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    use std::cell::{Cell, Ref, RefCell, RefMut};
    use std::cmp::Ordering;
    use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
    use std::fmt;
    use std::hash;
    use std::mem;
    use std::path::{Path, PathBuf};
    use std::rc::Rc;
    use std::time::{Duration, Instant};

    use anyhow::Context;
    use bytesize::ByteSize;
    use curl::easy::Easy;
    use curl::multi::{EasyHandle, Multi};
    use lazycell::LazyCell;
    use semver::Version;
    use serde::Serialize;
    use tracing::debug;

    use crate::core::compiler::{CompileKind, RustcTargetData};
    use crate::core::dependency::DepKind;
    use crate::core::resolver::features::ForceAllTargets;
    use crate::core::resolver::{HasDevUnits, Resolve};
    use crate::core::{Dependency, Manifest, PackageId, SourceId, Target};
    use crate::core::{Summary, Workspace};
    use crate::sources::source::{MaybePackage, SourceMap};
    use crate::util::cache_lock::{CacheLock, CacheLockMode};
    use crate::util::errors::{CargoResult, HttpNotSuccessful};
    use crate::util::interning::InternedString;
    use crate::util::network::http::http_handle_and_timeout;
    use crate::util::network::http::HttpTimeout;
    use crate::util::network::retry::{Retry, RetryResult};
    use crate::util::network::sleep::SleepTracker;
    use crate::util::toml::prepare_for_publish;
    use crate::util::{self, internal, Config, Progress, ProgressStyle};
    use crate::util_schemas::manifest::RustVersion;

    pub const MANIFEST_PREAMBLE: &str = "\
    # THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
    #
    # When uploading crates to the registry Cargo will automatically
    # \"normalize\" Cargo.toml files for maximal compatibility
    # with all versions of Cargo and also rewrite `path` dependencies
    # to registry (e.g., crates.io) dependencies.
    #
    # If you are reading this file be aware that the original Cargo.toml
    # will likely look very different (and much more reasonable).
    # See Cargo.toml.orig for the original contents.
    ";

    /// Information about a package that is available somewhere in the file system.
    ///
    /// A package is a `Cargo.toml` file plus all the files that are part of it.
    #[derive(Clone)]
    pub struct Package {
        inner: Rc<PackageInner>,
    }

    #[derive(Clone)]
    // TODO: is `manifest_path` a relic?
    struct PackageInner {
        /// The package's manifest.
        manifest: Manifest,
        /// The root of the package.
        manifest_path: PathBuf,
    }

    impl Ord for Package {
        fn cmp(&self, other: &Package) -> Ordering {
            self.package_id().cmp(&other.package_id())
        }
    }

    impl PartialOrd for Package {
        fn partial_cmp(&self, other: &Package) -> Option<Ordering> {
            Some(self.cmp(other))
        }
    }

    /// A Package in a form where `Serialize` can be derived.
    #[derive(Serialize)]
    pub struct SerializedPackage {
        name: InternedString,
        version: Version,
        id: PackageId,
        license: Option<String>,
        license_file: Option<String>,
        description: Option<String>,
        source: SourceId,
        dependencies: Vec<Dependency>,
        targets: Vec<Target>,
        features: BTreeMap<InternedString, Vec<InternedString>>,
        manifest_path: PathBuf,
        metadata: Option<toml::Value>,
        publish: Option<Vec<String>>,
        authors: Vec<String>,
        categories: Vec<String>,
        keywords: Vec<String>,
        readme: Option<String>,
        repository: Option<String>,
        homepage: Option<String>,
        documentation: Option<String>,
        edition: String,
        links: Option<String>,
        #[serde(skip_serializing_if = "Option::is_none")]
        metabuild: Option<Vec<String>>,
        default_run: Option<String>,
        rust_version: Option<RustVersion>,
    }

    impl Package {
        /// Creates a package from a manifest and its location.
        pub fn new(manifest: Manifest, manifest_path: &Path) -> Package {
            Package {
                inner: Rc::new(PackageInner {
                    manifest,
                    manifest_path: manifest_path.to_path_buf(),
                }),
            }
        }

        /// Gets the manifest dependencies.
        pub fn dependencies(&self) -> &[Dependency] {
            self.manifest().dependencies()
        }
        /// Gets the manifest.
        pub fn manifest(&self) -> &Manifest {
            &self.inner.manifest
        }
        /// Gets the manifest.
        pub fn manifest_mut(&mut self) -> &mut Manifest {
            &mut Rc::make_mut(&mut self.inner).manifest
        }
        /// Gets the path to the manifest.
        pub fn manifest_path(&self) -> &Path {
            &self.inner.manifest_path
        }
        /// Gets the name of the package.
        pub fn name(&self) -> InternedString {
            self.package_id().name()
        }
        /// Gets the `PackageId` object for the package (fully defines a package).
        pub fn package_id(&self) -> PackageId {
            self.manifest().package_id()
        }
        /// Gets the root folder of the package.
        pub fn root(&self) -> &Path {
            self.manifest_path().parent().unwrap()
        }
        /// Gets the summary for the package.
        pub fn summary(&self) -> &Summary {
            self.manifest().summary()
        }
        /// Gets the targets specified in the manifest.
        pub fn targets(&self) -> &[Target] {
            self.manifest().targets()
        }
        /// Gets the library crate for this package, if it exists.
        pub fn library(&self) -> Option<&Target> {
            self.targets().iter().find(|t| t.is_lib())
        }
        /// Gets the current package version.
        pub fn version(&self) -> &Version {
            self.package_id().version()
        }
        /// Gets the package authors.
        pub fn authors(&self) -> &Vec<String> {
            &self.manifest().metadata().authors
        }

