#
cargo/core/compiler/job_queue/
mod.rs

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    //! Management of the interaction between the main `cargo` and all spawned jobs.
    //!
    //! ## Overview
    //!
    //! This module implements a job queue. A job here represents a unit of work,
    //! which is roughly a rustc invocation, a build script run, or just a no-op.
    //! The job queue primarily handles the following things:
    //!
    //! * Spawns concurrent jobs. Depending on its [`Freshness`], a job could be
    //!     either executed on a spawned thread or ran on the same thread to avoid
    //!     the threading overhead.
    //! * Controls the number of concurrency. It allocates and manages [`jobserver`]
    //!     tokens to each spawned off rustc and build scripts.
    //! * Manages the communication between the main `cargo` process and its
    //!     spawned jobs. Those [`Message`]s are sent over a [`Queue`] shared
    //!     across threads.
    //! * Schedules the execution order of each [`Job`]. Priorities are determined
    //!     when calling [`JobQueue::enqueue`] to enqueue a job. The scheduling is
    //!     relatively rudimentary and could likely be improved.
    //!
    //! A rough outline of building a queue and executing jobs is:
    //!
    //! 1. [`JobQueue::new`] to simply create one queue.
    //! 2. [`JobQueue::enqueue`] to add new jobs onto the queue.
    //! 3. Consumes the queue and executes all jobs via [`JobQueue::execute`].
    //!
    //! The primary loop happens insides [`JobQueue::execute`], which is effectively
    //! [`DrainState::drain_the_queue`]. [`DrainState`] is, as its name tells,
    //! the running state of the job queue getting drained.
    //!
    //! ## Jobserver
    //!
    //! As of Feb. 2023, Cargo and rustc have a relatively simple jobserver
    //! relationship with each other. They share a single jobserver amongst what
    //! is potentially hundreds of threads of work on many-cored systems.
    //! The jobserver could come from either the environment (e.g., from a `make`
    //! invocation), or from Cargo creating its own jobserver server if there is no
    //! jobserver to inherit from.
    //!
    //! Cargo wants to complete the build as quickly as possible, fully saturating
    //! all cores (as constrained by the `-j=N`) parameter. Cargo also must not spawn
    //! more than N threads of work: the total amount of tokens we have floating
    //! around must always be limited to N.
    //!
    //! It is not really possible to optimally choose which crate should build
    //! first or last; nor is it possible to decide whether to give an additional
    //! token to rustc first or rather spawn a new crate of work. The algorithm in
    //! Cargo prioritizes spawning as many crates (i.e., rustc processes) as
    //! possible. In short, the jobserver relationship among Cargo and rustc
    //! processes is **1 `cargo` to N `rustc`**. Cargo knows nothing beyond rustc
    //! processes in terms of parallelism[^parallel-rustc].
    //!
    //! We integrate with the [jobserver] crate, originating from GNU make
    //! [POSIX jobserver], to make sure that build scripts which use make to
    //! build C code can cooperate with us on the number of used tokens and
    //! avoid overfilling the system we're on.
    //!
    //! ## Scheduling
    //!
    //! The current scheduling algorithm is not really polished. It is simply based
    //! on a dependency graph [`DependencyQueue`]. We continue adding nodes onto
    //! the graph until we finalize it. When the graph gets finalized, it finds the
    //! sum of the cost of each dependencies of each node, including transitively.
    //! The sum of dependency cost turns out to be the cost of each given node.
    //!
    //! At the time being, the cost is just passed as a fixed placeholder in
    //! [`JobQueue::enqueue`]. In the future, we could explore more possibilities
    //! around it. For instance, we start persisting timing information for each
    //! build somewhere. For a subsequent build, we can look into the historical
    //! data and perform a PGO-like optimization to prioritize jobs, making a build
    //! fully pipelined.
    //!
    //! ## Message queue
    //!
    //! Each spawned thread running a process uses the message queue [`Queue`] to
    //! send messages back to the main thread (the one running `cargo`).
    //! The main thread coordinates everything, and handles printing output.
    //!
    //! It is important to be careful which messages use [`push`] vs [`push_bounded`].
    //! `push` is for priority messages (like tokens, or "finished") where the
    //! sender shouldn't block. We want to handle those so real work can proceed
    //! ASAP.
    //!
    //! `push_bounded` is only for messages being printed to stdout/stderr. Being
    //! bounded prevents a flood of messages causing a large amount of memory
    //! being used.
    //!
    //! `push` also avoids blocking which helps avoid deadlocks. For example, when
    //! the diagnostic server thread is dropped, it waits for the thread to exit.
    //! But if the thread is blocked on a full queue, and there is a critical
    //! error, the drop will deadlock. This should be fixed at some point in the
    //! future. The jobserver thread has a similar problem, though it will time
    //! out after 1 second.
    //!
    //! To access the message queue, each running `Job` is given its own [`JobState`],
    //! containing everything it needs to communicate with the main thread.
    //!
    //! See [`Message`] for all available message kinds.
    //!
    //! [^parallel-rustc]: In fact, `jobserver` that Cargo uses also manages the
    //!     allocation of tokens to rustc beyond the implicit token each rustc owns
    //!     (i.e., the ones used for parallel LLVM work and parallel rustc threads).
    //!     See also ["Rust Compiler Development Guide: Parallel Compilation"]
    //!     and [this comment][rustc-codegen] in rust-lang/rust.
    //!
    //! ["Rust Compiler Development Guide: Parallel Compilation"]: https://rustc-dev-guide.rust-lang.org/parallel-rustc.html
    //! [rustc-codegen]: https://github.com/rust-lang/rust/blob/5423745db8b434fcde54888b35f518f00cce00e4/compiler/rustc_codegen_ssa/src/back/write.rs#L1204-L1217
    //! [jobserver]: https://docs.rs/jobserver
    //! [POSIX jobserver]: https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html
    //! [`push`]: Queue::push
    //! [`push_bounded`]: Queue::push_bounded

