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    //! Management of the index of a registry source
    //!
    //! This module contains management of the index and various operations, such as
    //! actually parsing the index, looking for crates, etc. This is intended to be
    //! abstract over remote indices (downloaded via git) and local registry indices
    //! (which are all just present on the filesystem).
    //!
    //! ## Index Performance
    //!
    //! One important aspect of the index is that we want to optimize the "happy
    //! path" as much as possible. Whenever you type `cargo build` Cargo will
    //! *always* reparse the registry and learn about dependency information. This
    //! is done because Cargo needs to learn about the upstream crates.io crates
    //! that you're using and ensure that the preexisting `Cargo.lock` still matches
    //! the current state of the world.
    //!
    //! Consequently, Cargo "null builds" (the index that Cargo adds to each build
    //! itself) need to be fast when accessing the index. The primary performance
    //! optimization here is to avoid parsing JSON blobs from the registry if we
    //! don't need them. Most secondary optimizations are centered around removing
    //! allocations and such, but avoiding parsing JSON is the #1 optimization.
    //!
    //! When we get queries from the resolver we're given a `Dependency`. This
    //! dependency in turn has a version requirement, and with lock files that
    //! already exist these version requirements are exact version requirements
    //! `=a.b.c`. This means that we in theory only need to parse one line of JSON
    //! per query in the registry, the one that matches version `a.b.c`.
    //!
    //! The crates.io index, however, is not amenable to this form of query. Instead
    //! the crates.io index simply is a file where each line is a JSON blob. To
    //! learn about the versions in each JSON blob we would need to parse the JSON,
    //! defeating the purpose of trying to parse as little as possible.
    //!
    //! > Note that as a small aside even *loading* the JSON from the registry is
    //! > actually pretty slow. For crates.io and remote registries we don't
    //! > actually check out the git index on disk because that takes quite some
    //! > time and is quite large. Instead we use `libgit2` to read the JSON from
    //! > the raw git objects. This in turn can be slow (aka show up high in
    //! > profiles) because libgit2 has to do deflate decompression and such.
    //!
    //! To solve all these issues a strategy is employed here where Cargo basically
    //! creates an index into the index. The first time a package is queried about
    //! (first time being for an entire computer) Cargo will load the contents
    //! (slowly via libgit2) from the registry. It will then (slowly) parse every
    //! single line to learn about its versions. Afterwards, however, Cargo will
    //! emit a new file (a cache) which is amenable for speedily parsing in future
    //! invocations.
    //!
    //! This cache file is currently organized by basically having the semver
    //! version extracted from each JSON blob. That way Cargo can quickly and easily
    //! parse all versions contained and which JSON blob they're associated with.
    //! The JSON blob then doesn't actually need to get parsed unless the version is
    //! parsed.
    //!
    //! Altogether the initial measurements of this shows a massive improvement for
    //! Cargo null build performance. It's expected that the improvements earned
    //! here will continue to grow over time in the sense that the previous
    //! implementation (parse all lines each time) actually continues to slow down
    //! over time as new versions of a crate are published. In any case when first
    //! implemented a null build of Cargo itself would parse 3700 JSON blobs from
    //! the registry and load 150 blobs from git. Afterwards it parses 150 JSON
    //! blobs and loads 0 files git. Removing 200ms or more from Cargo's startup
    //! time is certainly nothing to sneeze at!
    //!
    //! Note that this is just a high-level overview, there's of course lots of
    //! details like invalidating caches and whatnot which are handled below, but
    //! hopefully those are more obvious inline in the code itself.

    use crate::core::{PackageId, SourceId, Summary};
    use crate::sources::registry::{LoadResponse, RegistryData, RegistryPackage, INDEX_V_MAX};
    use crate::util::interning::InternedString;
    use crate::util::{internal, CargoResult, Config, Filesystem, OptVersionReq, ToSemver};
    use anyhow::bail;
    use cargo_util::{paths, registry::make_dep_path};
    use log::{debug, info};
    use semver::Version;
    use std::collections::{HashMap, HashSet};
    use std::fs;
    use std::io::ErrorKind;
    use std::path::Path;
    use std::str;
    use std::task::{ready, Poll};

