view rust/hg-core/src/vfs.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
author Arun Kulshreshtha <akulshreshtha@janestreet.com>
date Tue, 30 Aug 2022 15:29:55 -0400
parents ffd4b1f1c9cb
children be019ac8c1e4 58074252db3c
line wrap: on
line source

use crate::errors::{HgError, IoErrorContext, IoResultExt};
use memmap2::{Mmap, MmapOptions};
use std::io::{ErrorKind, Write};
use std::path::{Path, PathBuf};

/// Filesystem access abstraction for the contents of a given "base" diretory
#[derive(Clone, Copy)]
pub struct Vfs<'a> {
    pub(crate) base: &'a Path,
}

struct FileNotFound(std::io::Error, PathBuf);

impl Vfs<'_> {
    pub fn join(&self, relative_path: impl AsRef<Path>) -> PathBuf {
        self.base.join(relative_path)
    }

    pub fn symlink_metadata(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<std::fs::Metadata, HgError> {
        let path = self.join(relative_path);
        std::fs::symlink_metadata(&path).when_reading_file(&path)
    }

    pub fn read_link(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<PathBuf, HgError> {
        let path = self.join(relative_path);
        std::fs::read_link(&path).when_reading_file(&path)
    }

    pub fn read(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<Vec<u8>, HgError> {
        let path = self.join(relative_path);
        std::fs::read(&path).when_reading_file(&path)
    }

    /// Returns `Ok(None)` if the file does not exist.
    pub fn try_read(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<Option<Vec<u8>>, HgError> {
        match self.read(relative_path) {
            Err(e) => match &e {
                HgError::IoError { error, .. } => match error.kind() {
                    ErrorKind::NotFound => return Ok(None),
                    _ => Err(e),
                },
                _ => Err(e),
            },
            Ok(v) => Ok(Some(v)),
        }
    }

    fn mmap_open_gen(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<Result<Mmap, FileNotFound>, HgError> {
        let path = self.join(relative_path);
        let file = match std::fs::File::open(&path) {
            Err(err) => {
                if let ErrorKind::NotFound = err.kind() {
                    return Ok(Err(FileNotFound(err, path)));
                };
                return (Err(err)).when_reading_file(&path);
            }
            Ok(file) => file,
        };
        // TODO: what are the safety requirements here?
        let mmap = unsafe { MmapOptions::new().map(&file) }
            .when_reading_file(&path)?;
        Ok(Ok(mmap))
    }

    pub fn mmap_open_opt(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<Option<Mmap>, HgError> {
        self.mmap_open_gen(relative_path).map(|res| res.ok())
    }

    pub fn mmap_open(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<Mmap, HgError> {
        match self.mmap_open_gen(relative_path)? {
            Err(FileNotFound(err, path)) => Err(err).when_reading_file(&path),
            Ok(res) => Ok(res),
        }
    }

    pub fn rename(
        &self,
        relative_from: impl AsRef<Path>,
        relative_to: impl AsRef<Path>,
    ) -> Result<(), HgError> {
        let from = self.join(relative_from);
        let to = self.join(relative_to);
        std::fs::rename(&from, &to)
            .with_context(|| IoErrorContext::RenamingFile { from, to })
    }

    pub fn remove_file(
        &self,
        relative_path: impl AsRef<Path>,
    ) -> Result<(), HgError> {
        let path = self.join(relative_path);
        std::fs::remove_file(&path)
            .with_context(|| IoErrorContext::RemovingFile(path))
    }

    #[cfg(unix)]
    pub fn create_symlink(
        &self,
        relative_link_path: impl AsRef<Path>,
        target_path: impl AsRef<Path>,
    ) -> Result<(), HgError> {
        let link_path = self.join(relative_link_path);
        std::os::unix::fs::symlink(target_path, &link_path)
            .when_writing_file(&link_path)
    }

    /// Write `contents` into a temporary file, then rename to `relative_path`.
    /// This makes writing to a file "atomic": a reader opening that path will
    /// see either the previous contents of the file or the complete new
    /// content, never a partial write.
    pub fn atomic_write(
        &self,
        relative_path: impl AsRef<Path>,
        contents: &[u8],
    ) -> Result<(), HgError> {
        let mut tmp = tempfile::NamedTempFile::new_in(self.base)
            .when_writing_file(self.base)?;
        tmp.write_all(contents)
            .and_then(|()| tmp.flush())
            .when_writing_file(tmp.path())?;
        let path = self.join(relative_path);
        tmp.persist(&path)
            .map_err(|e| e.error)
            .when_writing_file(&path)?;
        Ok(())
    }
}

fn fs_metadata(
    path: impl AsRef<Path>,
) -> Result<Option<std::fs::Metadata>, HgError> {
    let path = path.as_ref();
    match std::fs::metadata(path) {
        Ok(meta) => Ok(Some(meta)),
        Err(error) => match error.kind() {
            // TODO: when we require a Rust version where `NotADirectory` is
            // stable, invert this logic and return None for it and `NotFound`
            // and propagate any other error.
            ErrorKind::PermissionDenied => Err(error).with_context(|| {
                IoErrorContext::ReadingMetadata(path.to_owned())
            }),
            _ => Ok(None),
        },
    }
}

pub(crate) fn is_dir(path: impl AsRef<Path>) -> Result<bool, HgError> {
    Ok(fs_metadata(path)?.map_or(false, |meta| meta.is_dir()))
}

pub(crate) fn is_file(path: impl AsRef<Path>) -> Result<bool, HgError> {
    Ok(fs_metadata(path)?.map_or(false, |meta| meta.is_file()))
}