Use chopsticks and limit philosophers. (#2655)

Use chopstick to explain why 2 are needed to eat.
Limit async to 2 philosophers so they can deadlock in tokio.
(Tested with [3, 4, 5] philosophers and they all were able to run
without deadlock
with lock ordering disabled.)

---------

Co-authored-by: Sterling Stein <scubed2+git@gmail.com>

src/concurrency/async-exercises/dining-philosophers.md

@@ -16,8 +16,8 @@ code below to a file called `src/main.rs`, fill out the blanks, and test that
 
 ```rust,compile_fail
 {{#include dining-philosophers.rs:Philosopher}}
-    // left_fork: ...
-    // right_fork: ...
+    // left_chopstick: ...
+    // right_chopstick: ...
     // thoughts: ...
 }
 
@@ -26,7 +26,7 @@ code below to a file called `src/main.rs`, fill out the blanks, and test that
 {{#include dining-philosophers.rs:Philosopher-eat}}
 {{#include dining-philosophers.rs:Philosopher-eat-body}}
 {{#include dining-philosophers.rs:Philosopher-eat-end}}
-    // Create forks
+    // Create chopsticks
 
     // Create philosophers
 

src/concurrency/async-exercises/dining-philosophers.rs

@@ -18,13 +18,13 @@ use std::sync::Arc;
 use tokio::sync::{mpsc, Mutex};
 use tokio::time;
 
-struct Fork;
+struct Chopstick;
 
 struct Philosopher {
     name: String,
     // ANCHOR_END: Philosopher
-    left_fork: Arc<Mutex<Fork>>,
-    right_fork: Arc<Mutex<Fork>>,
+    left_chopstick: Arc<Mutex<Chopstick>>,
+    right_chopstick: Arc<Mutex<Chopstick>>,
     thoughts: mpsc::Sender<String>,
 }
 
@@ -40,11 +40,11 @@ impl Philosopher {
 
     // ANCHOR: Philosopher-eat
     async fn eat(&self) {
-        // Keep trying until we have both forks
+        // Keep trying until we have both chopsticks
         // ANCHOR_END: Philosopher-eat
-        // Pick up forks...
-        let _left_fork = self.left_fork.lock().await;
-        let _right_fork = self.right_fork.lock().await;
+        // Pick up chopsticks...
+        let _left_chopstick = self.left_chopstick.lock().await;
+        let _right_chopstick = self.right_chopstick.lock().await;
 
         // ANCHOR: Philosopher-eat-body
         println!("{} is eating...", &self.name);
@@ -56,30 +56,33 @@ impl Philosopher {
     }
 }
 
-static PHILOSOPHERS: &[&str] =
-    &["Socrates", "Hypatia", "Plato", "Aristotle", "Pythagoras"];
+// tokio scheduler doesn't deadlock with 5 philosophers, so have 2.
+static PHILOSOPHERS: &[&str] = &["Socrates", "Hypatia"];
 
 #[tokio::main]
 async fn main() {
     // ANCHOR_END: Philosopher-eat-end
-    // Create forks
-    let mut forks = vec![];
-    (0..PHILOSOPHERS.len()).for_each(|_| forks.push(Arc::new(Mutex::new(Fork))));
+    // Create chopsticks
+    let mut chopsticks = vec![];
+    PHILOSOPHERS
+        .iter()
+        .for_each(|_| chopsticks.push(Arc::new(Mutex::new(Chopstick))));
 
     // Create philosophers
     let (philosophers, mut rx) = {
         let mut philosophers = vec![];
         let (tx, rx) = mpsc::channel(10);
         for (i, name) in PHILOSOPHERS.iter().enumerate() {
-            let mut left_fork = Arc::clone(&forks[i]);
-            let mut right_fork = Arc::clone(&forks[(i + 1) % PHILOSOPHERS.len()]);
+            let mut left_chopstick = Arc::clone(&chopsticks[i]);
+            let mut right_chopstick =
+                Arc::clone(&chopsticks[(i + 1) % PHILOSOPHERS.len()]);
             if i == PHILOSOPHERS.len() - 1 {
-                std::mem::swap(&mut left_fork, &mut right_fork);
+                std::mem::swap(&mut left_chopstick, &mut right_chopstick);
             }
             philosophers.push(Philosopher {
                 name: name.to_string(),
-                left_fork,
-                right_fork,
+                left_chopstick,
+                right_chopstick,
                 thoughts: tx.clone(),
             });
         }

src/concurrency/sync-exercises/dining-philosophers.md

@@ -7,12 +7,13 @@ minutes: 20
 The dining philosophers problem is a classic problem in concurrency:
 
 > Five philosophers dine together at the same table. Each philosopher has their
-> own place at the table. There is a fork between each plate. The dish served is
-> a kind of spaghetti which has to be eaten with two forks. Each philosopher can
+> own place at the table. There is a chopstick between each plate. The dish
+> served is spaghetti which requires two chopsticks to eat. Each philosopher can
 > only alternately think and eat. Moreover, a philosopher can only eat their
-> spaghetti when they have both a left and right fork. Thus two forks will only
-> be available when their two nearest neighbors are thinking, not eating. After
-> an individual philosopher finishes eating, they will put down both forks.
+> spaghetti when they have both a left and right chopstick. Thus two chopsticks
+> will only be available when their two nearest neighbors are thinking, not
+> eating. After an individual philosopher finishes eating, they will put down
+> both chopsticks.
 
 You will need a local [Cargo installation](../../cargo/running-locally.md) for
 this exercise. Copy the code below to a file called `src/main.rs`, fill out the
@@ -22,17 +23,17 @@ blanks, and test that `cargo run` does not deadlock:
 