    /// Manager for handling the on-disk index.
    ///
    /// Note that local and remote registries store the index differently. Local
    /// is a simple on-disk tree of files of the raw index. Remote registries are
    /// stored as a raw git repository. The different means of access are handled
    /// via the [`RegistryData`] trait abstraction.
    ///
    /// This transparently handles caching of the index in a more efficient format.
    pub struct RegistryIndex<'cfg> {
        source_id: SourceId,
        /// Root directory of the index for the registry.
        path: Filesystem,
        /// Cache of summary data.
        ///
        /// This is keyed off the package name. The [`Summaries`] value handles
        /// loading the summary data. It keeps an optimized on-disk representation
        /// of the JSON files, which is created in an as-needed fashion. If it
        /// hasn't been cached already, it uses [`RegistryData::load`] to access
        /// to JSON files from the index, and the creates the optimized on-disk
        /// summary cache.
        summaries_cache: HashMap<InternedString, Summaries>,
        /// [`Config`] reference for convenience.
        config: &'cfg Config,
    }

    /// An internal cache of summaries for a particular package.
    ///
    /// A list of summaries are loaded from disk via one of two methods:
    ///
    /// 1. Primarily Cargo will parse the corresponding file for a crate in the
    ///    upstream crates.io registry. That's just a JSON blob per line which we
    ///    can parse, extract the version, and then store here.
    ///
    /// 2. Alternatively, if Cargo has previously run, we'll have a cached index of
    ///    dependencies for the upstream index. This is a file that Cargo maintains
    ///    lazily on the local filesystem and is much faster to parse since it
    ///    doesn't involve parsing all of the JSON.
    ///
    /// The outward-facing interface of this doesn't matter too much where it's
    /// loaded from, but it's important when reading the implementation to note that
    /// we try to parse as little as possible!
    #[derive(Default)]
    struct Summaries {
        /// A raw vector of uninterpreted bytes. This is what `Unparsed` start/end
        /// fields are indexes into. If a `Summaries` is loaded from the crates.io
        /// index then this field will be empty since nothing is `Unparsed`.
        raw_data: Vec<u8>,

        /// All known versions of a crate, keyed from their `Version` to the
        /// possibly parsed or unparsed version of the full summary.
        versions: HashMap<Version, MaybeIndexSummary>,
    }

    /// A lazily parsed `IndexSummary`.
    enum MaybeIndexSummary {
        /// A summary which has not been parsed, The `start` and `end` are pointers
        /// into `Summaries::raw_data` which this is an entry of.
        Unparsed { start: usize, end: usize },

        /// An actually parsed summary.
        Parsed(IndexSummary),
    }

    /// A parsed representation of a summary from the index.
    ///
    /// In addition to a full `Summary` we have information on whether it is `yanked`.
    pub struct IndexSummary {
        pub summary: Summary,
        pub yanked: bool,
        /// Schema version, see [`RegistryPackage`].
        v: u32,
    }

    /// A representation of the cache on disk that Cargo maintains of summaries.
    /// Cargo will initially parse all summaries in the registry and will then
    /// serialize that into this form and place it in a new location on disk,
    /// ensuring that access in the future is much speedier.
    #[derive(Default)]
    struct SummariesCache<'a> {
        versions: Vec<(Version, &'a [u8])>,
        index_version: &'a str,
    }

    impl<'cfg> RegistryIndex<'cfg> {
        pub fn new(
            source_id: SourceId,
            path: &Filesystem,
            config: &'cfg Config,
        ) -> RegistryIndex<'cfg> {
            RegistryIndex {
                source_id,
                path: path.clone(),
                summaries_cache: HashMap::new(),
                config,
            }
        }

        /// Returns the hash listed for a specified `PackageId`.
        pub fn hash(&mut self, pkg: PackageId, load: &mut dyn RegistryData) -> Poll<CargoResult<&str>> {
            let req = OptVersionReq::exact(pkg.version());
            let summary = self.summaries(&pkg.name(), &req, load)?;
            let summary = ready!(summary).next();
            Poll::Ready(Ok(summary
                .ok_or_else(|| internal(format!("no hash listed for {}", pkg)))?
                .summary
                .checksum()
                .ok_or_else(|| internal(format!("no hash listed for {}", pkg)))?))
        }

