annotate rust/hg-core/src/repo.rs @ 51118:532e74ad3ff6

rust: run a clippy pass with the latest stable version

Our current version of clippy is older than the latest stable.
The newest version has new lints that are moslty good advice, so let's apply
them ahead of time. This has the added benefit of reducing the noise for
developpers like myself that use clippy as an IDE helper, as well as being
more prepared for a future clippy upgrade.

use crate::changelog::Changelog;
use crate::config::{Config, ConfigError, ConfigParseError};
use crate::dirstate::DirstateParents;
use crate::dirstate_tree::dirstate_map::DirstateMapWriteMode;
use crate::dirstate_tree::on_disk::Docket as DirstateDocket;
use crate::dirstate_tree::owning::OwningDirstateMap;
use crate::errors::HgResultExt;
use crate::errors::{HgError, IoResultExt};
use crate::lock::{try_with_lock_no_wait, LockError};
use crate::manifest::{Manifest, Manifestlog};
use crate::revlog::filelog::Filelog;
use crate::revlog::RevlogError;
use crate::utils::debug::debug_wait_for_file_or_print;
use crate::utils::files::get_path_from_bytes;
use crate::utils::hg_path::HgPath;
use crate::utils::SliceExt;
use crate::vfs::{is_dir, is_file, Vfs};
use crate::DirstateError;
use crate::{requirements, NodePrefix, UncheckedRevision};
use std::cell::{Ref, RefCell, RefMut};
use std::collections::HashSet;
use std::io::Seek;
use std::io::SeekFrom;
use std::io::Write as IoWrite;
use std::path::{Path, PathBuf};

const V2_MAX_READ_ATTEMPTS: usize = 5;

type DirstateMapIdentity = (Option<u64>, Option<Vec<u8>>, usize);

/// A repository on disk
pub struct Repo {
    working_directory: PathBuf,
    dot_hg: PathBuf,
    store: PathBuf,
    requirements: HashSet<String>,
    config: Config,
    dirstate_parents: LazyCell<DirstateParents>,
    dirstate_map: LazyCell<OwningDirstateMap>,
    changelog: LazyCell<Changelog>,
    manifestlog: LazyCell<Manifestlog>,
}

#[derive(Debug, derive_more::From)]
pub enum RepoError {
    NotFound {
        at: PathBuf,
    },
    #[from]
    ConfigParseError(ConfigParseError),
    #[from]
    Other(HgError),
}

impl From<ConfigError> for RepoError {
    fn from(error: ConfigError) -> Self {
        match error {
            ConfigError::Parse(error) => error.into(),
            ConfigError::Other(error) => error.into(),
        }
    }
}

impl Repo {
    /// tries to find nearest repository root in current working directory or
    /// its ancestors
    pub fn find_repo_root() -> Result<PathBuf, RepoError> {
        let current_directory = crate::utils::current_dir()?;
        // ancestors() is inclusive: it first yields `current_directory`
        // as-is.
        for ancestor in current_directory.ancestors() {
            if is_dir(ancestor.join(".hg"))? {
                return Ok(ancestor.to_path_buf());
            }
        }
        Err(RepoError::NotFound {
            at: current_directory,
        })
    }

    /// Find a repository, either at the given path (which must contain a `.hg`
    /// sub-directory) or by searching the current directory and its
    /// ancestors.
    ///
    /// A method with two very different "modes" like this usually a code smell
    /// to make two methods instead, but in this case an `Option` is what rhg
    /// sub-commands get from Clap for the `-R` / `--repository` CLI argument.
    /// Having two methods would just move that `if` to almost all callers.
    pub fn find(
        config: &Config,
        explicit_path: Option<PathBuf>,
    ) -> Result<Self, RepoError> {
        if let Some(root) = explicit_path {
            if is_dir(root.join(".hg"))? {
                Self::new_at_path(root, config)
            } else if is_file(&root)? {
                Err(HgError::unsupported("bundle repository").into())
            } else {
                Err(RepoError::NotFound { at: root })
            }
        } else {
            let root = Self::find_repo_root()?;
            Self::new_at_path(root, config)
        }
    }