 ```rust,compile_fail
 {{#include dining-philosophers.rs:Philosopher}}
-    // left_fork: ...
-    // right_fork: ...
+    // left_chopstick: ...
+    // right_chopstick: ...
     // thoughts: ...
 }
 
 {{#include dining-philosophers.rs:Philosopher-think}}
 
 {{#include dining-philosophers.rs:Philosopher-eat}}
-        // Pick up forks...
+        // Pick up chopsticks...
 {{#include dining-philosophers.rs:Philosopher-eat-end}}
-    // Create forks
+    // Create chopsticks
 
     // Create philosophers
 

src/concurrency/sync-exercises/dining-philosophers.rs

@@ -18,13 +18,13 @@ use std::sync::{mpsc, Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
-struct Fork;
+struct Chopstick;
 
 struct Philosopher {
     name: String,
     // ANCHOR_END: Philosopher
-    left_fork: Arc<Mutex<Fork>>,
-    right_fork: Arc<Mutex<Fork>>,
+    left_chopstick: Arc<Mutex<Chopstick>>,
+    right_chopstick: Arc<Mutex<Chopstick>>,
     thoughts: mpsc::SyncSender<String>,
 }
 
@@ -41,8 +41,8 @@ impl Philosopher {
     fn eat(&self) {
         // ANCHOR_END: Philosopher-eat
         println!("{} is trying to eat", &self.name);
-        let _left = self.left_fork.lock().unwrap();
-        let _right = self.right_fork.lock().unwrap();
+        let _left = self.left_chopstick.lock().unwrap();
+        let _right = self.right_chopstick.lock().unwrap();
 
         // ANCHOR: Philosopher-eat-end
         println!("{} is eating...", &self.name);
@@ -57,27 +57,29 @@ fn main() {
     // ANCHOR_END: Philosopher-eat-end
     let (tx, rx) = mpsc::sync_channel(10);
 
-    let forks = (0..PHILOSOPHERS.len())
-        .map(|_| Arc::new(Mutex::new(Fork)))
+    let chopsticks = PHILOSOPHERS
+        .iter()
+        .map(|_| Arc::new(Mutex::new(Chopstick)))
         .collect::<Vec<_>>();
 
-    for i in 0..forks.len() {
+    for i in 0..chopsticks.len() {
         let tx = tx.clone();
-        let mut left_fork = Arc::clone(&forks[i]);
-        let mut right_fork = Arc::clone(&forks[(i + 1) % forks.len()]);
+        let mut left_chopstick = Arc::clone(&chopsticks[i]);
+        let mut right_chopstick =
+            Arc::clone(&chopsticks[(i + 1) % chopsticks.len()]);
 
         // To avoid a deadlock, we have to break the symmetry
-        // somewhere. This will swap the forks without deinitializing
+        // somewhere. This will swap the chopsticks without deinitializing
         // either of them.
-        if i == forks.len() - 1 {
-            std::mem::swap(&mut left_fork, &mut right_fork);
+        if i == chopsticks.len() - 1 {
+            std::mem::swap(&mut left_chopstick, &mut right_chopstick);
         }
 
         let philosopher = Philosopher {
             name: PHILOSOPHERS[i].to_string(),
             thoughts: tx,
-            left_fork,
-            right_fork,
+            left_chopstick,
+            right_chopstick,
         };
 
         thread::spawn(move || {