        /// Returns `None` if the package is set to publish.
        /// Returns `Some(allowed_registries)` if publishing is limited to specified
        /// registries or if package is set to not publish.
        pub fn publish(&self) -> &Option<Vec<String>> {
            self.manifest().publish()
        }
        /// Returns `true` if this package is a proc-macro.
        pub fn proc_macro(&self) -> bool {
            self.targets().iter().any(|target| target.proc_macro())
        }
        /// Gets the package's minimum Rust version.
        pub fn rust_version(&self) -> Option<&RustVersion> {
            self.manifest().rust_version()
        }

        /// Returns `true` if the package uses a custom build script for any target.
        pub fn has_custom_build(&self) -> bool {
            self.targets().iter().any(|t| t.is_custom_build())
        }

        pub fn map_source(self, to_replace: SourceId, replace_with: SourceId) -> Package {
            Package {
                inner: Rc::new(PackageInner {
                    manifest: self.manifest().clone().map_source(to_replace, replace_with),
                    manifest_path: self.manifest_path().to_owned(),
                }),
            }
        }

        pub fn to_registry_toml(&self, ws: &Workspace<'_>) -> CargoResult<String> {
            let manifest = prepare_for_publish(self.manifest().original(), ws, self.root())?;
            let toml = toml::to_string_pretty(&manifest)?;
            Ok(format!("{}\n{}", MANIFEST_PREAMBLE, toml))
        }

        /// Returns if package should include `Cargo.lock`.
        pub fn include_lockfile(&self) -> bool {
            self.targets().iter().any(|t| t.is_example() || t.is_bin())
        }

        pub fn serialized(&self) -> SerializedPackage {
            let summary = self.manifest().summary();
            let package_id = summary.package_id();
            let manmeta = self.manifest().metadata();
            // Filter out metabuild targets. They are an internal implementation
            // detail that is probably not relevant externally. There's also not a
            // real path to show in `src_path`, and this avoids changing the format.
            let targets: Vec<Target> = self
                .manifest()
                .targets()
                .iter()
                .filter(|t| t.src_path().is_path())
                .cloned()
                .collect();
            // Convert Vec<FeatureValue> to Vec<InternedString>
            let features = summary
                .features()
                .iter()
                .map(|(k, v)| {
                    (
                        *k,
                        v.iter()
                            .map(|fv| InternedString::new(&fv.to_string()))
                            .collect(),
                    )
                })
                .collect();

            SerializedPackage {
                name: package_id.name(),
                version: package_id.version().clone(),
                id: package_id,
                license: manmeta.license.clone(),
                license_file: manmeta.license_file.clone(),
                description: manmeta.description.clone(),
                source: summary.source_id(),
                dependencies: summary.dependencies().to_vec(),
                targets,
                features,
                manifest_path: self.manifest_path().to_path_buf(),
                metadata: self.manifest().custom_metadata().cloned(),
                authors: manmeta.authors.clone(),
                categories: manmeta.categories.clone(),
                keywords: manmeta.keywords.clone(),
                readme: manmeta.readme.clone(),
                repository: manmeta.repository.clone(),
                homepage: manmeta.homepage.clone(),
                documentation: manmeta.documentation.clone(),
                edition: self.manifest().edition().to_string(),
                links: self.manifest().links().map(|s| s.to_owned()),
                metabuild: self.manifest().metabuild().cloned(),
                publish: self.publish().as_ref().cloned(),
                default_run: self.manifest().default_run().map(|s| s.to_owned()),
                rust_version: self.rust_version().cloned(),
            }
        }
    }

    impl fmt::Display for Package {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "{}", self.summary().package_id())
        }
    }

    impl fmt::Debug for Package {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Package")
                .field("id", &self.summary().package_id())
                .field("..", &"..")
                .finish()
        }
    }

    impl PartialEq for Package {
        fn eq(&self, other: &Package) -> bool {
            self.package_id() == other.package_id()
        }
    }

    impl Eq for Package {}

    impl hash::Hash for Package {
        fn hash<H: hash::Hasher>(&self, into: &mut H) {
            self.package_id().hash(into)
        }
    }

    /// A set of packages, with the intent to download.
    ///
    /// This is primarily used to convert a set of `PackageId`s to `Package`s. It
    /// will download as needed, or used the cached download if available.
    pub struct PackageSet<'cfg> {
        packages: HashMap<PackageId, LazyCell<Package>>,
        sources: RefCell<SourceMap<'cfg>>,
        config: &'cfg Config,
        multi: Multi,
        /// Used to prevent reusing the PackageSet to download twice.
        downloading: Cell<bool>,
        /// Whether or not to use curl HTTP/2 multiplexing.
        multiplexing: bool,
    }