        /// Load a list of summaries for `name` package in this registry which
        /// match `req`
        ///
        /// This function will semantically parse the on-disk index, match all
        /// versions, and then return an iterator over all summaries which matched.
        /// Internally there's quite a few layer of caching to amortize this cost
        /// though since this method is called quite a lot on null builds in Cargo.
        pub fn summaries<'a, 'b>(
            &'a mut self,
            name: &str,
            req: &'b OptVersionReq,
            load: &mut dyn RegistryData,
        ) -> Poll<CargoResult<impl Iterator<Item = &'a IndexSummary> + 'b>>
        where
            'a: 'b,
        {
            let source_id = self.source_id;
            let config = self.config;

            // First up actually parse what summaries we have available. If Cargo
            // has run previously this will parse a Cargo-specific cache file rather
            // than the registry itself. In effect this is intended to be a quite
            // cheap operation.
            let name = InternedString::new(name);
            let summaries = ready!(self.load_summaries(name, load)?);

            // Iterate over our summaries, extract all relevant ones which match our
            // version requirement, and then parse all corresponding rows in the
            // registry. As a reminder this `summaries` method is called for each
            // entry in a lock file on every build, so we want to absolutely
            // minimize the amount of work being done here and parse as little as
            // necessary.
            let raw_data = &summaries.raw_data;
            Poll::Ready(Ok(summaries
                .versions
                .iter_mut()
                .filter_map(move |(k, v)| if req.matches(k) { Some(v) } else { None })
                .filter_map(
                    move |maybe| match maybe.parse(config, raw_data, source_id) {
                        Ok(summary) => Some(summary),
                        Err(e) => {
                            info!("failed to parse `{}` registry package: {}", name, e);
                            None
                        }
                    },
                )
                .filter(move |is| {
                    if is.v > INDEX_V_MAX {
                        debug!(
                            "unsupported schema version {} ({} {})",
                            is.v,
                            is.summary.name(),
                            is.summary.version()
                        );
                        false
                    } else {
                        true
                    }
                })))
        }

        fn load_summaries(
            &mut self,
            name: InternedString,
            load: &mut dyn RegistryData,
        ) -> Poll<CargoResult<&mut Summaries>> {
            // If we've previously loaded what versions are present for `name`, just
            // return that since our cache should still be valid.
            if self.summaries_cache.contains_key(&name) {
                return Poll::Ready(Ok(self.summaries_cache.get_mut(&name).unwrap()));
            }

            // Prepare the `RegistryData` which will lazily initialize internal data
            // structures.
            load.prepare()?;

            let root = load.assert_index_locked(&self.path);
            let cache_root = root.join(".cache");

            // See module comment in `registry/mod.rs` for why this is structured
            // the way it is.
            let fs_name = name
                .chars()
                .flat_map(|c| c.to_lowercase())
                .collect::<String>();

            let path = make_dep_path(&fs_name, false);
            let summaries = ready!(Summaries::parse(
                root,
                &cache_root,
                path.as_ref(),
                self.source_id,
                load,
                self.config,
            ))?
            .unwrap_or_default();
            self.summaries_cache.insert(name, summaries);
            Poll::Ready(Ok(self.summaries_cache.get_mut(&name).unwrap()))
        }

        /// Clears the in-memory summaries cache.
        pub fn clear_summaries_cache(&mut self) {
            self.summaries_cache.clear();
        }

        pub fn query_inner(
            &mut self,
            name: &str,
            req: &OptVersionReq,
            load: &mut dyn RegistryData,
            yanked_whitelist: &HashSet<PackageId>,
            f: &mut dyn FnMut(Summary),
        ) -> Poll<CargoResult<()>> {
            if self.config.offline() {
                // This should only return `Poll::Ready(Ok(()))` if there is at least 1 match.
                //
                // If there are 0 matches it should fall through and try again with online.
                // This is necessary for dependencies that are not used (such as
                // target-cfg or optional), but are not downloaded. Normally the
                // build should succeed if they are not downloaded and not used,
                // but they still need to resolve. If they are actually needed
                // then cargo will fail to download and an error message
                // indicating that the required dependency is unavailable while
                // offline will be displayed.
                if ready!(self.query_inner_with_online(name, req, load, yanked_whitelist, f, false)?)
                    > 0
                {
                    return Poll::Ready(Ok(()));
                }
            }
            self.query_inner_with_online(name, req, load, yanked_whitelist, f, true)
                .map_ok(|_| ())
        }