    /// To be called after checking that `.hg` is a sub-directory
    fn new_at_path(
        working_directory: PathBuf,
        config: &Config,
    ) -> Result<Self, RepoError> {
        let dot_hg = working_directory.join(".hg");

        let mut repo_config_files =
            vec![dot_hg.join("hgrc"), dot_hg.join("hgrc-not-shared")];

        let hg_vfs = Vfs { base: &dot_hg };
        let mut reqs = requirements::load_if_exists(hg_vfs)?;
        let relative =
            reqs.contains(requirements::RELATIVE_SHARED_REQUIREMENT);
        let shared =
            reqs.contains(requirements::SHARED_REQUIREMENT) || relative;

        // From `mercurial/localrepo.py`:
        //
        // if .hg/requires contains the sharesafe requirement, it means
        // there exists a `.hg/store/requires` too and we should read it
        // NOTE: presence of SHARESAFE_REQUIREMENT imply that store requirement
        // is present. We never write SHARESAFE_REQUIREMENT for a repo if store
        // is not present, refer checkrequirementscompat() for that
        //
        // However, if SHARESAFE_REQUIREMENT is not present, it means that the
        // repository was shared the old way. We check the share source
        // .hg/requires for SHARESAFE_REQUIREMENT to detect whether the
        // current repository needs to be reshared
        let share_safe = reqs.contains(requirements::SHARESAFE_REQUIREMENT);

        let store_path;
        if !shared {
            store_path = dot_hg.join("store");
        } else {
            let bytes = hg_vfs.read("sharedpath")?;
            let mut shared_path =
                get_path_from_bytes(bytes.trim_end_matches(|b| b == b'\n'))
                    .to_owned();
            if relative {
                shared_path = dot_hg.join(shared_path)
            }
            if !is_dir(&shared_path)? {
                return Err(HgError::corrupted(format!(
                    ".hg/sharedpath points to nonexistent directory {}",
                    shared_path.display()
                ))
                .into());
            }

            store_path = shared_path.join("store");

            let source_is_share_safe =
                requirements::load(Vfs { base: &shared_path })?
                    .contains(requirements::SHARESAFE_REQUIREMENT);

            if share_safe != source_is_share_safe {
                return Err(HgError::unsupported("share-safe mismatch").into());
            }

            if share_safe {
                repo_config_files.insert(0, shared_path.join("hgrc"))
            }
        }
        if share_safe {
            reqs.extend(requirements::load(Vfs { base: &store_path })?);
        }

        let repo_config = if std::env::var_os("HGRCSKIPREPO").is_none() {
            config.combine_with_repo(&repo_config_files)?
        } else {
            config.clone()
        };

        let repo = Self {
            requirements: reqs,
            working_directory,
            store: store_path,
            dot_hg,
            config: repo_config,
            dirstate_parents: LazyCell::new(),
            dirstate_map: LazyCell::new(),
            changelog: LazyCell::new(),
            manifestlog: LazyCell::new(),
        };

        requirements::check(&repo)?;

        Ok(repo)
    }

    pub fn working_directory_path(&self) -> &Path {
        &self.working_directory
    }

    pub fn requirements(&self) -> &HashSet<String> {
        &self.requirements
    }

    pub fn config(&self) -> &Config {
        &self.config
    }

    /// For accessing repository files (in `.hg`), except for the store
    /// (`.hg/store`).
    pub fn hg_vfs(&self) -> Vfs<'_> {
        Vfs { base: &self.dot_hg }
    }

    /// For accessing repository store files (in `.hg/store`)
    pub fn store_vfs(&self) -> Vfs<'_> {
        Vfs { base: &self.store }
    }

    /// For accessing the working copy
    pub fn working_directory_vfs(&self) -> Vfs<'_> {
        Vfs {
            base: &self.working_directory,
        }
    }

    pub fn try_with_wlock_no_wait<R>(
        &self,
        f: impl FnOnce() -> R,
    ) -> Result<R, LockError> {
        try_with_lock_no_wait(self.hg_vfs(), "wlock", f)
    }

    /// Whether this repo should use dirstate-v2.
    /// The presence of `dirstate-v2` in the requirements does not mean that
    /// the on-disk dirstate is necessarily in version 2. In most cases,
    /// a dirstate-v2 file will indeed be found, but in rare cases (like the
    /// upgrade mechanism being cut short), the on-disk version will be a
    /// v1 file.
    /// Semantically, having a requirement only means that a client cannot
    /// properly understand or properly update the repo if it lacks the support
    /// for the required feature, but not that that feature is actually used
    /// in all occasions.
    pub fn use_dirstate_v2(&self) -> bool {
        self.requirements
            .contains(requirements::DIRSTATE_V2_REQUIREMENT)
    }

