view rust/hg-core/src/revlog/nodemap_docket.rs @ 49491:c6a1beba27e9

bisect: avoid copying ancestor list for non-merge commits

During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.

To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.

During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:

Setup:
  $ cd mozilla-central
  $ hg bisect --reset
  $ hg bisect --good 0
  $ hg log -r tip -T '{rev}\n'
  628417

Test:
  $ time hg bisect --bad tip --noupdate

Before:
  real    3m35.927s
  user    3m35.553s
  sys     0m0.319s

After:
  real    1m41.142s
  user    1m40.810s
  sys     0m0.285s

use crate::errors::{HgError, HgResultExt};
use bytes_cast::{unaligned, BytesCast};
use memmap2::Mmap;
use std::path::{Path, PathBuf};

use crate::utils::strip_suffix;
use crate::vfs::Vfs;

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(
        store_vfs: &Vfs,
        index_path: &Path,
    ) -> Result<Option<(Self, Mmap)>, HgError> {
        let docket_path = index_path.with_extension("n");
        let docket_bytes = if let Some(bytes) =
            store_vfs.read(&docket_path).io_not_found_as_none()?
        {
            bytes
        } else {
            return Ok(None);
        };

        let input = if let Some((&ONDISK_VERSION, rest)) =
            docket_bytes.split_first()
        {
            rest
        } else {
            return Ok(None);
        };

        /// Treat any error as a parse error
        fn parse<T, E>(result: Result<T, E>) -> Result<T, HgError> {
            result
                .map_err(|_| HgError::corrupted("nodemap docket parse error"))
        }

        let (header, rest) = parse(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 tip_node_size = header.tip_node_size.get() as usize;
        let data_length = header.data_length.get() as usize;
        let (uid, rest) = parse(u8::slice_from_bytes(rest, uid_size))?;
        let (_tip_node, _rest) =
            parse(u8::slice_from_bytes(rest, tip_node_size))?;
        let uid = parse(std::str::from_utf8(uid))?;
        let docket = NodeMapDocket { data_length };

        let data_path = rawdata_path(&docket_path, uid);
        // TODO: use `vfs.read()` here when the `persistent-nodemap.mmap`
        // config is false?
        if let Some(mmap) =
            store_vfs.mmap_open(&data_path).io_not_found_as_none()?
        {
            if mmap.len() >= data_length {
                Ok(Some((docket, mmap)))
            } else {
                Err(HgError::corrupted("persistent nodemap too short"))
            }
        } else {
            // Even if .hg/requires opted in, some revlogs are deemed small
            // enough to not need a persistent nodemap.
            Ok(None)
        }
    }
}

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)
}