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    use crate::core::source::{MaybePackage, QueryKind, Source, SourceId};
    use crate::core::GitReference;
    use crate::core::{Dependency, Package, PackageId, Summary};
    use crate::sources::git::utils::GitRemote;
    use crate::sources::PathSource;
    use crate::util::errors::CargoResult;
    use crate::util::hex::short_hash;
    use crate::util::Config;
    use anyhow::Context;
    use cargo_util::paths::exclude_from_backups_and_indexing;
    use log::trace;
    use std::fmt::{self, Debug, Formatter};
    use std::task::Poll;
    use url::Url;

    pub struct GitSource<'cfg> {
        remote: GitRemote,
        manifest_reference: GitReference,
        locked_rev: Option<git2::Oid>,
        source_id: SourceId,
        path_source: Option<PathSource<'cfg>>,
        ident: String,
        config: &'cfg Config,
        quiet: bool,
    }

    impl<'cfg> GitSource<'cfg> {
        pub fn new(source_id: SourceId, config: &'cfg Config) -> CargoResult<GitSource<'cfg>> {
            assert!(source_id.is_git(), "id is not git, id={}", source_id);

            let remote = GitRemote::new(source_id.url());
            let ident = ident(&source_id);

            let source = GitSource {
                remote,
                manifest_reference: source_id.git_reference().unwrap().clone(),
                locked_rev: match source_id.precise() {
                    Some(s) => Some(git2::Oid::from_str(s).with_context(|| {
                        format!("precise value for git is not a git revision: {}", s)
                    })?),
                    None => None,
                },
                source_id,
                path_source: None,
                ident,
                config,
                quiet: false,
            };

            Ok(source)
        }

        pub fn url(&self) -> &Url {
            self.remote.url()
        }

        pub fn read_packages(&mut self) -> CargoResult<Vec<Package>> {
            if self.path_source.is_none() {
                self.invalidate_cache();
                self.block_until_ready()?;
            }
            self.path_source.as_mut().unwrap().read_packages()
        }
    }

    fn ident(id: &SourceId) -> String {
        let ident = id
            .canonical_url()
            .raw_canonicalized_url()
            .path_segments()
            .and_then(|s| s.rev().next())
            .unwrap_or("");

        let ident = if ident.is_empty() { "_empty" } else { ident };

        format!("{}-{}", ident, short_hash(id.canonical_url()))
    }

    impl<'cfg> Debug for GitSource<'cfg> {
        fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
            write!(f, "git repo at {}", self.remote.url())?;

            match self.manifest_reference.pretty_ref() {
                Some(s) => write!(f, " ({})", s),
                None => Ok(()),
            }
        }
    }

    impl<'cfg> Source for GitSource<'cfg> {
        fn query(
            &mut self,
            dep: &Dependency,
            kind: QueryKind,
            f: &mut dyn FnMut(Summary),
        ) -> Poll<CargoResult<()>> {
            if let Some(src) = self.path_source.as_mut() {
                src.query(dep, kind, f)
            } else {
                Poll::Pending
            }
        }

        fn supports_checksums(&self) -> bool {
            false
        }

        fn requires_precise(&self) -> bool {
            true
        }

        fn source_id(&self) -> SourceId {
            self.source_id
        }

        fn block_until_ready(&mut self) -> CargoResult<()> {
            if self.path_source.is_some() {
                return Ok(());
            }

            let git_fs = self.config.git_path();
            // Ignore errors creating it, in case this is a read-only filesystem:
            // perhaps the later operations can succeed anyhow.
            let _ = git_fs.create_dir();
            let git_path = self.config.assert_package_cache_locked(&git_fs);

            // Before getting a checkout, make sure that `<cargo_home>/git` is
            // marked as excluded from indexing and backups. Older versions of Cargo
            // didn't do this, so we do it here regardless of whether `<cargo_home>`
            // exists.
            //
            // This does not use `create_dir_all_excluded_from_backups_atomic` for
            // the same reason: we want to exclude it even if the directory already
            // exists.
            exclude_from_backups_and_indexing(&git_path);

            let db_path = git_path.join("db").join(&self.ident);

            let db = self.remote.db_at(&db_path).ok();
            let (db, actual_rev) = match (self.locked_rev, db) {
                // If we have a locked revision, and we have a preexisting database
                // which has that revision, then no update needs to happen.
                (Some(rev), Some(db)) if db.contains(rev) => (db, rev),

                // If we're in offline mode, we're not locked, and we have a
                // database, then try to resolve our reference with the preexisting
                // repository.
                (None, Some(db)) if self.config.offline() => {
                    let rev = db.resolve(&self.manifest_reference).with_context(|| {
                        "failed to lookup reference in preexisting repository, and \
                             can't check for updates in offline mode (--offline)"
                    })?;
                    (db, rev)
                }

