[nextest-runner] spawn a task for each test

Previously we'd use the same task for all tests -- that turns out to be fine in
most cases, but doesn't quite provide per-test isolation, and is slower in
repos like clap that have lots of small tests. Switch to using the same task
instead.

It's cool that async-scoped kind of works here, but non-static tasks are such
a pain to work with in general.

This makes the clap tests significantly faster. On `fc55ad08`, runtime on my
desktop (Ryzen 7950X/Linux) goes from 0.36 seconds to 0.24.

nextest-runner/src/list/test_list.rs

@@ -31,7 +31,7 @@ use owo_colors::OwoColorize;
 use std::{
     collections::{BTreeMap, BTreeSet},
     ffi::{OsStr, OsString},
-    io,
+    fmt, io,
     path::PathBuf,
     sync::{Arc, OnceLock},
 };
@@ -1070,6 +1070,12 @@ pub struct TestInstanceId<'a> {
     pub test_name: &'a str,
 }
 
+impl fmt::Display for TestInstanceId<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{} {}", self.binary_id, self.test_name)
+    }
+}
+
 /// Context required for test execution.
 #[derive(Clone, Debug)]
 pub struct TestExecuteContext<'a> {

nextest-runner/src/runner/dispatcher.rs

@@ -7,7 +7,7 @@
 //! receives events from the executor and from other inputs (e.g. signal and
 //! input handling), and sends events to the reporter.
 
-use super::{RunUnitRequest, ShutdownRequest};
+use super::{RunUnitRequest, RunnerTaskState, ShutdownRequest};
 use crate::{
     config::{MaxFail, ScriptConfig, ScriptId},
     input::{InputEvent, InputHandler},
@@ -104,7 +104,7 @@ where
         report_cancel_rx: oneshot::Receiver<()>,
         cancelled_ref: &AtomicBool,
         cancellation_sender: broadcast::Sender<()>,
-    ) {
+    ) -> RunnerTaskState {
         let mut report_cancel_rx = std::pin::pin!(report_cancel_rx);
 
         let mut signals_done = false;
@@ -118,7 +118,7 @@ where
                         Some(event) => InternalEvent::Executor(event),
                         None => {
                             // All runs have been completed.
-                            break;
+                            break RunnerTaskState::finished_no_children();
                         }
                     }
                 },
@@ -705,6 +705,8 @@ where
                     if self.disable_signal_3_times_panic {
                         SignalCount::Twice
                     } else {
+                        // TODO: a panic here won't currently lead to other
+                        // tasks being cancelled. This should be fixed.
                         panic!("Signaled 3 times, exiting immediately");
                     }
                 }

nextest-runner/src/runner/imp.rs

@@ -1,7 +1,7 @@
 // Copyright (c) The nextest Contributors
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-use super::{DispatcherContext, ExecutorContext};
+use super::{DispatcherContext, ExecutorContext, RunnerTaskState};
 use crate::{
     config::{
         EvaluatableProfile, MaxFail, RetryPolicy, SetupScriptExecuteData, TestGroup, TestThreads,
@@ -30,7 +30,7 @@ use tokio::{
     sync::{broadcast, mpsc::unbounded_channel, oneshot},
     task::JoinError,
 };
-use tracing::debug;
+use tracing::{debug, warn};
 
 /// Test runner options.
 #[derive(Debug, Default)]
@@ -298,20 +298,23 @@ impl<'a> TestRunnerInner<'a> {
                 cancelled_ref,
                 cancellation_sender.clone(),
             );
-            scope.spawn_cancellable(dispatcher_fut, || ());
+            scope.spawn_cancellable(dispatcher_fut, || RunnerTaskState::Cancelled);
 
             let (script_tx, mut script_rx) = unbounded_channel::<SetupScriptExecuteData<'a>>();
             let script_resp_tx = resp_tx.clone();
             let run_scripts_fut = async move {
+                // Since script tasks are run serially, we just reuse the one
+                // script task.
                 let script_data = executor_cx_ref
                     .run_setup_scripts(script_resp_tx, cancelled_ref)
                     .await;
                 if script_tx.send(script_data).is_err() {
                     // The dispatcher has shut down, so we should too.
                     debug!("script_tx.send failed, shutting down");
                 }
+                RunnerTaskState::finished_no_children()
             };
-            scope.spawn_cancellable(run_scripts_fut, || ());
+            scope.spawn_cancellable(run_scripts_fut, || RunnerTaskState::Cancelled);
 