    /// Helper for downloading crates.
    pub struct Downloads<'a, 'cfg> {
        set: &'a PackageSet<'cfg>,
        /// When a download is started, it is added to this map. The key is a
        /// "token" (see `Download::token`). It is removed once the download is
        /// finished.
        pending: HashMap<usize, (Download<'cfg>, EasyHandle)>,
        /// Set of packages currently being downloaded. This should stay in sync
        /// with `pending`.
        pending_ids: HashSet<PackageId>,
        /// Downloads that have failed and are waiting to retry again later.
        sleeping: SleepTracker<(Download<'cfg>, Easy)>,
        /// The final result of each download. A pair `(token, result)`. This is a
        /// temporary holding area, needed because curl can report multiple
        /// downloads at once, but the main loop (`wait`) is written to only
        /// handle one at a time.
        results: Vec<(usize, Result<(), curl::Error>)>,
        /// The next ID to use for creating a token (see `Download::token`).
        next: usize,
        /// Progress bar.
        progress: RefCell<Option<Progress<'cfg>>>,
        /// Number of downloads that have successfully finished.
        downloads_finished: usize,
        /// Total bytes for all successfully downloaded packages.
        downloaded_bytes: u64,
        /// Size (in bytes) and package name of the largest downloaded package.
        largest: (u64, InternedString),
        /// Time when downloading started.
        start: Instant,
        /// Indicates *all* downloads were successful.
        success: bool,

        /// Timeout management, both of timeout thresholds as well as whether or not
        /// our connection has timed out (and accompanying message if it has).
        ///
        /// Note that timeout management is done manually here instead of in libcurl
        /// because we want to apply timeouts to an entire batch of operations, not
        /// any one particular single operation.
        timeout: HttpTimeout,
        /// Last time bytes were received.
        updated_at: Cell<Instant>,
        /// This is a slow-speed check. It is reset to `now + timeout_duration`
        /// every time at least `threshold` bytes are received. If the current
        /// time ever exceeds `next_speed_check`, then give up and report a
        /// timeout error.
        next_speed_check: Cell<Instant>,
        /// This is the slow-speed threshold byte count. It starts at the
        /// configured threshold value (default 10), and is decremented by the
        /// number of bytes received in each chunk. If it is <= zero, the
        /// threshold has been met and data is being received fast enough not to
        /// trigger a timeout; reset `next_speed_check` and set this back to the
        /// configured threshold.
        next_speed_check_bytes_threshold: Cell<u64>,
        /// Global filesystem lock to ensure only one Cargo is downloading at a
        /// time.
        _lock: CacheLock<'cfg>,
    }

    struct Download<'cfg> {
        /// The token for this download, used as the key of the `Downloads::pending` map
        /// and stored in `EasyHandle` as well.
        token: usize,

        /// The package that we're downloading.
        id: PackageId,

        /// Actual downloaded data, updated throughout the lifetime of this download.
        data: RefCell<Vec<u8>>,

        /// HTTP headers for debugging.
        headers: RefCell<Vec<String>>,

        /// The URL that we're downloading from, cached here for error messages and
        /// reenqueuing.
        url: String,

        /// A descriptive string to print when we've finished downloading this crate.
        descriptor: String,

        /// Statistics updated from the progress callback in libcurl.
        total: Cell<u64>,
        current: Cell<u64>,

        /// The moment we started this transfer at.
        start: Instant,
        timed_out: Cell<Option<String>>,

        /// Logic used to track retrying this download if it's a spurious failure.
        retry: Retry<'cfg>,
    }

    impl<'cfg> PackageSet<'cfg> {
        pub fn new(
            package_ids: &[PackageId],
            sources: SourceMap<'cfg>,
            config: &'cfg Config,
        ) -> CargoResult<PackageSet<'cfg>> {
            // We've enabled the `http2` feature of `curl` in Cargo, so treat
            // failures here as fatal as it would indicate a build-time problem.
            let mut multi = Multi::new();
            let multiplexing = config.http_config()?.multiplexing.unwrap_or(true);
            multi
                .pipelining(false, multiplexing)
                .with_context(|| "failed to enable multiplexing/pipelining in curl")?;

            // let's not flood crates.io with connections
            multi.set_max_host_connections(2)?;

