Mercurial > hg
view rust/hg-core/src/dirstate_tree/on_disk.rs @ 47283:2a9ddc8094c7
dirstate-v2: Change the on-disk format to be tree-shaped
Nodes are stored not only for tracked files but also for their ancestor
directories. A node has "pointers" (byte count from the start of the file)
to its direct child nodes. Everything can be accessed with zero copy.
Differential Revision: https://phab.mercurial-scm.org/D10722
author | Simon Sapin <simon.sapin@octobus.net> |
---|---|
date | Wed, 19 May 2021 13:15:00 +0200 |
parents | 1766130fe9ba |
children | 73f23e7610f8 |
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//! 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::parsers::clear_ambiguous_mtime; use crate::dirstate::parsers::Timestamp; use crate::dirstate_tree::dirstate_map::{self, DirstateMap}; 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]>; } pub(super) fn read<'on_disk>( on_disk: &'on_disk [u8], ) -> Result<(DirstateMap<'on_disk>, Option<DirstateParents>), DirstateError> { 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 { 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()); } 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> { 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", )) } } pub(super) fn copy_source<'on_disk>( &self, on_disk: &'on_disk [u8], ) -> Result<Option<Cow<'on_disk, HgPath>>, HgError> { 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> { Ok(if self.entry.state != b'\0' { Some(DirstateEntry { state: self.entry.state.try_into()?, 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> { 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> { read_slice::<Node>(on_disk, slice)? .iter() .map(|node| { Ok((node.path(on_disk)?, node.to_in_memory_node(on_disk)?)) }) .collect() } fn read_hg_path(on_disk: &[u8], slice: Slice) -> Result<Cow<HgPath>, HgError> { 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> 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") }) } pub(super) fn write( dirstate_map: &mut DirstateMap, 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 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(&mut dirstate_map.root, now, &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) } /// Serialize the dirstate to the `v2` format after clearing ambigous `mtime`s. fn write_nodes( nodes: &mut dirstate_map::ChildNodes, now: i32, 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 = dirstate_map::Node::sorted(nodes); // First accumulate serialized nodes in a `Vec` let mut on_disk_nodes = Vec::with_capacity(nodes.len()); for (full_path, node) in nodes { on_disk_nodes.push(Node { children: write_nodes(&mut node.children, now, out)?, tracked_descendants_count: node.tracked_descendants_count.into(), full_path: write_slice::<u8>( full_path.full_path().as_bytes(), out, ), base_name_start: u32::try_from(full_path.base_name_start()) // Could only panic for paths over 4 GiB .expect("dirstate-v2 offset overflow") .into(), copy_source: if let Some(source) = &node.copy_source { write_slice::<u8>(source.as_bytes(), out) } else { Slice { start: 0.into(), len: 0.into(), } }, entry: if let Some(entry) = &mut node.entry { clear_ambiguous_mtime(entry, now); 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 } }