    mod job;
    mod job_state;

    use std::cell::RefCell;
    use std::collections::{HashMap, HashSet};
    use std::fmt::Write as _;
    use std::io;
    use std::path::{Path, PathBuf};
    use std::sync::Arc;
    use std::thread::{self, Scope};
    use std::time::Duration;

    use anyhow::{format_err, Context as _};
    use cargo_util::ProcessBuilder;
    use jobserver::{Acquired, HelperThread};
    use semver::Version;
    use tracing::{debug, trace};

    pub use self::job::Freshness::{self, Dirty, Fresh};
    pub use self::job::{Job, Work};
    pub use self::job_state::JobState;
    use super::build_runner::OutputFile;
    use super::timings::Timings;
    use super::{BuildContext, BuildPlan, BuildRunner, CompileMode, Unit};
    use crate::core::compiler::descriptive_pkg_name;
    use crate::core::compiler::future_incompat::{
        self, FutureBreakageItem, FutureIncompatReportPackage,
    };
    use crate::core::resolver::ResolveBehavior;
    use crate::core::{PackageId, Shell, TargetKind};
    use crate::util::diagnostic_server::{self, DiagnosticPrinter};
    use crate::util::errors::AlreadyPrintedError;
    use crate::util::machine_message::{self, Message as _};
    use crate::util::CargoResult;
    use crate::util::{self, internal};
    use crate::util::{DependencyQueue, GlobalContext, Progress, ProgressStyle, Queue};

    /// This structure is backed by the `DependencyQueue` type and manages the
    /// queueing of compilation steps for each package. Packages enqueue units of
    /// work and then later on the entire graph is converted to DrainState and
    /// executed.
    pub struct JobQueue<'gctx> {
        queue: DependencyQueue<Unit, Artifact, Job>,
        counts: HashMap<PackageId, usize>,
        timings: Timings<'gctx>,
    }

    /// This structure is backed by the `DependencyQueue` type and manages the
    /// actual compilation step of each package. Packages enqueue units of work and
    /// then later on the entire graph is processed and compiled.
    ///
    /// It is created from JobQueue when we have fully assembled the crate graph
    /// (i.e., all package dependencies are known).
    struct DrainState<'gctx> {
        // This is the length of the DependencyQueue when starting out
        total_units: usize,

        queue: DependencyQueue<Unit, Artifact, Job>,
        messages: Arc<Queue<Message>>,
        /// Diagnostic deduplication support.
        diag_dedupe: DiagDedupe<'gctx>,
        /// Count of warnings, used to print a summary after the job succeeds
        warning_count: HashMap<JobId, WarningCount>,
        active: HashMap<JobId, Unit>,
        compiled: HashSet<PackageId>,
        documented: HashSet<PackageId>,
        scraped: HashSet<PackageId>,
        counts: HashMap<PackageId, usize>,
        progress: Progress<'gctx>,
        next_id: u32,
        timings: Timings<'gctx>,

        /// Tokens that are currently owned by this Cargo, and may be "associated"
        /// with a rustc process. They may also be unused, though if so will be
        /// dropped on the next loop iteration.
        ///
        /// Note that the length of this may be zero, but we will still spawn work,
        /// as we share the implicit token given to this Cargo process with a
        /// single rustc process.
        tokens: Vec<Acquired>,

        /// The list of jobs that we have not yet started executing, but have
        /// retrieved from the `queue`. We eagerly pull jobs off the main queue to
        /// allow us to request jobserver tokens pretty early.
        pending_queue: Vec<(Unit, Job, usize)>,
        print: DiagnosticPrinter<'gctx>,

        /// How many jobs we've finished
        finished: usize,
        per_package_future_incompat_reports: Vec<FutureIncompatReportPackage>,
    }

    /// Count of warnings, used to print a summary after the job succeeds
    #[derive(Default)]
    pub struct WarningCount {
        /// total number of warnings
        pub total: usize,
        /// number of warnings that were suppressed because they
        /// were duplicates of a previous warning
        pub duplicates: usize,
        /// number of fixable warnings set to `NotAllowed`
        /// if any errors have been seen ofr the current
        /// target
        pub fixable: FixableWarnings,
    }

    impl WarningCount {
        /// If an error is seen this should be called
        /// to set `fixable` to `NotAllowed`
        fn disallow_fixable(&mut self) {
            self.fixable = FixableWarnings::NotAllowed;
        }

        /// Checks fixable if warnings are allowed
        /// fixable warnings are allowed if no
        /// errors have been seen for the current
        /// target. If an error was seen `fixable`
        /// will be `NotAllowed`.
        fn fixable_allowed(&self) -> bool {
            match &self.fixable {
                FixableWarnings::NotAllowed => false,
                _ => true,
            }
        }
    }