            Ok(PackageSet {
                packages: package_ids
                    .iter()
                    .map(|&id| (id, LazyCell::new()))
                    .collect(),
                sources: RefCell::new(sources),
                config,
                multi,
                downloading: Cell::new(false),
                multiplexing,
            })
        }

        pub fn package_ids(&self) -> impl Iterator<Item = PackageId> + '_ {
            self.packages.keys().cloned()
        }

        pub fn packages(&self) -> impl Iterator<Item = &Package> {
            self.packages.values().filter_map(|p| p.borrow())
        }

        pub fn enable_download<'a>(&'a self) -> CargoResult<Downloads<'a, 'cfg>> {
            assert!(!self.downloading.replace(true));
            let timeout = HttpTimeout::new(self.config)?;
            Ok(Downloads {
                start: Instant::now(),
                set: self,
                next: 0,
                pending: HashMap::new(),
                pending_ids: HashSet::new(),
                sleeping: SleepTracker::new(),
                results: Vec::new(),
                progress: RefCell::new(Some(Progress::with_style(
                    "Downloading",
                    ProgressStyle::Ratio,
                    self.config,
                ))),
                downloads_finished: 0,
                downloaded_bytes: 0,
                largest: (0, InternedString::new("")),
                success: false,
                updated_at: Cell::new(Instant::now()),
                timeout,
                next_speed_check: Cell::new(Instant::now()),
                next_speed_check_bytes_threshold: Cell::new(0),
                _lock: self
                    .config
                    .acquire_package_cache_lock(CacheLockMode::DownloadExclusive)?,
            })
        }

        pub fn get_one(&self, id: PackageId) -> CargoResult<&Package> {
            if let Some(pkg) = self.packages.get(&id).and_then(|slot| slot.borrow()) {
                return Ok(pkg);
            }
            Ok(self.get_many(Some(id))?.remove(0))
        }

        pub fn get_many(&self, ids: impl IntoIterator<Item = PackageId>) -> CargoResult<Vec<&Package>> {
            let mut pkgs = Vec::new();
            let _lock = self
                .config
                .acquire_package_cache_lock(CacheLockMode::DownloadExclusive)?;
            let mut downloads = self.enable_download()?;
            for id in ids {
                pkgs.extend(downloads.start(id)?);
            }
            while downloads.remaining() > 0 {
                pkgs.push(downloads.wait()?);
            }
            downloads.success = true;
            drop(downloads);

            let mut deferred = self.config.deferred_global_last_use()?;
            deferred.save_no_error(self.config);
            Ok(pkgs)
        }

        /// Downloads any packages accessible from the give root ids.
        pub fn download_accessible(
            &self,
            resolve: &Resolve,
            root_ids: &[PackageId],
            has_dev_units: HasDevUnits,
            requested_kinds: &[CompileKind],
            target_data: &RustcTargetData<'cfg>,
            force_all_targets: ForceAllTargets,
        ) -> CargoResult<()> {
            fn collect_used_deps(
                used: &mut BTreeSet<PackageId>,
                resolve: &Resolve,
                pkg_id: PackageId,
                has_dev_units: HasDevUnits,
                requested_kinds: &[CompileKind],
                target_data: &RustcTargetData<'_>,
                force_all_targets: ForceAllTargets,
            ) -> CargoResult<()> {
                if !used.insert(pkg_id) {
                    return Ok(());
                }
                let filtered_deps = PackageSet::filter_deps(
                    pkg_id,
                    resolve,
                    has_dev_units,
                    requested_kinds,
                    target_data,
                    force_all_targets,
                );
                for (pkg_id, _dep) in filtered_deps {
                    collect_used_deps(
                        used,
                        resolve,
                        pkg_id,
                        has_dev_units,
                        requested_kinds,
                        target_data,
                        force_all_targets,
                    )?;
                }
                Ok(())
            }

            // This is sorted by PackageId to get consistent behavior and error
            // messages for Cargo's testsuite. Perhaps there is a better ordering
            // that optimizes download time?
            let mut to_download = BTreeSet::new();

            for id in root_ids {
                collect_used_deps(
                    &mut to_download,
                    resolve,
                    *id,
                    has_dev_units,
                    requested_kinds,
                    target_data,
                    force_all_targets,
                )?;
            }
            self.get_many(to_download.into_iter())?;
            Ok(())
        }

        /// Check if there are any dependency packages that violate artifact constraints
        /// to instantly abort, or that do not have any libs which results in warnings.
        pub(crate) fn warn_no_lib_packages_and_artifact_libs_overlapping_deps(
            &self,
            ws: &Workspace<'cfg>,
            resolve: &Resolve,
            root_ids: &[PackageId],
            has_dev_units: HasDevUnits,
            requested_kinds: &[CompileKind],
            target_data: &RustcTargetData<'_>,
            force_all_targets: ForceAllTargets,
        ) -> CargoResult<()> {
            let no_lib_pkgs: BTreeMap<PackageId, Vec<(&Package, &HashSet<Dependency>)>> = root_ids
                .iter()
                .map(|&root_id| {
                    let dep_pkgs_to_deps: Vec<_> = PackageSet::filter_deps(
                        root_id,
                        resolve,
                        has_dev_units,
                        requested_kinds,
                        target_data,
                        force_all_targets,
                    )
                    .collect();

                    let dep_pkgs_and_deps = dep_pkgs_to_deps
                        .into_iter()
                        .filter(|(_id, deps)| deps.iter().any(|dep| dep.maybe_lib()))
                        .filter_map(|(dep_package_id, deps)| {
                            self.get_one(dep_package_id).ok().and_then(|dep_pkg| {
                                (!dep_pkg.targets().iter().any(|t| t.is_lib())).then(|| (dep_pkg, deps))
                            })
                        })
                        .collect();
                    (root_id, dep_pkgs_and_deps)
                })
                .collect();