    pub fn has_sparse(&self) -> bool {
        self.requirements.contains(requirements::SPARSE_REQUIREMENT)
    }

    pub fn has_narrow(&self) -> bool {
        self.requirements.contains(requirements::NARROW_REQUIREMENT)
    }

    pub fn has_nodemap(&self) -> bool {
        self.requirements
            .contains(requirements::NODEMAP_REQUIREMENT)
    }

    fn dirstate_file_contents(&self) -> Result<Vec<u8>, HgError> {
        Ok(self
            .hg_vfs()
            .read("dirstate")
            .io_not_found_as_none()?
            .unwrap_or_default())
    }

    fn dirstate_identity(&self) -> Result<Option<u64>, HgError> {
        use std::os::unix::fs::MetadataExt;
        Ok(self
            .hg_vfs()
            .symlink_metadata("dirstate")
            .io_not_found_as_none()?
            .map(|meta| meta.ino()))
    }

    pub fn dirstate_parents(&self) -> Result<DirstateParents, HgError> {
        Ok(*self
            .dirstate_parents
            .get_or_init(|| self.read_dirstate_parents())?)
    }

    fn read_dirstate_parents(&self) -> Result<DirstateParents, HgError> {
        let dirstate = self.dirstate_file_contents()?;
        let parents = if dirstate.is_empty() {
            DirstateParents::NULL
        } else if self.use_dirstate_v2() {
            let docket_res =
                crate::dirstate_tree::on_disk::read_docket(&dirstate);
            match docket_res {
                Ok(docket) => docket.parents(),
                Err(_) => {
                    log::info!(
                        "Parsing dirstate docket failed, \
                        falling back to dirstate-v1"
                    );
                    *crate::dirstate::parsers::parse_dirstate_parents(
                        &dirstate,
                    )?
                }
            }
        } else {
            *crate::dirstate::parsers::parse_dirstate_parents(&dirstate)?
        };
        self.dirstate_parents.set(parents);
        Ok(parents)
    }

    /// Returns the information read from the dirstate docket necessary to
    /// check if the data file has been updated/deleted by another process
    /// since we last read the dirstate.
    /// Namely, the inode, data file uuid and the data size.
    fn get_dirstate_data_file_integrity(
        &self,
    ) -> Result<DirstateMapIdentity, HgError> {
        assert!(
            self.use_dirstate_v2(),
            "accessing dirstate data file ID without dirstate-v2"
        );
        // Get the identity before the contents since we could have a race
        // between the two. Having an identity that is too old is fine, but
        // one that is younger than the content change is bad.
        let identity = self.dirstate_identity()?;
        let dirstate = self.dirstate_file_contents()?;
        if dirstate.is_empty() {
            self.dirstate_parents.set(DirstateParents::NULL);
            Ok((identity, None, 0))
        } else {
            let docket_res =
                crate::dirstate_tree::on_disk::read_docket(&dirstate);
            match docket_res {
                Ok(docket) => {
                    self.dirstate_parents.set(docket.parents());
                    Ok((
                        identity,
                        Some(docket.uuid.to_owned()),
                        docket.data_size(),
                    ))
                }
                Err(_) => {
                    log::info!(
                        "Parsing dirstate docket failed, \
                        falling back to dirstate-v1"
                    );
                    let parents =
                        *crate::dirstate::parsers::parse_dirstate_parents(
                            &dirstate,
                        )?;
                    self.dirstate_parents.set(parents);
                    Ok((identity, None, 0))
                }
            }
        }
    }

    fn new_dirstate_map(&self) -> Result<OwningDirstateMap, DirstateError> {
        if self.use_dirstate_v2() {
            // The v2 dirstate is split into a docket and a data file.
            // Since we don't always take the `wlock` to read it
            // (like in `hg status`), it is susceptible to races.
            // A simple retry method should be enough since full rewrites
            // only happen when too much garbage data is present and
            // this race is unlikely.
            let mut tries = 0;

            while tries < V2_MAX_READ_ATTEMPTS {
                tries += 1;
                match self.read_docket_and_data_file() {
                    Ok(m) => {
                        return Ok(m);
                    }
                    Err(e) => match e {
                        DirstateError::Common(HgError::RaceDetected(
                            context,
                        )) => {
                            log::info!(
                                "dirstate read race detected {} (retry {}/{})",
                                context,
                                tries,
                                V2_MAX_READ_ATTEMPTS,
                            );
                            continue;
                        }
                        _ => {
                            log::info!(
                                "Reading dirstate v2 failed, \
                                falling back to v1"
                            );
                            return self.new_dirstate_map_v1();
                        }
                    },
                }
            }
            let error = HgError::abort(
                format!("dirstate read race happened {tries} times in a row"),
                255,
                None,
            );
            Err(DirstateError::Common(error))
        } else {
            self.new_dirstate_map_v1()
        }
    }