        fn query_inner_with_online(
            &mut self,
            name: &str,
            req: &OptVersionReq,
            load: &mut dyn RegistryData,
            yanked_whitelist: &HashSet<PackageId>,
            f: &mut dyn FnMut(Summary),
            online: bool,
        ) -> Poll<CargoResult<usize>> {
            let source_id = self.source_id;

            let summaries = ready!(self.summaries(name, req, load))?;

            let summaries = summaries
                // First filter summaries for `--offline`. If we're online then
                // everything is a candidate, otherwise if we're offline we're only
                // going to consider candidates which are actually present on disk.
                //
                // Note: This particular logic can cause problems with
                // optional dependencies when offline. If at least 1 version
                // of an optional dependency is downloaded, but that version
                // does not satisfy the requirements, then resolution will
                // fail. Unfortunately, whether or not something is optional
                // is not known here.
                .filter(|s| (online || load.is_crate_downloaded(s.summary.package_id())))
                // Next filter out all yanked packages. Some yanked packages may
                // leak through if they're in a whitelist (aka if they were
                // previously in `Cargo.lock`
                .filter(|s| !s.yanked || yanked_whitelist.contains(&s.summary.package_id()))
                .map(|s| s.summary.clone());

            // Handle `cargo update --precise` here. If specified, our own source
            // will have a precise version listed of the form
            // `<pkg>=<p_req>o-><f_req>` where `<pkg>` is the name of a crate on
            // this source, `<p_req>` is the version installed and `<f_req> is the
            // version requested (argument to `--precise`).
            let precise = match source_id.precise() {
                Some(p) if p.starts_with(name) && p[name.len()..].starts_with('=') => {
                    let mut vers = p[name.len() + 1..].splitn(2, "->");
                    let current_vers = vers.next().unwrap().to_semver().unwrap();
                    let requested_vers = vers.next().unwrap().to_semver().unwrap();
                    Some((current_vers, requested_vers))
                }
                _ => None,
            };
            let summaries = summaries.filter(|s| match &precise {
                Some((current, requested)) => {
                    if req.matches(current) {
                        // Unfortunately crates.io allows versions to differ only
                        // by build metadata. This shouldn't be allowed, but since
                        // it is, this will honor it if requested. However, if not
                        // specified, then ignore it.
                        let s_vers = s.version();
                        match (s_vers.build.is_empty(), requested.build.is_empty()) {
                            (true, true) => s_vers == requested,
                            (true, false) => false,
                            (false, true) => {
                                // Strip out the metadata.
                                s_vers.major == requested.major
                                    && s_vers.minor == requested.minor
                                    && s_vers.patch == requested.patch
                                    && s_vers.pre == requested.pre
                            }
                            (false, false) => s_vers == requested,
                        }
                    } else {
                        true
                    }
                }
                None => true,
            });

            let mut count = 0;
            for summary in summaries {
                f(summary);
                count += 1;
            }
            Poll::Ready(Ok(count))
        }

        pub fn is_yanked(
            &mut self,
            pkg: PackageId,
            load: &mut dyn RegistryData,
        ) -> Poll<CargoResult<bool>> {
            let req = OptVersionReq::exact(pkg.version());
            let found = self
                .summaries(&pkg.name(), &req, load)
                .map_ok(|mut p| p.any(|summary| summary.yanked));
            found
        }
    }

