(),
             base_rev: if index_entry.base_revision() == rev {
                 None
             } else {
                 Some(index_entry.base_revision())
             },
         };
         Ok(entry)
     }
 }
 
 /// The revlog entry's bytes and the necessary informations to extract
 /// the entry's data.
 #[derive(Debug)]
 pub struct RevlogEntry<'a> {
     rev: Revision,
     bytes: &'a [u8],
     compressed_len: usize,
     uncompressed_len: usize,
     base_rev: Option<Revision>,
 }
 
 impl<'a> RevlogEntry<'a> {
     /// Extract the data contained in the entry.
     pub fn data(&self) -> Result<Cow<'_, [u8]>, RevlogError> {
         if self.bytes.is_empty() {
             return Ok(Cow::Borrowed(&[]));
         }
         match self.bytes[0] {
             // Revision data is the entirety of the entry, including this
             // header.
             b'\0' => Ok(Cow::Borrowed(self.bytes)),
             // Raw revision data follows.
             b'u' => Ok(Cow::Borrowed(&self.bytes[1..])),
             // zlib (RFC 1950) data.
             b'x' => Ok(Cow::Owned(self.uncompressed_zlib_data()?)),
             // zstd data.
             b'\x28' => Ok(Cow::Owned(self.uncompressed_zstd_data()?)),
             format_type => Err(RevlogError::UnknowDataFormat(format_type)),
         }
     }
 
     fn uncompressed_zlib_data(&self) -> Result<Vec<u8>, RevlogError> {
         let mut decoder = ZlibDecoder::new(self.bytes);
         if self.is_delta() {
             let mut buf = Vec::with_capacity(self.compressed_len);
             decoder
                 .read_to_end(&mut buf)
                 .or(Err(RevlogError::Corrupted))?;
             Ok(buf)
         } else {
             let mut buf = vec![0; self.uncompressed_len];
             decoder
                 .read_exact(&mut buf)
                 .or(Err(RevlogError::Corrupted))?;
             Ok(buf)
         }
     }
 
     fn uncompressed_zstd_data(&self) -> Result<Vec<u8>, RevlogError> {
         if self.is_delta() {
             let mut buf = Vec::with_capacity(self.compressed_len);
             zstd::stream::copy_decode(self.bytes, &mut buf)
                 .or(Err(RevlogError::Corrupted))?;
             Ok(buf)
         } else {
             let mut buf = vec![0; self.uncompressed_len];
             let len = zstd::block::decompress_to_buffer(self.bytes, &mut buf)
                 .or(Err(RevlogError::Corrupted))?;
             if len != self.uncompressed_len {
                 Err(RevlogError::Corrupted)
             } else {
                 Ok(buf)
             }
         }
     }
 
     /// Tell if the entry is a snapshot or a delta
     /// (influences on decompression).
     fn is_delta(&self) -> bool {
         self.base_rev.is_some()
     }
 }
 
 /// Format version of the revlog.
 pub fn get_version(index_bytes: &[u8]) -> u16 {
     BigEndian::read_u16(&index_bytes[2..=3])
 }
 
 /// Calculate the hash of a revision given its data and its parents.
 fn hash(data: &[u8], p1_hash: &[u8], p2_hash: &[u8]) -> Vec<u8> {
     let mut hasher = Sha1::new();
     let (a, b) = (p1_hash, p2_hash);
     if a > b {
         hasher.input(b);
         hasher.input(a);
     } else {
         hasher.input(a);
         hasher.input(b);
     }
     hasher.input(data);
     let mut hash = vec![0; NODE_BYTES_LENGTH];
     hasher.result(&mut hash);
     hash
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
     use super::super::index::IndexEntryBuilder;
 
     #[test]
     fn version_test() {
         let bytes = IndexEntryBuilder::new()
             .is_first(true)
             .with_version(1)
             .build();
 
         assert_eq!(get_version(&bytes), 1)
     }
 }