Limit the amount of parallelism in check_all

Don't create more threads than there are CPU cores.

Author: Nahor

src/app_state.rs

@@ -5,7 +5,7 @@ use std::{
     io::{self, Read, Seek, StdoutLock, Write},
     path::{Path, MAIN_SEPARATOR_STR},
     process::{Command, Stdio},
-    sync::mpsc,
+    sync::{atomic::AtomicUsize, mpsc, Arc},
     thread,
 };
 
@@ -20,6 +20,7 @@ use crate::{
 };
 
 const STATE_FILE_NAME: &str = ".rustlings-state.txt";
+const DEFAULT_CHECK_PARALLELISM: usize = 8;
 
 #[must_use]
 pub enum ExercisesProgress {
@@ -411,17 +412,31 @@ impl AppState {
 
         let (mut checked_count, mut results) = thread::scope(|s| {
             let (tx, rx) = mpsc::channel();
-
-            self.exercises
-                .iter()
-                .enumerate()
-                .for_each(|(index, exercise)| {
-                    let tx = tx.clone();
-                    let cmd_runner = &self.cmd_runner;
-                    let _ = thread::Builder::new().spawn_scoped(s, move || {
-                        tx.send((index, exercise.run_exercise(None, cmd_runner)))
-                    });
+            let exercise_ind = Arc::new(AtomicUsize::default());
+
+            let num_core = thread::available_parallelism()
+                .map_or(DEFAULT_CHECK_PARALLELISM, |count| count.get());
+            (0..num_core).for_each(|_| {
+                let tx = tx.clone();
+                let exercise_ind = exercise_ind.clone();
+                let this = &self;
+                let _ = thread::Builder::new().spawn_scoped(s, move || {
+                    loop {
+                        let exercise_ind =
+                            exercise_ind.fetch_add(1, std::sync::atomic::Ordering::AcqRel);
+                        let Some(exercise) = this.exercises.get(exercise_ind) else {
+                            // No more exercises
+                            break;
+                        };
+                        if tx
+                            .send((exercise_ind, exercise.run_exercise(None, &this.cmd_runner)))
+                            .is_err()
+                        {
+                            break;
+                        }
+                    }
                 });
+            });
 
             // Drop this `tx`, since the `rx` loop will not stop while there is
             // at least one tx alive (i.e. we want the loop to block only while