            for (pkg_id, dep_pkgs) in no_lib_pkgs {
                for (_dep_pkg_without_lib_target, deps) in dep_pkgs {
                    for dep in deps.iter().filter(|dep| {
                        dep.artifact()
                            .map(|artifact| artifact.is_lib())
                            .unwrap_or(true)
                    }) {
                        ws.config().shell().warn(&format!(
                            "{} ignoring invalid dependency `{}` which is missing a lib target",
                            pkg_id,
                            dep.name_in_toml(),
                        ))?;
                    }
                }
            }
            Ok(())
        }

        fn filter_deps<'a>(
            pkg_id: PackageId,
            resolve: &'a Resolve,
            has_dev_units: HasDevUnits,
            requested_kinds: &'a [CompileKind],
            target_data: &'a RustcTargetData<'_>,
            force_all_targets: ForceAllTargets,
        ) -> impl Iterator<Item = (PackageId, &'a HashSet<Dependency>)> + 'a {
            resolve
                .deps(pkg_id)
                .filter(move |&(_id, deps)| {
                    deps.iter().any(|dep| {
                        if dep.kind() == DepKind::Development && has_dev_units == HasDevUnits::No {
                            return false;
                        }
                        if force_all_targets == ForceAllTargets::No {
                            let activated = requested_kinds
                                .iter()
                                .chain(Some(&CompileKind::Host))
                                .any(|kind| target_data.dep_platform_activated(dep, *kind));
                            if !activated {
                                return false;
                            }
                        }
                        true
                    })
                })
                .into_iter()
        }

        pub fn sources(&self) -> Ref<'_, SourceMap<'cfg>> {
            self.sources.borrow()
        }

        pub fn sources_mut(&self) -> RefMut<'_, SourceMap<'cfg>> {
            self.sources.borrow_mut()
        }

        /// Merge the given set into self.
        pub fn add_set(&mut self, set: PackageSet<'cfg>) {
            assert!(!self.downloading.get());
            assert!(!set.downloading.get());
            for (pkg_id, p_cell) in set.packages {
                self.packages.entry(pkg_id).or_insert(p_cell);
            }
            let mut sources = self.sources.borrow_mut();
            let other_sources = set.sources.into_inner();
            sources.add_source_map(other_sources);
        }
    }

    impl<'a, 'cfg> Downloads<'a, 'cfg> {
        /// Starts to download the package for the `id` specified.
        ///
        /// Returns `None` if the package is queued up for download and will
        /// eventually be returned from `wait_for_download`. Returns `Some(pkg)` if
        /// the package is ready and doesn't need to be downloaded.
        pub fn start(&mut self, id: PackageId) -> CargoResult<Option<&'a Package>> {
            self.start_inner(id)
                .with_context(|| format!("failed to download `{}`", id))
        }

        fn start_inner(&mut self, id: PackageId) -> CargoResult<Option<&'a Package>> {
            // First up see if we've already cached this package, in which case
            // there's nothing to do.
            let slot = self
                .set
                .packages
                .get(&id)
                .ok_or_else(|| internal(format!("couldn't find `{}` in package set", id)))?;
            if let Some(pkg) = slot.borrow() {
                return Ok(Some(pkg));
            }

            // Ask the original source for this `PackageId` for the corresponding
            // package. That may immediately come back and tell us that the package
            // is ready, or it could tell us that it needs to be downloaded.
            let mut sources = self.set.sources.borrow_mut();
            let source = sources
                .get_mut(id.source_id())
                .ok_or_else(|| internal(format!("couldn't find source for `{}`", id)))?;
            let pkg = source
                .download(id)
                .with_context(|| "unable to get packages from source")?;
            let (url, descriptor, authorization) = match pkg {
                MaybePackage::Ready(pkg) => {
                    debug!("{} doesn't need a download", id);
                    assert!(slot.fill(pkg).is_ok());
                    return Ok(Some(slot.borrow().unwrap()));
                }
                MaybePackage::Download {
                    url,
                    descriptor,
                    authorization,
                } => (url, descriptor, authorization),
            };

            // Ok we're going to download this crate, so let's set up all our
            // internal state and hand off an `Easy` handle to our libcurl `Multi`
            // handle. This won't actually start the transfer, but later it'll
            // happen during `wait_for_download`
            let token = self.next;
            self.next += 1;
            debug!(target: "network", "downloading {} as {}", id, token);
            assert!(self.pending_ids.insert(id));

            let (mut handle, _timeout) = http_handle_and_timeout(self.set.config)?;
            handle.get(true)?;
            handle.url(&url)?;
            handle.follow_location(true)?; // follow redirects

            // Add authorization header.
            if let Some(authorization) = authorization {
                let mut headers = curl::easy::List::new();
                headers.append(&format!("Authorization: {}", authorization))?;
                handle.http_headers(headers)?;
            }