    fn new_dirstate_map_v1(&self) -> Result<OwningDirstateMap, DirstateError> {
        debug_wait_for_file_or_print(self.config(), "dirstate.pre-read-file");
        let identity = self.dirstate_identity()?;
        let dirstate_file_contents = self.dirstate_file_contents()?;
        if dirstate_file_contents.is_empty() {
            self.dirstate_parents.set(DirstateParents::NULL);
            Ok(OwningDirstateMap::new_empty(Vec::new()))
        } else {
            let (map, parents) =
                OwningDirstateMap::new_v1(dirstate_file_contents, identity)?;
            self.dirstate_parents.set(parents);
            Ok(map)
        }
    }

    fn read_docket_and_data_file(
        &self,
    ) -> Result<OwningDirstateMap, DirstateError> {
        debug_wait_for_file_or_print(self.config(), "dirstate.pre-read-file");
        let dirstate_file_contents = self.dirstate_file_contents()?;
        let identity = self.dirstate_identity()?;
        if dirstate_file_contents.is_empty() {
            self.dirstate_parents.set(DirstateParents::NULL);
            return Ok(OwningDirstateMap::new_empty(Vec::new()));
        }
        let docket = crate::dirstate_tree::on_disk::read_docket(
            &dirstate_file_contents,
        )?;
        debug_wait_for_file_or_print(
            self.config(),
            "dirstate.post-docket-read-file",
        );
        self.dirstate_parents.set(docket.parents());
        let uuid = docket.uuid.to_owned();
        let data_size = docket.data_size();

        let context = "between reading dirstate docket and data file";
        let race_error = HgError::RaceDetected(context.into());
        let metadata = docket.tree_metadata();

        let mut map = if crate::vfs::is_on_nfs_mount(docket.data_filename()) {
            // Don't mmap on NFS to prevent `SIGBUS` error on deletion
            let contents = self.hg_vfs().read(docket.data_filename());
            let contents = match contents {
                Ok(c) => c,
                Err(HgError::IoError { error, context }) => {
                    match error.raw_os_error().expect("real os error") {
                        // 2 = ENOENT, No such file or directory
                        // 116 = ESTALE, Stale NFS file handle
                        //
                        // TODO match on `error.kind()` when
                        // `ErrorKind::StaleNetworkFileHandle` is stable.
                        2 | 116 => {
                            // Race where the data file was deleted right after
                            // we read the docket, try again
                            return Err(race_error.into());
                        }
                        _ => {
                            return Err(
                                HgError::IoError { error, context }.into()
                            )
                        }
                    }
                }
                Err(e) => return Err(e.into()),
            };
            OwningDirstateMap::new_v2(
                contents, data_size, metadata, uuid, identity,
            )
        } else {
            match self
                .hg_vfs()
                .mmap_open(docket.data_filename())
                .io_not_found_as_none()
            {
                Ok(Some(data_mmap)) => OwningDirstateMap::new_v2(
                    data_mmap, data_size, metadata, uuid, identity,
                ),
                Ok(None) => {
                    // Race where the data file was deleted right after we
                    // read the docket, try again
                    return Err(race_error.into());
                }
                Err(e) => return Err(e.into()),
            }
        }?;

        let write_mode_config = self
            .config()
            .get_str(b"devel", b"dirstate.v2.data_update_mode")
            .unwrap_or(Some("auto"))
            .unwrap_or("auto"); // don't bother for devel options
        let write_mode = match write_mode_config {
            "auto" => DirstateMapWriteMode::Auto,
            "force-new" => DirstateMapWriteMode::ForceNewDataFile,
            "force-append" => DirstateMapWriteMode::ForceAppend,
            _ => DirstateMapWriteMode::Auto,
        };

        map.with_dmap_mut(|m| m.set_write_mode(write_mode));