    /// Used to keep track of how many fixable warnings there are
    /// and if fixable warnings are allowed
    #[derive(Default)]
    pub enum FixableWarnings {
        NotAllowed,
        #[default]
        Zero,
        Positive(usize),
    }

    pub struct ErrorsDuringDrain {
        pub count: usize,
    }

    struct ErrorToHandle {
        error: anyhow::Error,

        /// This field is true for "interesting" errors and false for "mundane"
        /// errors. If false, we print the above error only if it's the first one
        /// encountered so far while draining the job queue.
        ///
        /// At most places that an error is propagated, we set this to false to
        /// avoid scenarios where Cargo might end up spewing tons of redundant error
        /// messages. For example if an i/o stream got closed somewhere, we don't
        /// care about individually reporting every thread that it broke; just the
        /// first is enough.
        ///
        /// The exception where print_always is true is that we do report every
        /// instance of a rustc invocation that failed with diagnostics. This
        /// corresponds to errors from Message::Finish.
        print_always: bool,
    }

    impl<E> From<E> for ErrorToHandle
    where
        anyhow::Error: From<E>,
    {
        fn from(error: E) -> Self {
            ErrorToHandle {
                error: anyhow::Error::from(error),
                print_always: false,
            }
        }
    }

    #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
    pub struct JobId(pub u32);

    impl std::fmt::Display for JobId {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, "{}", self.0)
        }
    }

    /// Handler for deduplicating diagnostics.
    struct DiagDedupe<'gctx> {
        seen: RefCell<HashSet<u64>>,
        gctx: &'gctx GlobalContext,
    }

    impl<'gctx> DiagDedupe<'gctx> {
        fn new(gctx: &'gctx GlobalContext) -> Self {
            DiagDedupe {
                seen: RefCell::new(HashSet::new()),
                gctx,
            }
        }

        /// Emits a diagnostic message.
        ///
        /// Returns `true` if the message was emitted, or `false` if it was
        /// suppressed for being a duplicate.
        fn emit_diag(&self, diag: &str) -> CargoResult<bool> {
            let h = util::hash_u64(diag);
            if !self.seen.borrow_mut().insert(h) {
                return Ok(false);
            }
            let mut shell = self.gctx.shell();
            shell.print_ansi_stderr(diag.as_bytes())?;
            shell.err().write_all(b"\n")?;
            Ok(true)
        }
    }

    /// Possible artifacts that can be produced by compilations, used as edge values
    /// in the dependency graph.
    ///
    /// As edge values we can have multiple kinds of edges depending on one node,
    /// for example some units may only depend on the metadata for an rlib while
    /// others depend on the full rlib. This `Artifact` enum is used to distinguish
    /// this case and track the progress of compilations as they proceed.
    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
    enum Artifact {
        /// A generic placeholder for "depends on everything run by a step" and
        /// means that we can't start the next compilation until the previous has
        /// finished entirely.
        All,

        /// A node indicating that we only depend on the metadata of a compilation,
        /// but the compilation is typically also producing an rlib. We can start
        /// our step, however, before the full rlib is available.
        Metadata,
    }

    enum Message {
        Run(JobId, String),
        BuildPlanMsg(String, ProcessBuilder, Arc<Vec<OutputFile>>),
        Stdout(String),
        Stderr(String),

        // This is for general stderr output from subprocesses
        Diagnostic {
            id: JobId,
            level: String,
            diag: String,
            fixable: bool,
        },
        // This handles duplicate output that is suppressed, for showing
        // only a count of duplicate messages instead
        WarningCount {
            id: JobId,
            emitted: bool,
            fixable: bool,
        },
        // This is for warnings generated by Cargo's interpretation of the
        // subprocess output, e.g. scrape-examples prints a warning if a
        // unit fails to be scraped
        Warning {
            id: JobId,
            warning: String,
        },

        FixDiagnostic(diagnostic_server::Message),
        Token(io::Result<Acquired>),
        Finish(JobId, Artifact, CargoResult<()>),
        FutureIncompatReport(JobId, Vec<FutureBreakageItem>),
    }

    impl<'gctx> JobQueue<'gctx> {
        pub fn new(bcx: &BuildContext<'_, 'gctx>) -> JobQueue<'gctx> {
            JobQueue {
                queue: DependencyQueue::new(),
                counts: HashMap::new(),
                timings: Timings::new(bcx, &bcx.roots),
            }
        }

        pub fn enqueue(
            &mut self,
            build_runner: &BuildRunner<'_, 'gctx>,
            unit: &Unit,
            job: Job,
        ) -> CargoResult<()> {
            let dependencies = build_runner.unit_deps(unit);
            let mut queue_deps = dependencies
                .iter()
                .filter(|dep| {
                    // Binaries aren't actually needed to *compile* tests, just to run
                    // them, so we don't include this dependency edge in the job graph.
                    // But we shouldn't filter out dependencies being scraped for Rustdoc.
                    (!dep.unit.target.is_test() && !dep.unit.target.is_bin())
                        || dep.unit.artifact.is_true()
                        || dep.unit.mode.is_doc_scrape()
                })
                .map(|dep| {
                    // Handle the case here where our `unit -> dep` dependency may
                    // only require the metadata, not the full compilation to
                    // finish. Use the tables in `build_runner` to figure out what
                    // kind of artifact is associated with this dependency.
                    let artifact = if build_runner.only_requires_rmeta(unit, &dep.unit) {
                        Artifact::Metadata
                    } else {
                        Artifact::All
                    };
                    (dep.unit.clone(), artifact)
                })
                .collect::<HashMap<_, _>>();