            // Enable HTTP/2 if possible.
            crate::try_old_curl_http2_pipewait!(self.set.multiplexing, handle);

            handle.write_function(move |buf| {
                debug!(target: "network", "{} - {} bytes of data", token, buf.len());
                tls::with(|downloads| {
                    if let Some(downloads) = downloads {
                        downloads.pending[&token]
                            .0
                            .data
                            .borrow_mut()
                            .extend_from_slice(buf);
                    }
                });
                Ok(buf.len())
            })?;
            handle.header_function(move |data| {
                tls::with(|downloads| {
                    if let Some(downloads) = downloads {
                        // Headers contain trailing \r\n, trim them to make it easier
                        // to work with.
                        let h = String::from_utf8_lossy(data).trim().to_string();
                        downloads.pending[&token].0.headers.borrow_mut().push(h);
                    }
                });
                true
            })?;

            handle.progress(true)?;
            handle.progress_function(move |dl_total, dl_cur, _, _| {
                tls::with(|downloads| match downloads {
                    Some(d) => d.progress(token, dl_total as u64, dl_cur as u64),
                    None => false,
                })
            })?;

            // If the progress bar isn't enabled then it may be awhile before the
            // first crate finishes downloading so we inform immediately that we're
            // downloading crates here.
            if self.downloads_finished == 0
                && self.pending.is_empty()
                && !self.progress.borrow().as_ref().unwrap().is_enabled()
            {
                self.set
                    .config
                    .shell()
                    .status("Downloading", "crates ...")?;
            }

            let dl = Download {
                token,
                data: RefCell::new(Vec::new()),
                headers: RefCell::new(Vec::new()),
                id,
                url,
                descriptor,
                total: Cell::new(0),
                current: Cell::new(0),
                start: Instant::now(),
                timed_out: Cell::new(None),
                retry: Retry::new(self.set.config)?,
            };
            self.enqueue(dl, handle)?;
            self.tick(WhyTick::DownloadStarted)?;

            Ok(None)
        }

        /// Returns the number of crates that are still downloading.
        pub fn remaining(&self) -> usize {
            self.pending.len() + self.sleeping.len()
        }

        /// Blocks the current thread waiting for a package to finish downloading.
        ///
        /// This method will wait for a previously enqueued package to finish
        /// downloading and return a reference to it after it's done downloading.
        ///
        /// # Panics
        ///
        /// This function will panic if there are no remaining downloads.
        pub fn wait(&mut self) -> CargoResult<&'a Package> {
            let (dl, data) = loop {
                assert_eq!(self.pending.len(), self.pending_ids.len());
                let (token, result) = self.wait_for_curl()?;
                debug!(target: "network", "{} finished with {:?}", token, result);

                let (mut dl, handle) = self
                    .pending
                    .remove(&token)
                    .expect("got a token for a non-in-progress transfer");
                let data = mem::take(&mut *dl.data.borrow_mut());
                let headers = mem::take(&mut *dl.headers.borrow_mut());
                let mut handle = self.set.multi.remove(handle)?;
                self.pending_ids.remove(&dl.id);

                // Check if this was a spurious error. If it was a spurious error
                // then we want to re-enqueue our request for another attempt and
                // then we wait for another request to finish.
                let ret = {
                    let timed_out = &dl.timed_out;
                    let url = &dl.url;
                    dl.retry.r#try(|| {
                        if let Err(e) = result {
                            // If this error is "aborted by callback" then that's
                            // probably because our progress callback aborted due to
                            // a timeout. We'll find out by looking at the
                            // `timed_out` field, looking for a descriptive message.
                            // If one is found we switch the error code (to ensure
                            // it's flagged as spurious) and then attach our extra
                            // information to the error.
                            if !e.is_aborted_by_callback() {
                                return Err(e.into());
                            }

                            return Err(match timed_out.replace(None) {
                                Some(msg) => {
                                    let code = curl_sys::CURLE_OPERATION_TIMEDOUT;
                                    let mut err = curl::Error::new(code);
                                    err.set_extra(msg);
                                    err
                                }
                                None => e,
                            }
                            .into());
                        }

                        let code = handle.response_code()?;
                        if code != 200 && code != 0 {
                            return Err(HttpNotSuccessful::new_from_handle(
                                &mut handle,
                                &url,
                                data,
                                headers,
                            )
                            .into());
                        }
                        Ok(data)
                    })
                };
                match ret {
                    RetryResult::Success(data) => break (dl, data),
                    RetryResult::Err(e) => {
                        return Err(e.context(format!("failed to download from `{}`", dl.url)))
                    }
                    RetryResult::Retry(sleep) => {
                        debug!(target: "network", "download retry {} for {sleep}ms", dl.url);
                        self.sleeping.push(sleep, (dl, handle));
                    }
                }
            };

            // If the progress bar isn't enabled then we still want to provide some
            // semblance of progress of how we're downloading crates, and if the
            // progress bar is enabled this provides a good log of what's happening.
            self.progress.borrow_mut().as_mut().unwrap().clear();
            self.set
                .config
                .shell()
                .status("Downloaded", &dl.descriptor)?;

            self.downloads_finished += 1;
            self.downloaded_bytes += dl.total.get();
            if dl.total.get() > self.largest.0 {
                self.largest = (dl.total.get(), dl.id.name());
            }