                // ... otherwise we use this state to update the git database. Note
                // that we still check for being offline here, for example in the
                // situation that we have a locked revision but the database
                // doesn't have it.
                (locked_rev, db) => {
                    if self.config.offline() {
                        anyhow::bail!(
                            "can't checkout from '{}': you are in the offline mode (--offline)",
                            self.remote.url()
                        );
                    }
                    if !self.quiet {
                        self.config.shell().status(
                            "Updating",
                            format!("git repository `{}`", self.remote.url()),
                        )?;
                    }

                    trace!("updating git source `{:?}`", self.remote);

                    self.remote.checkout(
                        &db_path,
                        db,
                        &self.manifest_reference,
                        locked_rev,
                        self.config,
                    )?
                }
            };

            // Don’t use the full hash, in order to contribute less to reaching the
            // path length limit on Windows. See
            // <https://github.com/servo/servo/pull/14397>.
            let short_id = db.to_short_id(actual_rev)?;

            // Check out `actual_rev` from the database to a scoped location on the
            // filesystem. This will use hard links and such to ideally make the
            // checkout operation here pretty fast.
            let checkout_path = git_path
                .join("checkouts")
                .join(&self.ident)
                .join(short_id.as_str());
            let parent_remote_url = self.url();
            db.copy_to(actual_rev, &checkout_path, self.config, parent_remote_url)?;

            let source_id = self.source_id.with_precise(Some(actual_rev.to_string()));
            let path_source = PathSource::new_recursive(&checkout_path, source_id, self.config);

            self.path_source = Some(path_source);
            self.locked_rev = Some(actual_rev);
            self.path_source.as_mut().unwrap().update()
        }

        fn download(&mut self, id: PackageId) -> CargoResult<MaybePackage> {
            trace!(
                "getting packages for package ID `{}` from `{:?}`",
                id,
                self.remote
            );
            self.path_source
                .as_mut()
                .expect("BUG: `update()` must be called before `get()`")
                .download(id)
        }

        fn finish_download(&mut self, _id: PackageId, _data: Vec<u8>) -> CargoResult<Package> {
            panic!("no download should have started")
        }

        fn fingerprint(&self, _pkg: &Package) -> CargoResult<String> {
            Ok(self.locked_rev.as_ref().unwrap().to_string())
        }

        fn describe(&self) -> String {
            format!("Git repository {}", self.source_id)
        }

        fn add_to_yanked_whitelist(&mut self, _pkgs: &[PackageId]) {}

        fn is_yanked(&mut self, _pkg: PackageId) -> Poll<CargoResult<bool>> {
            Poll::Ready(Ok(false))
        }

        fn invalidate_cache(&mut self) {}

        fn set_quiet(&mut self, quiet: bool) {
            self.quiet = quiet;
        }
    }

    #[cfg(test)]
    mod test {
        use super::ident;
        use crate::core::{GitReference, SourceId};
        use crate::util::IntoUrl;

        #[test]
        pub fn test_url_to_path_ident_with_path() {
            let ident = ident(&src("https://github.com/carlhuda/cargo"));
            assert!(ident.starts_with("cargo-"));
        }

        #[test]
        pub fn test_url_to_path_ident_without_path() {
            let ident = ident(&src("https://github.com"));
            assert!(ident.starts_with("_empty-"));
        }

        #[test]
        fn test_canonicalize_idents_by_stripping_trailing_url_slash() {
            let ident1 = ident(&src("https://github.com/PistonDevelopers/piston/"));
            let ident2 = ident(&src("https://github.com/PistonDevelopers/piston"));
            assert_eq!(ident1, ident2);
        }

        #[test]
        fn test_canonicalize_idents_by_lowercasing_github_urls() {
            let ident1 = ident(&src("https://github.com/PistonDevelopers/piston"));
            let ident2 = ident(&src("https://github.com/pistondevelopers/piston"));
            assert_eq!(ident1, ident2);
        }

        #[test]
        fn test_canonicalize_idents_by_stripping_dot_git() {
            let ident1 = ident(&src("https://github.com/PistonDevelopers/piston"));
            let ident2 = ident(&src("https://github.com/PistonDevelopers/piston.git"));
            assert_eq!(ident1, ident2);
        }

        #[test]
        fn test_canonicalize_idents_different_protocols() {
            let ident1 = ident(&src("https://github.com/PistonDevelopers/piston"));
            let ident2 = ident(&src("git://github.com/PistonDevelopers/piston"));
            assert_eq!(ident1, ident2);
        }

        fn src(s: &str) -> SourceId {
            SourceId::for_git(&s.into_url().unwrap(), GitReference::DefaultBranch).unwrap()
        }
    }