object: fix race in appension to MPSC queue cleanup list

Fixes hexops/mach#1414

Signed-off-by: Emi <emi@hexops.com>

src/mpsc.zig

@@ -24,6 +24,7 @@ pub fn Pool(comptime Node: type) type {
         head: ?*Node = null,
 
         // Tracks chunks of allocated nodes, used for freeing them at deinit() time.
+        cleanup_mu: std.Thread.Mutex = .{},
         cleanup: std.ArrayListUnmanaged([*]Node) = .{},
 
         // How many nodes to allocate at once for each chunk in the pool.
@@ -72,10 +73,25 @@ pub fn Pool(comptime Node: type) type {
                 break; // Pool is empty
             }
 
-            // Pool is empty, allocate new chunk of nodes, and track the pointer for later cleanup
+            // Pool is empty, we need to allocate new nodes
+            // This is the rare path where we need a lock to ensure thread safety only for the
+            // pool.cleanup tracking list.
+            pool.cleanup_mu.lock();
+
+            // Check the pool again after acquiring the lock
+            // Another thread might have already allocated nodes while we were waiting
+            const head2 = @atomicLoad(?*Node, &pool.head, .acquire);
+            if (head2) |_| {
+                // Pool is no longer empty, release the lock and try to acquire a node again
+                pool.cleanup_mu.unlock();
+                return pool.acquire(allocator);
+            }
+
+            // Pool still empty, allocate new chunk of nodes, and track the pointer for later cleanup
             const new_nodes = try allocator.alloc(Node, pool.chunk_size);
             errdefer allocator.free(new_nodes);
             try pool.cleanup.append(allocator, @ptrCast(new_nodes.ptr));
+            pool.cleanup_mu.unlock();
 
             // Link all our new nodes (except the first one acquired by the caller) into a chain
             // with eachother.
@@ -311,3 +327,43 @@ test "basic" {
     try std.testing.expectEqual(queue.pop(), 3);
     try std.testing.expectEqual(queue.pop(), null);
 }
+
+test "concurrent producers" {
+    const allocator = std.testing.allocator;
+
+    var queue: Queue(u32) = undefined;
+    try queue.init(allocator, 32);
+    defer queue.deinit(allocator);
+
+    const n_jobs = 100;
+    const n_entries: u32 = 10000;
+
+    var pool: std.Thread.Pool = undefined;
+    try std.Thread.Pool.init(&pool, .{ .allocator = allocator, .n_jobs = n_jobs });
+    defer pool.deinit();
+
+    var wg: std.Thread.WaitGroup = .{};
+    for (0..n_jobs) |_| {
+        pool.spawnWg(
+            &wg,
+            struct {
+                pub fn run(q: *Queue(u32)) void {
+                    var i: u32 = 0;
+                    while (i < n_entries) : (i += 1) {
+                        q.push(allocator, i) catch unreachable;
+                    }
+                }
+            }.run,
+            .{&queue},
+        );
+    }
+
+    wg.wait();
+
+    // Verify we can read some values without crashing
+    var count: usize = 0;
+    while (queue.pop()) |_| {
+        count += 1;
+        if (count >= n_jobs * n_entries) break;
+    }
+}