            // We're about to synchronously extract the crate below. While we're
            // doing that our download progress won't actually be updated, nor do we
            // have a great view into the progress of the extraction. Let's prepare
            // the user for this CPU-heavy step if it looks like it'll take some
            // time to do so.
            if dl.total.get() < ByteSize::kb(400).0 {
                self.tick(WhyTick::DownloadFinished)?;
            } else {
                self.tick(WhyTick::Extracting(&dl.id.name()))?;
            }

            // Inform the original source that the download is finished which
            // should allow us to actually get the package and fill it in now.
            let mut sources = self.set.sources.borrow_mut();
            let source = sources
                .get_mut(dl.id.source_id())
                .ok_or_else(|| internal(format!("couldn't find source for `{}`", dl.id)))?;
            let start = Instant::now();
            let pkg = source.finish_download(dl.id, data)?;

            // Assume that no time has passed while we were calling
            // `finish_download`, update all speed checks and timeout limits of all
            // active downloads to make sure they don't fire because of a slowly
            // extracted tarball.
            let finish_dur = start.elapsed();
            self.updated_at.set(self.updated_at.get() + finish_dur);
            self.next_speed_check
                .set(self.next_speed_check.get() + finish_dur);

            let slot = &self.set.packages[&dl.id];
            assert!(slot.fill(pkg).is_ok());
            Ok(slot.borrow().unwrap())
        }

        fn enqueue(&mut self, dl: Download<'cfg>, handle: Easy) -> CargoResult<()> {
            let mut handle = self.set.multi.add(handle)?;
            let now = Instant::now();
            handle.set_token(dl.token)?;
            self.updated_at.set(now);
            self.next_speed_check.set(now + self.timeout.dur);
            self.next_speed_check_bytes_threshold
                .set(u64::from(self.timeout.low_speed_limit));
            dl.timed_out.set(None);
            dl.current.set(0);
            dl.total.set(0);
            self.pending.insert(dl.token, (dl, handle));
            Ok(())
        }

        /// Block, waiting for curl. Returns a token and a `Result` for that token
        /// (`Ok` means the download successfully finished).
        fn wait_for_curl(&mut self) -> CargoResult<(usize, Result<(), curl::Error>)> {
            // This is the main workhorse loop. We use libcurl's portable `wait`
            // method to actually perform blocking. This isn't necessarily too
            // efficient in terms of fd management, but we should only be juggling
            // a few anyway.
            //
            // Here we start off by asking the `multi` handle to do some work via
            // the `perform` method. This will actually do I/O work (non-blocking)
            // and attempt to make progress. Afterwards we ask about the `messages`
            // contained in the handle which will inform us if anything has finished
            // transferring.
            //
            // If we've got a finished transfer after all that work we break out
            // and process the finished transfer at the end. Otherwise we need to
            // actually block waiting for I/O to happen, which we achieve with the
            // `wait` method on `multi`.
            loop {
                self.add_sleepers()?;
                let n = tls::set(self, || {
                    self.set
                        .multi
                        .perform()
                        .with_context(|| "failed to perform http requests")
                })?;
                debug!(target: "network", "handles remaining: {}", n);
                let results = &mut self.results;
                let pending = &self.pending;
                self.set.multi.messages(|msg| {
                    let token = msg.token().expect("failed to read token");
                    let handle = &pending[&token].1;
                    if let Some(result) = msg.result_for(handle) {
                        results.push((token, result));
                    } else {
                        debug!(target: "network", "message without a result (?)");
                    }
                });

                if let Some(pair) = results.pop() {
                    break Ok(pair);
                }
                assert_ne!(self.remaining(), 0);
                if self.pending.is_empty() {
                    let delay = self.sleeping.time_to_next().unwrap();
                    debug!(target: "network", "sleeping main thread for {delay:?}");
                    std::thread::sleep(delay);
                } else {
                    let min_timeout = Duration::new(1, 0);
                    let timeout = self.set.multi.get_timeout()?.unwrap_or(min_timeout);
                    let timeout = timeout.min(min_timeout);
                    self.set
                        .multi
                        .wait(&mut [], timeout)
                        .with_context(|| "failed to wait on curl `Multi`")?;
                }
            }
        }

        fn add_sleepers(&mut self) -> CargoResult<()> {
            for (dl, handle) in self.sleeping.to_retry() {
                self.pending_ids.insert(dl.id);
                self.enqueue(dl, handle)?;
            }
            Ok(())
        }

        fn progress(&self, token: usize, total: u64, cur: u64) -> bool {
            let dl = &self.pending[&token].0;
            dl.total.set(total);
            let now = Instant::now();
            if cur > dl.current.get() {
                let delta = cur - dl.current.get();
                let threshold = self.next_speed_check_bytes_threshold.get();

                dl.current.set(cur);
                self.updated_at.set(now);

                if delta >= threshold {
                    self.next_speed_check.set(now + self.timeout.dur);
                    self.next_speed_check_bytes_threshold
                        .set(u64::from(self.timeout.low_speed_limit));
                } else {
                    self.next_speed_check_bytes_threshold.set(threshold - delta);
                }
            }
            if self.tick(WhyTick::DownloadUpdate).is_err() {
                return false;
            }

