From 45a807ac79dae1c56bee9c6fde28b93ed6a54d9c Mon Sep 17 00:00:00 2001
From: Frances Wingerter <fw@immunant.com>
Date: Mon, 5 May 2025 18:26:42 +0000
Subject: [PATCH] concurrency: also show shared ownership when combining
 Arc/Mutex

---
 src/concurrency/shared-state/example.md | 38 ++++++++++++-------------
 1 file changed, 18 insertions(+), 20 deletions(-)

diff --git a/src/concurrency/shared-state/example.md b/src/concurrency/shared-state/example.md
index 180d9f94..419ea189 100644
--- a/src/concurrency/shared-state/example.md
+++ b/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());
 }
 ```
 
@@ -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 v = v.lock().unwrap();
-        v.push(1000);
+    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(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.