    impl Summaries {
        /// Parse out a `Summaries` instances from on-disk state.
        ///
        /// This will attempt to prefer parsing a previous cache file that already
        /// exists from a previous invocation of Cargo (aka you're typing `cargo
        /// build` again after typing it previously). If parsing fails or the cache
        /// isn't found, then we take a slower path which loads the full descriptor
        /// for `relative` from the underlying index (aka typically libgit2 with
        /// crates.io) and then parse everything in there.
        ///
        /// * `root` - this is the root argument passed to `load`
        /// * `cache_root` - this is the root on the filesystem itself of where to
        ///   store cache files.
        /// * `relative` - this is the file we're loading from cache or the index
        ///   data
        /// * `source_id` - the registry's SourceId used when parsing JSON blobs to
        ///   create summaries.
        /// * `load` - the actual index implementation which may be very slow to
        ///   call. We avoid this if we can.
        pub fn parse(
            root: &Path,
            cache_root: &Path,
            relative: &Path,
            source_id: SourceId,
            load: &mut dyn RegistryData,
            config: &Config,
        ) -> Poll<CargoResult<Option<Summaries>>> {
            // First up, attempt to load the cache. This could fail for all manner
            // of reasons, but consider all of them non-fatal and just log their
            // occurrence in case anyone is debugging anything.
            let cache_path = cache_root.join(relative);
            let mut cached_summaries = None;
            let mut index_version = None;
            match fs::read(&cache_path) {
                Ok(contents) => match Summaries::parse_cache(contents) {
                    Ok((s, v)) => {
                        cached_summaries = Some(s);
                        index_version = Some(v);
                    }
                    Err(e) => {
                        log::debug!("failed to parse {:?} cache: {}", relative, e);
                    }
                },
                Err(e) => log::debug!("cache missing for {:?} error: {}", relative, e),
            }

            let response = ready!(load.load(root, relative, index_version.as_deref())?);

            match response {
                LoadResponse::CacheValid => {
                    log::debug!("fast path for registry cache of {:?}", relative);
                    return Poll::Ready(Ok(cached_summaries));
                }
                LoadResponse::NotFound => {
                    if let Err(e) = fs::remove_file(cache_path) {
                        if e.kind() != ErrorKind::NotFound {
                            log::debug!("failed to remove from cache: {}", e);
                        }
                    }
                    return Poll::Ready(Ok(None));
                }
                LoadResponse::Data {
                    raw_data,
                    index_version,
                } => {
                    // This is the fallback path where we actually talk to the registry backend to load
                    // information. Here we parse every single line in the index (as we need
                    // to find the versions)
                    log::debug!("slow path for {:?}", relative);
                    let mut cache = SummariesCache::default();
                    let mut ret = Summaries::default();
                    ret.raw_data = raw_data;
                    for line in split(&ret.raw_data, b'\n') {
                        // Attempt forwards-compatibility on the index by ignoring
                        // everything that we ourselves don't understand, that should
                        // allow future cargo implementations to break the
                        // interpretation of each line here and older cargo will simply
                        // ignore the new lines.
                        let summary = match IndexSummary::parse(config, line, source_id) {
                            Ok(summary) => summary,
                            Err(e) => {
                                // This should only happen when there is an index
                                // entry from a future version of cargo that this
                                // version doesn't understand. Hopefully, those future
                                // versions of cargo correctly set INDEX_V_MAX and
                                // CURRENT_CACHE_VERSION, otherwise this will skip
                                // entries in the cache preventing those newer
                                // versions from reading them (that is, until the
                                // cache is rebuilt).
                                log::info!("failed to parse {:?} registry package: {}", relative, e);
                                continue;
                            }
                        };
                        let version = summary.summary.package_id().version().clone();
                        cache.versions.push((version.clone(), line));
                        ret.versions.insert(version, summary.into());
                    }
                    if let Some(index_version) = index_version {
                        log::trace!("caching index_version {}", index_version);
                        let cache_bytes = cache.serialize(index_version.as_str());
                        // Once we have our `cache_bytes` which represents the `Summaries` we're
                        // about to return, write that back out to disk so future Cargo
                        // invocations can use it.
                        //
                        // This is opportunistic so we ignore failure here but are sure to log
                        // something in case of error.
                        if paths::create_dir_all(cache_path.parent().unwrap()).is_ok() {
                            let path = Filesystem::new(cache_path.clone());
                            config.assert_package_cache_locked(&path);
                            if let Err(e) = fs::write(cache_path, &cache_bytes) {
                                log::info!("failed to write cache: {}", e);
                            }
                        }

                        // If we've got debug assertions enabled read back in the cached values
                        // and assert they match the expected result.
                        #[cfg(debug_assertions)]
                        {
                            let readback = SummariesCache::parse(&cache_bytes)
                                .expect("failed to parse cache we just wrote");
                            assert_eq!(
                                readback.index_version, index_version,
                                "index_version mismatch"
                            );
                            assert_eq!(readback.versions, cache.versions, "versions mismatch");
                        }
                    }
                    Poll::Ready(Ok(Some(ret)))
                }
            }
        }