            // This is somewhat tricky, but we may need to synthesize some
            // dependencies for this target if it requires full upstream
            // compilations to have completed. Because of pipelining, some
            // dependency edges may be `Metadata` due to the above clause (as
            // opposed to everything being `All`). For example consider:
            //
            //    a (binary)
            //    └ b (lib)
            //        └ c (lib)
            //
            // Here the dependency edge from B to C will be `Metadata`, and the
            // dependency edge from A to B will be `All`. For A to be compiled,
            // however, it currently actually needs the full rlib of C. This means
            // that we need to synthesize a dependency edge for the dependency graph
            // from A to C. That's done here.
            //
            // This will walk all dependencies of the current target, and if any of
            // *their* dependencies are `Metadata` then we depend on the `All` of
            // the target as well. This should ensure that edges changed to
            // `Metadata` propagate upwards `All` dependencies to anything that
            // transitively contains the `Metadata` edge.
            if unit.requires_upstream_objects() {
                for dep in dependencies {
                    depend_on_deps_of_deps(build_runner, &mut queue_deps, dep.unit.clone());
                }

                fn depend_on_deps_of_deps(
                    build_runner: &BuildRunner<'_, '_>,
                    deps: &mut HashMap<Unit, Artifact>,
                    unit: Unit,
                ) {
                    for dep in build_runner.unit_deps(&unit) {
                        if deps.insert(dep.unit.clone(), Artifact::All).is_none() {
                            depend_on_deps_of_deps(build_runner, deps, dep.unit.clone());
                        }
                    }
                }
            }

            // For now we use a fixed placeholder value for the cost of each unit, but
            // in the future this could be used to allow users to provide hints about
            // relative expected costs of units, or this could be automatically set in
            // a smarter way using timing data from a previous compilation.
            self.queue.queue(unit.clone(), job, queue_deps, 100);
            *self.counts.entry(unit.pkg.package_id()).or_insert(0) += 1;
            Ok(())
        }

        /// Executes all jobs necessary to build the dependency graph.
        ///
        /// This function will spawn off `config.jobs()` workers to build all of the
        /// necessary dependencies, in order. Freshness is propagated as far as
        /// possible along each dependency chain.
        #[tracing::instrument(skip_all)]
        pub fn execute(
            mut self,
            build_runner: &mut BuildRunner<'_, '_>,
            plan: &mut BuildPlan,
        ) -> CargoResult<()> {
            self.queue.queue_finished();

            let progress =
                Progress::with_style("Building", ProgressStyle::Ratio, build_runner.bcx.gctx);
            let state = DrainState {
                total_units: self.queue.len(),
                queue: self.queue,
                // 100 here is somewhat arbitrary. It is a few screenfulls of
                // output, and hopefully at most a few megabytes of memory for
                // typical messages. If you change this, please update the test
                // caching_large_output, too.
                messages: Arc::new(Queue::new(100)),
                diag_dedupe: DiagDedupe::new(build_runner.bcx.gctx),
                warning_count: HashMap::new(),
                active: HashMap::new(),
                compiled: HashSet::new(),
                documented: HashSet::new(),
                scraped: HashSet::new(),
                counts: self.counts,
                progress,
                next_id: 0,
                timings: self.timings,
                tokens: Vec::new(),
                pending_queue: Vec::new(),
                print: DiagnosticPrinter::new(
                    build_runner.bcx.gctx,
                    &build_runner.bcx.rustc().workspace_wrapper,
                ),
                finished: 0,
                per_package_future_incompat_reports: Vec::new(),
            };

            // Create a helper thread for acquiring jobserver tokens
            let messages = state.messages.clone();
            let helper = build_runner
                .jobserver
                .clone()
                .into_helper_thread(move |token| {
                    messages.push(Message::Token(token));
                })
                .context("failed to create helper thread for jobserver management")?;

            // Create a helper thread to manage the diagnostics for rustfix if
            // necessary.
            let messages = state.messages.clone();
            // It is important that this uses `push` instead of `push_bounded` for
            // now. If someone wants to fix this to be bounded, the `drop`
            // implementation needs to be changed to avoid possible deadlocks.
            let _diagnostic_server = build_runner
                .bcx
                .build_config
                .rustfix_diagnostic_server
                .borrow_mut()
                .take()
                .map(move |srv| srv.start(move |msg| messages.push(Message::FixDiagnostic(msg))));

            thread::scope(move |scope| {
                match state.drain_the_queue(build_runner, plan, scope, &helper) {
                    Some(err) => Err(err),
                    None => Ok(()),
                }
            })
        }
    }

    impl<'gctx> DrainState<'gctx> {
        fn spawn_work_if_possible<'s>(
            &mut self,
            build_runner: &mut BuildRunner<'_, '_>,
            jobserver_helper: &HelperThread,
            scope: &'s Scope<'s, '_>,
        ) -> CargoResult<()> {
            // Dequeue as much work as we can, learning about everything
            // possible that can run. Note that this is also the point where we
            // start requesting job tokens. Each job after the first needs to
            // request a token.
            while let Some((unit, job, priority)) = self.queue.dequeue() {
                // We want to keep the pieces of work in the `pending_queue` sorted
                // by their priorities, and insert the current job at its correctly
                // sorted position: following the lower priority jobs, and the ones
                // with the same priority (since they were dequeued before the
                // current one, we also keep that relation).
                let idx = self
                    .pending_queue
                    .partition_point(|&(_, _, p)| p <= priority);
                self.pending_queue.insert(idx, (unit, job, priority));
                if self.active.len() + self.pending_queue.len() > 1 {
                    jobserver_helper.request_token();
                }
            }

            // Now that we've learned of all possible work that we can execute
            // try to spawn it so long as we've got a jobserver token which says
            // we're able to perform some parallel work.
            // The `pending_queue` is sorted in ascending priority order, and we
            // remove items from its end to schedule the highest priority items
            // sooner.
            while self.has_extra_tokens() && !self.pending_queue.is_empty() {
                let (unit, job, _) = self.pending_queue.pop().unwrap();
                *self.counts.get_mut(&unit.pkg.package_id()).unwrap() -= 1;
                if !build_runner.bcx.build_config.build_plan {
                    // Print out some nice progress information.
                    // NOTE: An error here will drop the job without starting it.
                    // That should be OK, since we want to exit as soon as
                    // possible during an error.
                    self.note_working_on(
                        build_runner.bcx.gctx,
                        build_runner.bcx.ws.root(),
                        &unit,
                        job.freshness(),
                    )?;
                }
                self.run(&unit, job, build_runner, scope);
            }

