annotate 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

//! 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 }
}