        Ok(map)
    }

    pub fn dirstate_map(
        &self,
    ) -> Result<Ref<OwningDirstateMap>, DirstateError> {
        self.dirstate_map.get_or_init(|| self.new_dirstate_map())
    }

    pub fn dirstate_map_mut(
        &self,
    ) -> Result<RefMut<OwningDirstateMap>, DirstateError> {
        self.dirstate_map
            .get_mut_or_init(|| self.new_dirstate_map())
    }

    fn new_changelog(&self) -> Result<Changelog, HgError> {
        Changelog::open(&self.store_vfs(), self.has_nodemap())
    }

    pub fn changelog(&self) -> Result<Ref<Changelog>, HgError> {
        self.changelog.get_or_init(|| self.new_changelog())
    }

    pub fn changelog_mut(&self) -> Result<RefMut<Changelog>, HgError> {
        self.changelog.get_mut_or_init(|| self.new_changelog())
    }

    fn new_manifestlog(&self) -> Result<Manifestlog, HgError> {
        Manifestlog::open(&self.store_vfs(), self.has_nodemap())
    }

    pub fn manifestlog(&self) -> Result<Ref<Manifestlog>, HgError> {
        self.manifestlog.get_or_init(|| self.new_manifestlog())
    }

    pub fn manifestlog_mut(&self) -> Result<RefMut<Manifestlog>, HgError> {
        self.manifestlog.get_mut_or_init(|| self.new_manifestlog())
    }

    /// Returns the manifest of the *changeset* with the given node ID
    pub fn manifest_for_node(
        &self,
        node: impl Into<NodePrefix>,
    ) -> Result<Manifest, RevlogError> {
        self.manifestlog()?.data_for_node(
            self.changelog()?
                .data_for_node(node.into())?
                .manifest_node()?
                .into(),
        )
    }

    /// Returns the manifest of the *changeset* with the given revision number
    pub fn manifest_for_rev(
        &self,
        revision: UncheckedRevision,
    ) -> Result<Manifest, RevlogError> {
        self.manifestlog()?.data_for_node(
            self.changelog()?
                .data_for_rev(revision)?
                .manifest_node()?
                .into(),
        )
    }

    pub fn has_subrepos(&self) -> Result<bool, DirstateError> {
        if let Some(entry) = self.dirstate_map()?.get(HgPath::new(".hgsub"))? {
            Ok(entry.tracked())
        } else {
            Ok(false)
        }
    }

    pub fn filelog(&self, path: &HgPath) -> Result<Filelog, HgError> {
        Filelog::open(self, path)
    }

    /// Write to disk any updates that were made through `dirstate_map_mut`.
    ///
    /// The "wlock" must be held while calling this.
    /// See for example `try_with_wlock_no_wait`.
    ///
    /// TODO: have a `WritableRepo` type only accessible while holding the
    /// lock?
    pub fn write_dirstate(&self) -> Result<(), DirstateError> {
        let map = self.dirstate_map()?;
        // TODO: Maintain a `DirstateMap::dirty` flag, and return early here if
        // it’s unset
        let parents = self.dirstate_parents()?;
        let (packed_dirstate, old_uuid_to_remove) = if self.use_dirstate_v2() {
            let (identity, uuid, data_size) =
                self.get_dirstate_data_file_integrity()?;
            let identity_changed = identity != map.old_identity();
            let uuid_changed = uuid.as_deref() != map.old_uuid();
            let data_length_changed = data_size != map.old_data_size();

            if identity_changed || uuid_changed || data_length_changed {
                // If any of identity, uuid or length have changed since
                // last disk read, don't write.
                // This is fine because either we're in a command that doesn't
                // write anything too important (like `hg status`), or we're in
                // `hg add` and we're supposed to have taken the lock before
                // reading anyway.
                //
                // TODO complain loudly if we've changed anything important
                // without taking the lock.
                // (see `hg help config.format.use-dirstate-tracked-hint`)
                log::debug!(
                    "dirstate has changed since last read, not updating."
                );
                return Ok(());
            }

            let uuid_opt = map.old_uuid();
            let write_mode = if uuid_opt.is_some() {
                DirstateMapWriteMode::Auto
            } else {
                DirstateMapWriteMode::ForceNewDataFile
            };
            let (data, tree_metadata, append, old_data_size) =
                map.pack_v2(write_mode)?;

            // Reuse the uuid, or generate a new one, keeping the old for
            // deletion.
            let (uuid, old_uuid) = match uuid_opt {
                Some(uuid) => {
                    let as_str = std::str::from_utf8(uuid)
                        .map_err(|_| {
                            HgError::corrupted(
                                "non-UTF-8 dirstate data file ID",
                            )
                        })?
                        .to_owned();
                    if append {
                        (as_str, None)
                    } else {
                        (DirstateDocket::new_uid(), Some(as_str))
                    }
                }
                None => (DirstateDocket::new_uid(), None),
            };

            let data_filename = format!("dirstate.{}", uuid);
            let data_filename = self.hg_vfs().join(data_filename);
            let mut options = std::fs::OpenOptions::new();
            options.write(true);