            Ok(())
        }

        fn has_extra_tokens(&self) -> bool {
            self.active.len() < self.tokens.len() + 1
        }

        fn handle_event(
            &mut self,
            build_runner: &mut BuildRunner<'_, '_>,
            plan: &mut BuildPlan,
            event: Message,
        ) -> Result<(), ErrorToHandle> {
            match event {
                Message::Run(id, cmd) => {
                    build_runner
                        .bcx
                        .gctx
                        .shell()
                        .verbose(|c| c.status("Running", &cmd))?;
                    self.timings.unit_start(id, self.active[&id].clone());
                }
                Message::BuildPlanMsg(module_name, cmd, filenames) => {
                    plan.update(&module_name, &cmd, &filenames)?;
                }
                Message::Stdout(out) => {
                    writeln!(build_runner.bcx.gctx.shell().out(), "{}", out)?;
                }
                Message::Stderr(err) => {
                    let mut shell = build_runner.bcx.gctx.shell();
                    shell.print_ansi_stderr(err.as_bytes())?;
                    shell.err().write_all(b"\n")?;
                }
                Message::Diagnostic {
                    id,
                    level,
                    diag,
                    fixable,
                } => {
                    let emitted = self.diag_dedupe.emit_diag(&diag)?;
                    if level == "warning" {
                        self.bump_warning_count(id, emitted, fixable);
                    }
                    if level == "error" {
                        let cnts = self.warning_count.entry(id).or_default();
                        // If there is an error, the `cargo fix` message should not show
                        cnts.disallow_fixable();
                    }
                }
                Message::Warning { id, warning } => {
                    build_runner.bcx.gctx.shell().warn(warning)?;
                    self.bump_warning_count(id, true, false);
                }
                Message::WarningCount {
                    id,
                    emitted,
                    fixable,
                } => {
                    self.bump_warning_count(id, emitted, fixable);
                }
                Message::FixDiagnostic(msg) => {
                    self.print.print(&msg)?;
                }
                Message::Finish(id, artifact, result) => {
                    let unit = match artifact {
                        // If `id` has completely finished we remove it
                        // from the `active` map ...
                        Artifact::All => {
                            trace!("end: {:?}", id);
                            self.finished += 1;
                            self.report_warning_count(
                                build_runner.bcx.gctx,
                                id,
                                &build_runner.bcx.rustc().workspace_wrapper,
                            );
                            self.active.remove(&id).unwrap()
                        }
                        // ... otherwise if it hasn't finished we leave it
                        // in there as we'll get another `Finish` later on.
                        Artifact::Metadata => {
                            trace!("end (meta): {:?}", id);
                            self.active[&id].clone()
                        }
                    };
                    debug!("end ({:?}): {:?}", unit, result);
                    match result {
                        Ok(()) => self.finish(id, &unit, artifact, build_runner)?,
                        Err(_) if build_runner.bcx.unit_can_fail_for_docscraping(&unit) => {
                            build_runner
                                .failed_scrape_units
                                .lock()
                                .unwrap()
                                .insert(build_runner.files().metadata(&unit));
                            self.queue.finish(&unit, &artifact);
                        }
                        Err(error) => {
                            let msg = "The following warnings were emitted during compilation:";
                            self.emit_warnings(Some(msg), &unit, build_runner)?;
                            self.back_compat_notice(build_runner, &unit)?;
                            return Err(ErrorToHandle {
                                error,
                                print_always: true,
                            });
                        }
                    }
                }
                Message::FutureIncompatReport(id, items) => {
                    let package_id = self.active[&id].pkg.package_id();
                    self.per_package_future_incompat_reports
                        .push(FutureIncompatReportPackage { package_id, items });
                }
                Message::Token(acquired_token) => {
                    let token = acquired_token.context("failed to acquire jobserver token")?;
                    self.tokens.push(token);
                }
            }

            Ok(())
        }

        // This will also tick the progress bar as appropriate
        fn wait_for_events(&mut self) -> Vec<Message> {
            // Drain all events at once to avoid displaying the progress bar
            // unnecessarily. If there's no events we actually block waiting for
            // an event, but we keep a "heartbeat" going to allow `record_cpu`
            // to run above to calculate CPU usage over time. To do this we
            // listen for a message with a timeout, and on timeout we run the
            // previous parts of the loop again.
            let mut events = self.messages.try_pop_all();
            if events.is_empty() {
                loop {
                    self.tick_progress();
                    self.tokens.truncate(self.active.len() - 1);
                    match self.messages.pop(Duration::from_millis(500)) {
                        Some(message) => {
                            events.push(message);
                            break;
                        }
                        None => continue,
                    }
                }
            }
            events
        }

        /// This is the "main" loop, where Cargo does all work to run the
        /// compiler.
        ///
        /// This returns an Option to prevent the use of `?` on `Result` types
        /// because it is important for the loop to carefully handle errors.
        fn drain_the_queue<'s>(
            mut self,
            build_runner: &mut BuildRunner<'_, '_>,
            plan: &mut BuildPlan,
            scope: &'s Scope<'s, '_>,
            jobserver_helper: &HelperThread,
        ) -> Option<anyhow::Error> {
            trace!("queue: {:#?}", self.queue);

