Small fixes to concurrency material (#2737)

See individual commits.

src/concurrency/shared-state/arc.md

@@ -4,25 +4,40 @@ minutes: 5
 
 # `Arc`
 
-[`Arc<T>`][1] allows shared read-only access via `Arc::clone`:
+[`Arc<T>`][1] allows shared, read-only ownership via `Arc::clone`:
 
 ```rust,editable
 use std::sync::Arc;
 use std::thread;
 
+/// A struct that prints which thread drops it.
+#[derive(Debug)]
+struct WhereDropped(Vec<i32>);
+
+impl Drop for WhereDropped {
+    fn drop(&mut self) {
+        println!("Dropped by {:?}", thread::current().id())
+    }
+}
+
 fn main() {
-    let v = Arc::new(vec![10, 20, 30]);
+    let v = Arc::new(WhereDropped(vec![10, 20, 30]));
     let mut handles = Vec::new();
-    for _ in 0..5 {
+    for i in 0..5 {
         let v = Arc::clone(&v);
         handles.push(thread::spawn(move || {
+            // Sleep for 0-500ms.
+            std::thread::sleep(std::time::Duration::from_millis(500 - i * 100));
             let thread_id = thread::current().id();
             println!("{thread_id:?}: {v:?}");
         }));
     }
 
+    // Now only the spawned threads will hold clones of `v`.
+    drop(v);
+
+    // When the last spawned thread finishes, it will drop `v`'s contents.
     handles.into_iter().for_each(|h| h.join().unwrap());
-    println!("v: {v:?}");
 }
 ```
 

src/concurrency/shared-state/example.md

@@ -12,13 +12,15 @@ use std::thread;
 
 fn main() {
     let v = vec![10, 20, 30];
-    let handle = thread::spawn(|| {
-        v.push(10);
-    });
-    v.push(1000);
-
-    handle.join().unwrap();
+    let mut handles = Vec::new();
+    for i in 0..5 {
+        handles.push(thread::spawn(|| {
+            v.push(10 * i);
             println!("v: {v:?}");
+        }));
+    }
+
+    handles.into_iter().for_each(|h| h.join().unwrap());
 }
 ```
 
@@ -32,21 +34,17 @@ use std::thread;
 
 fn main() {
     let v = Arc::new(Mutex::new(vec![10, 20, 30]));
-
-    let v2 = Arc::clone(&v);
-    let handle = thread::spawn(move || {
-        let mut v2 = v2.lock().unwrap();
-        v2.push(10);
-    });
-
-    {
+    let mut handles = Vec::new();
+    for i in 0..5 {
+        let v = Arc::clone(&v);
+        handles.push(thread::spawn(move || {
             let mut v = v.lock().unwrap();
-        v.push(1000);
+            v.push(10 * i);
+            println!("v: {v:?}");
+        }));
     }
 
-    handle.join().unwrap();
-
-    println!("v: {v:?}");
+    handles.into_iter().for_each(|h| h.join().unwrap());
 }
 ```
 
@@ -56,7 +54,7 @@ Notable parts:
   orthogonal.
   - Wrapping a `Mutex` in an `Arc` is a common pattern to share mutable state
     between threads.
-- `v: Arc<_>` needs to be cloned as `v2` before it can be moved into another
+- `v: Arc<_>` needs to be cloned to make a new reference for each new spawned
   thread. Note `move` was added to the lambda signature.
 - Blocks are introduced to narrow the scope of the `LockGuard` as much as
   possible.

src/concurrency/sync-exercises/link-checker.rs

@@ -105,11 +105,12 @@ fn spawn_crawler_threads(
     result_sender: mpsc::Sender<CrawlResult>,
     thread_count: u32,
 ) {
+    // To multiplex the non-cloneable Receiver, wrap it in Arc<Mutex<_>>.
     let command_receiver = Arc::new(Mutex::new(command_receiver));
 
     for _ in 0..thread_count {
         let result_sender = result_sender.clone();
-        let command_receiver = command_receiver.clone();
+        let command_receiver = Arc::clone(&command_receiver);
         thread::spawn(move || {
             let client = Client::new();
             loop {