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    //! A layer of on-disk index cache for 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, aka
    //! [`IndexPackage`]. To learn about the versions in each JSON blob we would
    //! need to parse the JSON via [`IndexSummary::parse`], 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 [`RemoteRegistry`] 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, representing as [`SummariesCache`]) 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.
    //!
    //! [`Dependency`]: crate::core::Dependency
    //! [`IndexPackage`]: super::IndexPackage
    //! [`IndexSummary::parse`]: super::IndexSummary::parse
    //! [`RemoteRegistry`]: crate::sources::registry::remote::RemoteRegistry

    use std::fs;
    use std::io;
    use std::path::PathBuf;
    use std::str;

    use anyhow::bail;
    use cargo_util::registry::make_dep_path;
    use semver::Version;

    use crate::util::cache_lock::CacheLockMode;
    use crate::util::Filesystem;
    use crate::CargoResult;
    use crate::GlobalContext;

    use super::split;
    use super::INDEX_V_MAX;

    /// The current version of [`SummariesCache`].
    const CURRENT_CACHE_VERSION: u8 = 3;

    /// 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.
    ///
    /// For serialization and deserialization of this on-disk index cache of
    /// summaries, see [`SummariesCache::serialize`]  and [`SummariesCache::parse`].
    ///
    /// # The format of the index cache
    ///
    /// The idea of this format is that it's a very easy file for Cargo to parse in
    /// future invocations. The read from disk should be fast and then afterwards
    /// all we need to know is what versions correspond to which JSON blob.
    ///
    /// Currently the format looks like:
    ///
    /// ```text
    /// +---------------+----------------------+--------------------+---+
    /// | cache version | index schema version | index file version | 0 |
    /// +---------------+----------------------+--------------------+---+
    /// ```
    ///
    /// followed by one or more (version + JSON blob) pairs...
    ///
    /// ```text
    /// +----------------+---+-----------+---+
    /// | semver version | 0 | JSON blob | 0 | ...
    /// +----------------+---+-----------+---+
    /// ```
    ///
    /// Each field represents:
    ///
    /// * _cache version_ --- 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.
    /// * _index schema version_ --- The schema version of the raw index file.
    ///   See [`IndexPackage::v`] for the detail.
    /// * _index file version_ --- Tracks when a cache needs to be regenerated.
    ///   A cache regeneration is required whenever the index file itself updates.
    /// * _semver version_ --- The version for each JSON blob. Extracted from the
    ///   blob for fast queries without parsing the entire blob.
    /// * _JSON blob_ --- The actual metadata for each version of the package. It
    ///   has the same representation as [`IndexPackage`].
    ///
    /// # Changes between each cache version
    ///
    /// * `1`: The original version.
    /// * `2`: Added the "index schema version" field so that if the index schema
    ///   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.
    ///
    /// See [`CURRENT_CACHE_VERSION`] for the current cache version.
    ///
    /// [`IndexPackage::v`]: super::IndexPackage::v
    /// [`IndexPackage`]: super::IndexPackage
    #[derive(Default)]
    pub struct SummariesCache<'a> {
        /// JSON blobs of the summaries. Each JSON blob has a [`Version`] beside,
        /// so that Cargo can query a version without full JSON parsing.
        pub versions: Vec<(Version, &'a [u8])>,
        /// For cache invalidation, we tracks the index file version to determine
        /// when to regenerate the cache itself.
        pub index_version: &'a str,
    }

    impl<'a> SummariesCache<'a> {
        /// Deserializes an on-disk cache.
        pub 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 schema version"))?;
            let index_v = u32::from_le_bytes(index_v_bytes.try_into().unwrap());
            if index_v != INDEX_V_MAX {
                bail!(
                    "index schema version {index_v} doesn't match the version I know ({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)
        }

        /// Serializes itself with a given `index_version`.
        pub 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
        }
    }

    /// Manages the on-disk index caches.
    pub struct CacheManager<'gctx> {
        /// The root path where caches are located.
        cache_root: Filesystem,
        /// [`GlobalContext`] reference for convenience.
        gctx: &'gctx GlobalContext,
    }

    impl<'gctx> CacheManager<'gctx> {
        /// Creates a new instance of the on-disk index cache manager.
        ///
        /// `root` --- The root path where caches are located.
        pub fn new(cache_root: Filesystem, gctx: &'gctx GlobalContext) -> CacheManager<'gctx> {
            CacheManager { cache_root, gctx }
        }

        /// Gets the cache associated with the key.
        pub fn get(&self, key: &str) -> Option<Vec<u8>> {
            let cache_path = &self.cache_path(key);
            match fs::read(cache_path) {
                Ok(contents) => Some(contents),
                Err(e) => {
                    tracing::debug!(?cache_path, "cache missing: {e}");
                    None
                }
            }
        }

        /// Associates the value with the key.
        pub fn put(&self, key: &str, value: &[u8]) {
            let cache_path = &self.cache_path(key);
            if fs::create_dir_all(cache_path.parent().unwrap()).is_ok() {
                let path = Filesystem::new(cache_path.clone());
                self.gctx
                    .assert_package_cache_locked(CacheLockMode::DownloadExclusive, &path);
                if let Err(e) = fs::write(cache_path, value) {
                    tracing::info!(?cache_path, "failed to write cache: {e}");
                }
            }
        }

        /// Invalidates the cache associated with the key.
        pub fn invalidate(&self, key: &str) {
            let cache_path = &self.cache_path(key);
            if let Err(e) = fs::remove_file(cache_path) {
                if e.kind() != io::ErrorKind::NotFound {
                    tracing::debug!(?cache_path, "failed to remove from cache: {e}");
                }
            }
        }

        fn cache_path(&self, key: &str) -> PathBuf {
            let relative = make_dep_path(key, false);
            // This is the file we're loading from cache or the index data.
            // See module comment in `registry/mod.rs` for why this is structured
            // the way it is.
            self.cache_root.join(relative).into_path_unlocked()
        }
    }