            // Iteratively execute the entire dependency graph. Each turn of the
            // loop starts out by scheduling as much work as possible (up to the
            // maximum number of parallel jobs we have tokens for). A local queue
            // is maintained separately from the main dependency queue as one
            // dequeue may actually dequeue quite a bit of work (e.g., 10 binaries
            // in one package).
            //
            // After a job has finished we update our internal state if it was
            // successful and otherwise wait for pending work to finish if it failed
            // and then immediately return (or keep going, if requested by the build
            // config).
            let mut errors = ErrorsDuringDrain { count: 0 };
            // CAUTION! Do not use `?` or break out of the loop early. Every error
            // must be handled in such a way that the loop is still allowed to
            // drain event messages.
            loop {
                if errors.count == 0 || build_runner.bcx.build_config.keep_going {
                    if let Err(e) = self.spawn_work_if_possible(build_runner, jobserver_helper, scope) {
                        self.handle_error(&mut build_runner.bcx.gctx.shell(), &mut errors, e);
                    }
                }

                // If after all that we're not actually running anything then we're
                // done!
                if self.active.is_empty() {
                    break;
                }

                // And finally, before we block waiting for the next event, drop any
                // excess tokens we may have accidentally acquired. Due to how our
                // jobserver interface is architected we may acquire a token that we
                // don't actually use, and if this happens just relinquish it back
                // to the jobserver itself.
                for event in self.wait_for_events() {
                    if let Err(event_err) = self.handle_event(build_runner, plan, event) {
                        self.handle_error(&mut build_runner.bcx.gctx.shell(), &mut errors, event_err);
                    }
                }
            }
            self.progress.clear();

            let profile_name = build_runner.bcx.build_config.requested_profile;
            // NOTE: this may be a bit inaccurate, since this may not display the
            // profile for what was actually built. Profile overrides can change
            // these settings, and in some cases different targets are built with
            // different profiles. To be accurate, it would need to collect a
            // list of Units built, and maybe display a list of the different
            // profiles used. However, to keep it simple and compatible with old
            // behavior, we just display what the base profile is.
            let profile = build_runner.bcx.profiles.base_profile();
            let mut opt_type = String::from(if profile.opt_level.as_str() == "0" {
                "unoptimized"
            } else {
                "optimized"
            });
            if profile.debuginfo.is_turned_on() {
                opt_type += " + debuginfo";
            }

            let time_elapsed = util::elapsed(build_runner.bcx.gctx.creation_time().elapsed());
            if let Err(e) = self.timings.finished(build_runner, &errors.to_error()) {
                self.handle_error(&mut build_runner.bcx.gctx.shell(), &mut errors, e);
            }
            if build_runner.bcx.build_config.emit_json() {
                let mut shell = build_runner.bcx.gctx.shell();
                let msg = machine_message::BuildFinished {
                    success: errors.count == 0,
                }
                .to_json_string();
                if let Err(e) = writeln!(shell.out(), "{}", msg) {
                    self.handle_error(&mut shell, &mut errors, e);
                }
            }

            if let Some(error) = errors.to_error() {
                // Any errors up to this point have already been printed via the
                // `display_error` inside `handle_error`.
                Some(anyhow::Error::new(AlreadyPrintedError::new(error)))
            } else if self.queue.is_empty() && self.pending_queue.is_empty() {
                let profile_link = build_runner.bcx.gctx.shell().err_hyperlink(
                    "https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles",
                );
                let message = format!(
                    "{profile_link}`{profile_name}` profile [{opt_type}]{profile_link:#} target(s) in {time_elapsed}",
                );
                if !build_runner.bcx.build_config.build_plan {
                    // It doesn't really matter if this fails.
                    let _ = build_runner.bcx.gctx.shell().status("Finished", message);
                    future_incompat::save_and_display_report(
                        build_runner.bcx,
                        &self.per_package_future_incompat_reports,
                    );
                }

                None
            } else {
                debug!("queue: {:#?}", self.queue);
                Some(internal("finished with jobs still left in the queue"))
            }
        }

        fn handle_error(
            &self,
            shell: &mut Shell,
            err_state: &mut ErrorsDuringDrain,
            new_err: impl Into<ErrorToHandle>,
        ) {
            let new_err = new_err.into();
            if new_err.print_always || err_state.count == 0 {
                crate::display_error(&new_err.error, shell);
                if err_state.count == 0 && !self.active.is_empty() {
                    let _ = shell.warn("build failed, waiting for other jobs to finish...");
                }
                err_state.count += 1;
            } else {
                tracing::warn!("{:?}", new_err.error);
            }
        }

        // This also records CPU usage and marks concurrency; we roughly want to do
        // this as often as we spin on the events receiver (at least every 500ms or
        // so).
        fn tick_progress(&mut self) {
            // Record some timing information if `--timings` is enabled, and
            // this'll end up being a noop if we're not recording this
            // information.
            self.timings.mark_concurrency(
                self.active.len(),
                self.pending_queue.len(),
                self.queue.len(),
            );
            self.timings.record_cpu();

            let active_names = self
                .active
                .values()
                .map(|u| self.name_for_progress(u))
                .collect::<Vec<_>>();
            let _ = self.progress.tick_now(
                self.finished,
                self.total_units,
                &format!(": {}", active_names.join(", ")),
            );
        }

