concurrency: also show shared ownership when combining Arc/Mutex

Author: fw-immunant

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);
+    let mut handles = Vec::new();
+    for i in 0..5 {
+        handles.push(thread::spawn(|| {
+            v.push(10 * i);
+            println!("v: {v:?}");
+        }));
+    }
 
-    handle.join().unwrap();
-    println!("v: {v:?}");
+    handles.into_iter().for_each(|h| h.join().unwrap());
 }
 ```
 
@@ -33,20 +35,17 @@ 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 v = v.lock().unwrap();
-        v.push(1000);
+    for i in 0..5 {
+        let v = Arc::clone(&v);
+        let mut handles = Vec::new();
+        handles.push(thread::spawn(move || {
+            let mut v = v.lock().unwrap();
+            v.push(10 * i);
+            println!("v: {v:?}");
+        }));
     }
 
-    handle.join().unwrap();
-
-    println!("v: {v:?}");
+    handles.into_iter().for_each(|h| h.join().unwrap());
 }
 ```
 
@@ -56,7 +55,7 @@ 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.