             let Some(script_data) = script_rx.blocking_recv() else {
                 // Most likely the harness is shutting down, so we should too.
@@ -337,33 +340,107 @@ impl<'a> TestRunnerInner<'a> {
                         TestGroup::Global => None,
                         TestGroup::Custom(name) => Some(name.clone()),
                     };
-
-                    let fut = executor_cx_ref.run_test_instance(
-                        test_instance,
-                        settings,
-                        resp_tx.clone(),
-                        cancelled_ref,
-                        cancellation_sender.subscribe(),
-                        setup_script_data.clone(),
-                    );
+                    let cancel_rx = cancellation_sender.subscribe();
+                    let resp_tx = resp_tx.clone();
+                    let setup_script_data = setup_script_data.clone();
+
+                    // Use a separate Tokio task for each test. For repos with
+                    // lots of small tests, this has been observed to be much
+                    // faster than using a single task for all tests (what we
+                    // used to do). It also provides some degree of per-test
+                    // isolation.
+                    let fut = async move {
+                        // SAFETY: Within an outer scope_and_block (which we
+                        // have here), scope_and_collect is safe as long as the
+                        // returned future isn't forgotten. We're not forgetting
+                        // it below -- we're running it to completion
+                        // immediately.
+                        //
+                        // But recursive scoped calls really feel like pushing
+                        // against the limits of async-scoped. For example,
+                        // there's no way built into async-scoped to propagate a
+                        // cancellation signal from the outer scope to the inner
+                        // scope. (But there could be, right? That seems
+                        // solvable via channels. And we could likely do our own
+                        // channels here.)
+                        let ((), mut ret) = unsafe {
+                            TokioScope::scope_and_collect(move |scope| {
+                                scope.spawn(executor_cx_ref.run_test_instance(
+                                    test_instance,
+                                    settings,
+                                    resp_tx.clone(),
+                                    cancelled_ref,
+                                    cancel_rx,
+                                    setup_script_data,
+                                ))
+                            })
+                        }
+                        .await;
+
+                        // If no future was started, that's really strange.
+                        // Worth at least logging.
+                        let Some(result) = ret.pop() else {
+                            warn!(
+                                "no task was started for test instance: {}",
+                                test_instance.id()
+                            );
+                            return None;
+                        };
+                        match result {
+                            Ok(()) => None,
+                            Err(join_error) => Some(join_error),
+                        }
+                    };
 
                     (threads_required, test_group, fut)
                 })
-                // future_queue_grouped means tests are spawned in order but returned in
-                // any order.
+                // future_queue_grouped means tests are spawned in the order
+                // defined, but returned in any order.
                 .future_queue_grouped(self.test_threads, groups)
-                .collect::<()>();
-
-            scope.spawn_cancellable(run_tests_fut, || ());
+                // Drop the None values.
+                .filter_map(std::future::ready)
+                .collect::<Vec<_>>()
+                // Interestingly, using a more idiomatic `async move {
+                // run_tests_fut.await ... }` block causes Rust 1.83 to complain
+                // about a weird lifetime mismatch. FutureExt::map as used below
+                // does not.
+                .map(|child_join_errors| RunnerTaskState::Finished { child_join_errors });
+
+            scope.spawn_cancellable(run_tests_fut, || RunnerTaskState::Cancelled);
         });
 
         dispatcher_cx.run_finished();
 
-        // Were there any join errors?
+        // Were there any join errors in tasks?
+        //
+        // If one of the tasks panics, we likely end up stuck because the
+        // dispatcher, which is spawned in the same async-scoped block, doesn't
+        // get relayed the panic immediately. That should probably be fixed at
+        // some point.
+        let mut cancelled_count = 0;
         let join_errors = results
             .into_iter()
-            .filter_map(|r| r.err())
+            .flat_map(|r| {
+                match r {
+                    Ok(RunnerTaskState::Finished { child_join_errors }) => child_join_errors,
+                    // Largely ignore cancelled tasks since it most likely means
+                    // shutdown -- we don't cancel tasks manually.
+                    Ok(RunnerTaskState::Cancelled) => {
+                        cancelled_count += 1;
+                        Vec::new()
+                    }
+                    Err(join_error) => vec![join_error],
+                }
+            })
             .collect::<Vec<_>>();
+
+        if cancelled_count > 0 {
+            debug!(
+                "{} tasks were cancelled -- this \
+                 generally should only happen due to panics",
+                cancelled_count
+            );
+        }
         if !join_errors.is_empty() {
             return Err(join_errors);
         }

nextest-runner/src/runner/internal_events.rs

@@ -24,9 +24,12 @@ use crate::{
 };
 use nextest_metadata::MismatchReason;
 use std::time::Duration;
-use tokio::sync::{
-    mpsc::{UnboundedReceiver, UnboundedSender},
-    oneshot,
+use tokio::{
+    sync::{
+        mpsc::{UnboundedReceiver, UnboundedSender},
+        oneshot,
+    },
+    task::JoinError,
 };
 
 /// An internal event.
@@ -230,3 +233,17 @@ pub(super) enum InternalTerminateReason {
     Timeout,
     Signal(ShutdownRequest),
 }
+
+pub(super) enum RunnerTaskState {
+    Finished { child_join_errors: Vec<JoinError> },
+    Cancelled,
+}
+
+impl RunnerTaskState {
+    /// Mark a runner task as finished and having not run any children.
+    pub(super) fn finished_no_children() -> Self {
+        Self::Finished {
+            child_join_errors: Vec::new(),
+        }
+    }
+}