        fn name_for_progress(&self, unit: &Unit) -> String {
            let pkg_name = unit.pkg.name();
            let target_name = unit.target.name();
            match unit.mode {
                CompileMode::Doc { .. } => format!("{}(doc)", pkg_name),
                CompileMode::RunCustomBuild => format!("{}(build)", pkg_name),
                CompileMode::Test | CompileMode::Check { test: true } => match unit.target.kind() {
                    TargetKind::Lib(_) => format!("{}(test)", target_name),
                    TargetKind::CustomBuild => panic!("cannot test build script"),
                    TargetKind::Bin => format!("{}(bin test)", target_name),
                    TargetKind::Test => format!("{}(test)", target_name),
                    TargetKind::Bench => format!("{}(bench)", target_name),
                    TargetKind::ExampleBin | TargetKind::ExampleLib(_) => {
                        format!("{}(example test)", target_name)
                    }
                },
                _ => match unit.target.kind() {
                    TargetKind::Lib(_) => pkg_name.to_string(),
                    TargetKind::CustomBuild => format!("{}(build.rs)", pkg_name),
                    TargetKind::Bin => format!("{}(bin)", target_name),
                    TargetKind::Test => format!("{}(test)", target_name),
                    TargetKind::Bench => format!("{}(bench)", target_name),
                    TargetKind::ExampleBin | TargetKind::ExampleLib(_) => {
                        format!("{}(example)", target_name)
                    }
                },
            }
        }

        /// Executes a job.
        ///
        /// Fresh jobs block until finished (which should be very fast!), Dirty
        /// jobs will spawn a thread in the background and return immediately.
        fn run<'s>(
            &mut self,
            unit: &Unit,
            job: Job,
            build_runner: &BuildRunner<'_, '_>,
            scope: &'s Scope<'s, '_>,
        ) {
            let id = JobId(self.next_id);
            self.next_id = self.next_id.checked_add(1).unwrap();

            debug!("start {}: {:?}", id, unit);

            assert!(self.active.insert(id, unit.clone()).is_none());

            let messages = self.messages.clone();
            let is_fresh = job.freshness().is_fresh();
            let rmeta_required = build_runner.rmeta_required(unit);

            let doit = move |diag_dedupe| {
                let state = JobState::new(id, messages, diag_dedupe, rmeta_required);
                state.run_to_finish(job);
            };

            match is_fresh {
                true => {
                    self.timings.add_fresh();
                    // Running a fresh job on the same thread is often much faster than spawning a new
                    // thread to run the job.
                    doit(Some(&self.diag_dedupe));
                }
                false => {
                    self.timings.add_dirty();
                    scope.spawn(move || doit(None));
                }
            }
        }

        fn emit_warnings(
            &mut self,
            msg: Option<&str>,
            unit: &Unit,
            build_runner: &mut BuildRunner<'_, '_>,
        ) -> CargoResult<()> {
            let outputs = build_runner.build_script_outputs.lock().unwrap();
            let Some(metadata) = build_runner.find_build_script_metadata(unit) else {
                return Ok(());
            };
            let bcx = &mut build_runner.bcx;
            if let Some(output) = outputs.get(metadata) {
                if !output.warnings.is_empty() {
                    if let Some(msg) = msg {
                        writeln!(bcx.gctx.shell().err(), "{}\n", msg)?;
                    }

                    for warning in output.warnings.iter() {
                        let warning_with_package =
                            format!("{}@{}: {}", unit.pkg.name(), unit.pkg.version(), warning);

                        bcx.gctx.shell().warn(warning_with_package)?;
                    }

                    if msg.is_some() {
                        // Output an empty line.
                        writeln!(bcx.gctx.shell().err())?;
                    }
                }
            }

            Ok(())
        }

        fn bump_warning_count(&mut self, id: JobId, emitted: bool, fixable: bool) {
            let cnts = self.warning_count.entry(id).or_default();
            cnts.total += 1;
            if !emitted {
                cnts.duplicates += 1;
            // Don't add to fixable if it's already been emitted
            } else if fixable {
                // Do not add anything to the fixable warning count if
                // is `NotAllowed` since that indicates there was an
                // error while building this `Unit`
                if cnts.fixable_allowed() {
                    cnts.fixable = match cnts.fixable {
                        FixableWarnings::NotAllowed => FixableWarnings::NotAllowed,
                        FixableWarnings::Zero => FixableWarnings::Positive(1),
                        FixableWarnings::Positive(fixable) => FixableWarnings::Positive(fixable + 1),
                    };
                }
            }
        }

        /// Displays a final report of the warnings emitted by a particular job.
        fn report_warning_count(
            &mut self,
            gctx: &GlobalContext,
            id: JobId,
            rustc_workspace_wrapper: &Option<PathBuf>,
        ) {
            let count = match self.warning_count.remove(&id) {
                // An error could add an entry for a `Unit`
                // with 0 warnings but having fixable
                // warnings be disallowed
                Some(count) if count.total > 0 => count,
                None | Some(_) => return,
            };
            let unit = &self.active[&id];
            let mut message = descriptive_pkg_name(&unit.pkg.name(), &unit.target, &unit.mode);
            message.push_str(" generated ");
            match count.total {
                1 => message.push_str("1 warning"),
                n => {
                    let _ = write!(message, "{} warnings", n);
                }
            };
            match count.duplicates {
                0 => {}
                1 => message.push_str(" (1 duplicate)"),
                n => {
                    let _ = write!(message, " ({} duplicates)", n);
                }
            }
            // Only show the `cargo fix` message if its a local `Unit`
            if unit.is_local() {
                // Do not show this if there are any errors or no fixable warnings
                if let FixableWarnings::Positive(fixable) = count.fixable {
                    // `cargo fix` doesn't have an option for custom builds
                    if !unit.target.is_custom_build() {
                        // To make sure the correct command is shown for `clippy` we
                        // check if `RUSTC_WORKSPACE_WRAPPER` is set and pointing towards
                        // `clippy-driver`.
                        let clippy = std::ffi::OsStr::new("clippy-driver");
                        let command = match rustc_workspace_wrapper.as_ref().and_then(|x| x.file_stem())
                        {
                            Some(wrapper) if wrapper == clippy => "cargo clippy --fix",
                            _ => "cargo fix",
                        };
                        let mut args = {
                            let named = unit.target.description_named();
                            // if its a lib we need to add the package to fix
                            if unit.target.is_lib() {
                                format!("{} -p {}", named, unit.pkg.name())
                            } else {
                                named
                            }
                        };
                        if unit.mode.is_rustc_test()
                            && !(unit.target.is_test() || unit.target.is_bench())
                        {
                            args.push_str(" --tests");
                        }
                        let mut suggestions = format!("{} suggestion", fixable);
                        if fixable > 1 {
                            suggestions.push_str("s")
                        }
                        let _ = write!(
                            message,
                            " (run `{command} --{args}` to apply {suggestions})"
                        );
                    }
                }
            }
            // Errors are ignored here because it is tricky to handle them
            // correctly, and they aren't important.
            let _ = gctx.shell().warn(message);
        }