            // Why are we not using the O_APPEND flag when appending?
            //
            // - O_APPEND makes it trickier to deal with garbage at the end of
            //   the file, left by a previous uncommitted transaction. By
            //   starting the write at [old_data_size] we make sure we erase
            //   all such garbage.
            //
            // - O_APPEND requires to special-case 0-byte writes, whereas we
            //   don't need that.
            //
            // - Some OSes have bugs in implementation O_APPEND:
            //   revlog.py talks about a Solaris bug, but we also saw some ZFS
            //   bug: https://github.com/openzfs/zfs/pull/3124,
            //   https://github.com/openzfs/zfs/issues/13370
            //
            if !append {
                log::trace!("creating a new dirstate data file");
                options.create_new(true);
            } else {
                log::trace!("appending to the dirstate data file");
            }

            let data_size = (|| {
                // TODO: loop and try another random ID if !append and this
                // returns `ErrorKind::AlreadyExists`? Collision chance of two
                // random IDs is one in 2**32
                let mut file = options.open(&data_filename)?;
                if append {
                    file.seek(SeekFrom::Start(old_data_size as u64))?;
                }
                file.write_all(&data)?;
                file.flush()?;
                file.stream_position()
            })()
            .when_writing_file(&data_filename)?;

            let packed_dirstate = DirstateDocket::serialize(
                parents,
                tree_metadata,
                data_size,
                uuid.as_bytes(),
            )
            .map_err(|_: std::num::TryFromIntError| {
                HgError::corrupted("overflow in dirstate docket serialization")
            })?;

            (packed_dirstate, old_uuid)
        } else {
            let identity = self.dirstate_identity()?;
            if identity != map.old_identity() {
                // If identity changed since last disk read, don't write.
                // This is fine because either we're in a command that doesn't
                // write anything too important (like `hg status`), or we're in
                // `hg add` and we're supposed to have taken the lock before
                // reading anyway.
                //
                // TODO complain loudly if we've changed anything important
                // without taking the lock.
                // (see `hg help config.format.use-dirstate-tracked-hint`)
                log::debug!(
                    "dirstate has changed since last read, not updating."
                );
                return Ok(());
            }
            (map.pack_v1(parents)?, None)
        };

        let vfs = self.hg_vfs();
        vfs.atomic_write("dirstate", &packed_dirstate)?;
        if let Some(uuid) = old_uuid_to_remove {
            // Remove the old data file after the new docket pointing to the
            // new data file was written.
            vfs.remove_file(format!("dirstate.{}", uuid))?;
        }
        Ok(())
    }
}

/// Lazily-initialized component of `Repo` with interior mutability
///
/// This differs from `OnceCell` in that the value can still be "deinitialized"
/// later by setting its inner `Option` to `None`. It also takes the
/// initialization function as an argument when the value is requested, not
/// when the instance is created.
struct LazyCell<T> {
    value: RefCell<Option<T>>,
}

impl<T> LazyCell<T> {
    fn new() -> Self {
        Self {
            value: RefCell::new(None),
        }
    }

    fn set(&self, value: T) {
        *self.value.borrow_mut() = Some(value)
    }

    fn get_or_init<E>(
        &self,
        init: impl Fn() -> Result<T, E>,
    ) -> Result<Ref<T>, E> {
        let mut borrowed = self.value.borrow();
        if borrowed.is_none() {
            drop(borrowed);
            // Only use `borrow_mut` if it is really needed to avoid panic in
            // case there is another outstanding borrow but mutation is not
            // needed.
            *self.value.borrow_mut() = Some(init()?);
            borrowed = self.value.borrow()
        }
        Ok(Ref::map(borrowed, |option| option.as_ref().unwrap()))
    }

    fn get_mut_or_init<E>(
        &self,
        init: impl Fn() -> Result<T, E>,
    ) -> Result<RefMut<T>, E> {
        let mut borrowed = self.value.borrow_mut();
        if borrowed.is_none() {
            *borrowed = Some(init()?);
        }
        Ok(RefMut::map(borrowed, |option| option.as_mut().unwrap()))
    }
}