        /// Parses an open `File` which represents information previously cached by
        /// Cargo.
        pub fn parse_cache(contents: Vec<u8>) -> CargoResult<(Summaries, InternedString)> {
            let cache = SummariesCache::parse(&contents)?;
            let index_version = InternedString::new(cache.index_version);
            let mut ret = Summaries::default();
            for (version, summary) in cache.versions {
                let (start, end) = subslice_bounds(&contents, summary);
                ret.versions
                    .insert(version, MaybeIndexSummary::Unparsed { start, end });
            }
            ret.raw_data = contents;
            return Ok((ret, index_version));

            // Returns the start/end offsets of `inner` with `outer`. Asserts that
            // `inner` is a subslice of `outer`.
            fn subslice_bounds(outer: &[u8], inner: &[u8]) -> (usize, usize) {
                let outer_start = outer.as_ptr() as usize;
                let outer_end = outer_start + outer.len();
                let inner_start = inner.as_ptr() as usize;
                let inner_end = inner_start + inner.len();
                assert!(inner_start >= outer_start);
                assert!(inner_end <= outer_end);
                (inner_start - outer_start, inner_end - outer_start)
            }
        }
    }

    // Implementation of serializing/deserializing the cache of summaries on disk.
    // Currently the format looks like:
    //
    // +--------------------+----------------------+-------------+---+
    // | cache version byte | index format version | git sha rev | 0 |
    // +--------------------+----------------------+-------------+---+
    //
    // followed by...
    //
    // +----------------+---+------------+---+
    // | semver version | 0 |  JSON blob | 0 | ...
    // +----------------+---+------------+---+
    //
    // The idea is that this is a very easy file for Cargo to parse in future
    // invocations. The read from disk should be quite fast and then afterwards all
    // we need to know is what versions correspond to which JSON blob.
    //
    // The leading version byte is intended to ensure that there's some level of
    // future compatibility against changes to this cache format so if different
    // versions of Cargo share the same cache they don't get too confused. The git
    // sha lets us know when the file needs to be regenerated (it needs regeneration
    // whenever the index itself updates).
    //
    // Cache versions:
    // * `1`: The original version.
    // * `2`: Added the "index format version" field so that if the index format
    //   changes, different versions of cargo won't get confused reading each
    //   other's caches.
    // * `3`: Bumped the version to work around an issue where multiple versions of
    //   a package were published that differ only by semver metadata. For
    //   example, openssl-src 110.0.0 and 110.0.0+1.1.0f. Previously, the cache
    //   would be incorrectly populated with two entries, both 110.0.0. After
    //   this, the metadata will be correctly included. This isn't really a format
    //   change, just a version bump to clear the incorrect cache entries. Note:
    //   the index shouldn't allow these, but unfortunately crates.io doesn't
    //   check it.

    const CURRENT_CACHE_VERSION: u8 = 3;

    impl<'a> SummariesCache<'a> {
        fn parse(data: &'a [u8]) -> CargoResult<SummariesCache<'a>> {
            // NB: keep this method in sync with `serialize` below
            let (first_byte, rest) = data
                .split_first()
                .ok_or_else(|| anyhow::format_err!("malformed cache"))?;
            if *first_byte != CURRENT_CACHE_VERSION {
                bail!("looks like a different Cargo's cache, bailing out");
            }
            let index_v_bytes = rest
                .get(..4)
                .ok_or_else(|| anyhow::anyhow!("cache expected 4 bytes for index version"))?;
            let index_v = u32::from_le_bytes(index_v_bytes.try_into().unwrap());
            if index_v != INDEX_V_MAX {
                bail!(
                    "index format version {} doesn't match the version I know ({})",
                    index_v,
                    INDEX_V_MAX
                );
            }
            let rest = &rest[4..];

            let mut iter = split(rest, 0);
            let last_index_update = if let Some(update) = iter.next() {
                str::from_utf8(update)?
            } else {
                bail!("malformed file");
            };
            let mut ret = SummariesCache::default();
            ret.index_version = last_index_update;
            while let Some(version) = iter.next() {
                let version = str::from_utf8(version)?;
                let version = Version::parse(version)?;
                let summary = iter.next().unwrap();
                ret.versions.push((version, summary));
            }
            Ok(ret)
        }