        fn finish(
            &mut self,
            id: JobId,
            unit: &Unit,
            artifact: Artifact,
            build_runner: &mut BuildRunner<'_, '_>,
        ) -> CargoResult<()> {
            if unit.mode.is_run_custom_build() && unit.show_warnings(build_runner.bcx.gctx) {
                self.emit_warnings(None, unit, build_runner)?;
            }
            let unlocked = self.queue.finish(unit, &artifact);
            match artifact {
                Artifact::All => self.timings.unit_finished(id, unlocked),
                Artifact::Metadata => self.timings.unit_rmeta_finished(id, unlocked),
            }
            Ok(())
        }

        // This isn't super trivial because we don't want to print loads and
        // loads of information to the console, but we also want to produce a
        // faithful representation of what's happening. This is somewhat nuanced
        // as a package can start compiling *very* early on because of custom
        // build commands and such.
        //
        // In general, we try to print "Compiling" for the first nontrivial task
        // run for a package, regardless of when that is. We then don't print
        // out any more information for a package after we've printed it once.
        fn note_working_on(
            &mut self,
            gctx: &GlobalContext,
            ws_root: &Path,
            unit: &Unit,
            fresh: &Freshness,
        ) -> CargoResult<()> {
            if (self.compiled.contains(&unit.pkg.package_id())
                && !unit.mode.is_doc()
                && !unit.mode.is_doc_scrape())
                || (self.documented.contains(&unit.pkg.package_id()) && unit.mode.is_doc())
                || (self.scraped.contains(&unit.pkg.package_id()) && unit.mode.is_doc_scrape())
            {
                return Ok(());
            }

            match fresh {
                // Any dirty stage which runs at least one command gets printed as
                // being a compiled package.
                Dirty(dirty_reason) => {
                    if !dirty_reason.is_fresh_build() {
                        gctx.shell()
                            .verbose(|shell| dirty_reason.present_to(shell, unit, ws_root))?;
                    }

                    if unit.mode.is_doc() {
                        self.documented.insert(unit.pkg.package_id());
                        gctx.shell().status("Documenting", &unit.pkg)?;
                    } else if unit.mode.is_doc_test() {
                        // Skip doc test.
                    } else if unit.mode.is_doc_scrape() {
                        self.scraped.insert(unit.pkg.package_id());
                        gctx.shell().status("Scraping", &unit.pkg)?;
                    } else {
                        self.compiled.insert(unit.pkg.package_id());
                        if unit.mode.is_check() {
                            gctx.shell().status("Checking", &unit.pkg)?;
                        } else {
                            gctx.shell().status("Compiling", &unit.pkg)?;
                        }
                    }
                }
                Fresh => {
                    // If doc test are last, only print "Fresh" if nothing has been printed.
                    if self.counts[&unit.pkg.package_id()] == 0
                        && !(unit.mode.is_doc_test() && self.compiled.contains(&unit.pkg.package_id()))
                    {
                        self.compiled.insert(unit.pkg.package_id());
                        gctx.shell().verbose(|c| c.status("Fresh", &unit.pkg))?;
                    }
                }
            }
            Ok(())
        }

        fn back_compat_notice(
            &self,
            build_runner: &BuildRunner<'_, '_>,
            unit: &Unit,
        ) -> CargoResult<()> {
            if unit.pkg.name() != "diesel"
                || unit.pkg.version() >= &Version::new(1, 4, 8)
                || build_runner.bcx.ws.resolve_behavior() == ResolveBehavior::V1
                || !unit.pkg.package_id().source_id().is_registry()
                || !unit.features.is_empty()
            {
                return Ok(());
            }
            if !build_runner
                .bcx
                .unit_graph
                .keys()
                .any(|unit| unit.pkg.name() == "diesel" && !unit.features.is_empty())
            {
                return Ok(());
            }
            build_runner.bcx.gctx.shell().note(
                "\
    This error may be due to an interaction between diesel and Cargo's new
    feature resolver. Try updating to diesel 1.4.8 to fix this error.
    ",
            )?;
            Ok(())
        }
    }

    impl ErrorsDuringDrain {
        fn to_error(&self) -> Option<anyhow::Error> {
            match self.count {
                0 => None,
                1 => Some(format_err!("1 job failed")),
                n => Some(format_err!("{} jobs failed", n)),
            }
        }
    }