            // If we've spent too long not actually receiving any data we time out.
            if now > self.updated_at.get() + self.timeout.dur {
                self.updated_at.set(now);
                let msg = format!(
                    "failed to download any data for `{}` within {}s",
                    dl.id,
                    self.timeout.dur.as_secs()
                );
                dl.timed_out.set(Some(msg));
                return false;
            }

            // If we reached the point in time that we need to check our speed
            // limit, see if we've transferred enough data during this threshold. If
            // it fails this check then we fail because the download is going too
            // slowly.
            if now >= self.next_speed_check.get() {
                self.next_speed_check.set(now + self.timeout.dur);
                assert!(self.next_speed_check_bytes_threshold.get() > 0);
                let msg = format!(
                    "download of `{}` failed to transfer more \
                     than {} bytes in {}s",
                    dl.id,
                    self.timeout.low_speed_limit,
                    self.timeout.dur.as_secs()
                );
                dl.timed_out.set(Some(msg));
                return false;
            }

            true
        }

        fn tick(&self, why: WhyTick<'_>) -> CargoResult<()> {
            let mut progress = self.progress.borrow_mut();
            let progress = progress.as_mut().unwrap();

            if let WhyTick::DownloadUpdate = why {
                if !progress.update_allowed() {
                    return Ok(());
                }
            }
            let pending = self.remaining();
            let mut msg = if pending == 1 {
                format!("{} crate", pending)
            } else {
                format!("{} crates", pending)
            };
            match why {
                WhyTick::Extracting(krate) => {
                    msg.push_str(&format!(", extracting {} ...", krate));
                }
                _ => {
                    let mut dur = Duration::new(0, 0);
                    let mut remaining = 0;
                    for (dl, _) in self.pending.values() {
                        dur += dl.start.elapsed();
                        // If the total/current look weird just throw out the data
                        // point, sounds like curl has more to learn before we have
                        // the true information.
                        if dl.total.get() >= dl.current.get() {
                            remaining += dl.total.get() - dl.current.get();
                        }
                    }
                    if remaining > 0 && dur > Duration::from_millis(500) {
                        msg.push_str(&format!(", remaining bytes: {}", ByteSize(remaining)));
                    }
                }
            }
            progress.print_now(&msg)
        }
    }

    #[derive(Copy, Clone)]
    enum WhyTick<'a> {
        DownloadStarted,
        DownloadUpdate,
        DownloadFinished,
        Extracting(&'a str),
    }

    impl<'a, 'cfg> Drop for Downloads<'a, 'cfg> {
        fn drop(&mut self) {
            self.set.downloading.set(false);
            let progress = self.progress.get_mut().take().unwrap();
            // Don't print a download summary if we're not using a progress bar,
            // we've already printed lots of `Downloading...` items.
            if !progress.is_enabled() {
                return;
            }
            // If we didn't download anything, no need for a summary.
            if self.downloads_finished == 0 {
                return;
            }
            // If an error happened, let's not clutter up the output.
            if !self.success {
                return;
            }
            // pick the correct plural of crate(s)
            let crate_string = if self.downloads_finished == 1 {
                "crate"
            } else {
                "crates"
            };
            let mut status = format!(
                "{} {} ({}) in {}",
                self.downloads_finished,
                crate_string,
                ByteSize(self.downloaded_bytes),
                util::elapsed(self.start.elapsed())
            );
            // print the size of largest crate if it was >1mb
            // however don't print if only a single crate was downloaded
            // because it is obvious that it will be the largest then
            if self.largest.0 > ByteSize::mb(1).0 && self.downloads_finished > 1 {
                status.push_str(&format!(
                    " (largest was `{}` at {})",
                    self.largest.1,
                    ByteSize(self.largest.0),
                ));
            }
            // Clear progress before displaying final summary.
            drop(progress);
            drop(self.set.config.shell().status("Downloaded", status));
        }
    }

    mod tls {
        use std::cell::Cell;

        use super::Downloads;

        thread_local!(static PTR: Cell<usize> = Cell::new(0));

        pub(crate) fn with<R>(f: impl FnOnce(Option<&Downloads<'_, '_>>) -> R) -> R {
            let ptr = PTR.with(|p| p.get());
            if ptr == 0 {
                f(None)
            } else {
                unsafe { f(Some(&*(ptr as *const Downloads<'_, '_>))) }
            }
        }

        pub(crate) fn set<R>(dl: &Downloads<'_, '_>, f: impl FnOnce() -> R) -> R {
            struct Reset<'a, T: Copy>(&'a Cell<T>, T);

            impl<'a, T: Copy> Drop for Reset<'a, T> {
                fn drop(&mut self) {
                    self.0.set(self.1);
                }
            }

            PTR.with(|p| {
                let _reset = Reset(p, p.get());
                p.set(dl as *const Downloads<'_, '_> as usize);
                f()
            })
        }
    }