        fn serialize(&self, index_version: &str) -> Vec<u8> {
            // NB: keep this method in sync with `parse` above
            let size = self
                .versions
                .iter()
                .map(|(_version, data)| (10 + data.len()))
                .sum();
            let mut contents = Vec::with_capacity(size);
            contents.push(CURRENT_CACHE_VERSION);
            contents.extend(&u32::to_le_bytes(INDEX_V_MAX));
            contents.extend_from_slice(index_version.as_bytes());
            contents.push(0);
            for (version, data) in self.versions.iter() {
                contents.extend_from_slice(version.to_string().as_bytes());
                contents.push(0);
                contents.extend_from_slice(data);
                contents.push(0);
            }
            contents
        }
    }

    impl MaybeIndexSummary {
        /// Parses this "maybe a summary" into a `Parsed` for sure variant.
        ///
        /// Does nothing if this is already `Parsed`, and otherwise the `raw_data`
        /// passed in is sliced with the bounds in `Unparsed` and then actually
        /// parsed.
        fn parse(
            &mut self,
            config: &Config,
            raw_data: &[u8],
            source_id: SourceId,
        ) -> CargoResult<&IndexSummary> {
            let (start, end) = match self {
                MaybeIndexSummary::Unparsed { start, end } => (*start, *end),
                MaybeIndexSummary::Parsed(summary) => return Ok(summary),
            };
            let summary = IndexSummary::parse(config, &raw_data[start..end], source_id)?;
            *self = MaybeIndexSummary::Parsed(summary);
            match self {
                MaybeIndexSummary::Unparsed { .. } => unreachable!(),
                MaybeIndexSummary::Parsed(summary) => Ok(summary),
            }
        }
    }

    impl From<IndexSummary> for MaybeIndexSummary {
        fn from(summary: IndexSummary) -> MaybeIndexSummary {
            MaybeIndexSummary::Parsed(summary)
        }
    }

    impl IndexSummary {
        /// Parses a line from the registry's index file into an `IndexSummary` for
        /// a package.
        ///
        /// The `line` provided is expected to be valid JSON.
        fn parse(config: &Config, line: &[u8], source_id: SourceId) -> CargoResult<IndexSummary> {
            // ****CAUTION**** Please be extremely careful with returning errors
            // from this function. Entries that error are not included in the
            // index cache, and can cause cargo to get confused when switching
            // between different versions that understand the index differently.
            // Make sure to consider the INDEX_V_MAX and CURRENT_CACHE_VERSION
            // values carefully when making changes here.
            let RegistryPackage {
                name,
                vers,
                cksum,
                deps,
                mut features,
                features2,
                yanked,
                links,
                rust_version,
                v,
            } = serde_json::from_slice(line)?;
            let v = v.unwrap_or(1);
            log::trace!("json parsed registry {}/{}", name, vers);
            let pkgid = PackageId::new(name, &vers, source_id)?;
            let deps = deps
                .into_iter()
                .map(|dep| dep.into_dep(source_id))
                .collect::<CargoResult<Vec<_>>>()?;
            if let Some(features2) = features2 {
                for (name, values) in features2 {
                    features.entry(name).or_default().extend(values);
                }
            }
            let mut summary = Summary::new(config, pkgid, deps, &features, links, rust_version)?;
            summary.set_checksum(cksum);
            Ok(IndexSummary {
                summary,
                yanked: yanked.unwrap_or(false),
                v,
            })
        }
    }

    fn split(haystack: &[u8], needle: u8) -> impl Iterator<Item = &[u8]> {
        struct Split<'a> {
            haystack: &'a [u8],
            needle: u8,
        }

        impl<'a> Iterator for Split<'a> {
            type Item = &'a [u8];

            fn next(&mut self) -> Option<&'a [u8]> {
                if self.haystack.is_empty() {
                    return None;
                }
                let (ret, remaining) = match memchr::memchr(self.needle, self.haystack) {
                    Some(pos) => (&self.haystack[..pos], &self.haystack[pos + 1..]),
                    None => (self.haystack, &[][..]),
                };
                self.haystack = remaining;
                Some(ret)
            }
        }

        Split { haystack, needle }
    }