remove the allocator from std.event.Loop

closes #3539

lib/std/event/loop.zig

@@ -12,15 +12,18 @@ const maxInt = std.math.maxInt;
 const Thread = std.Thread;
 
 pub const Loop = struct {
-    allocator: *mem.Allocator,
     next_tick_queue: std.atomic.Queue(anyframe),
     os_data: OsData,
     final_resume_node: ResumeNode,
     pending_event_count: usize,
     extra_threads: []*Thread,
 
-    // pre-allocated eventfds. all permanently active.
+    /// For resources that have the same lifetime as the `Loop`.
-    // this is how we send promises to be resumed on other threads.
+    /// This is only used by `Loop` for the thread pool and associated resources.
+    arena: std.heap.ArenaAllocator,
+
+    /// Pre-allocated eventfds. All permanently active.
+    /// This is how `Loop` sends promises to be resumed on other threads.
     available_eventfd_resume_nodes: std.atomic.Stack(ResumeNode.EventFd),
     eventfd_resume_nodes: []std.atomic.Stack(ResumeNode.EventFd).Node,
 
@@ -127,11 +130,9 @@ pub const Loop = struct {
     /// Thread count is the total thread count. The thread pool size will be
     /// max(thread_count - 1, 0)
     pub fn initThreadPool(self: *Loop, thread_count: usize) !void {
-        // TODO: https://github.com/ziglang/zig/issues/3539
-        const allocator = std.heap.page_allocator;
         self.* = Loop{
+            .arena = std.heap.ArenaAllocator.init(std.heap.page_allocator),
             .pending_event_count = 1,
-            .allocator = allocator,
             .os_data = undefined,
             .next_tick_queue = std.atomic.Queue(anyframe).init(),
             .extra_threads = undefined,
@@ -143,17 +144,17 @@ pub const Loop = struct {
                 .overlapped = ResumeNode.overlapped_init,
             },
         };
+        errdefer self.arena.deinit();
+
         // We need at least one of these in case the fs thread wants to use onNextTick
         const extra_thread_count = thread_count - 1;
         const resume_node_count = std.math.max(extra_thread_count, 1);
-        self.eventfd_resume_nodes = try self.allocator.alloc(
+        self.eventfd_resume_nodes = try self.arena.allocator.alloc(
             std.atomic.Stack(ResumeNode.EventFd).Node,
             resume_node_count,
         );
-        errdefer self.allocator.free(self.eventfd_resume_nodes);
 
-        self.extra_threads = try self.allocator.alloc(*Thread, extra_thread_count);
+        self.extra_threads = try self.arena.allocator.alloc(*Thread, extra_thread_count);
-        errdefer self.allocator.free(self.extra_threads);
 
         try self.initOsData(extra_thread_count);
         errdefer self.deinitOsData();
@@ -161,7 +162,8 @@ pub const Loop = struct {
 
     pub fn deinit(self: *Loop) void {
         self.deinitOsData();
-        self.allocator.free(self.extra_threads);
+        self.arena.deinit();
+        self.* = undefined;
     }
 
     const InitOsDataError = os.EpollCreateError || mem.Allocator.Error || os.EventFdError ||
@@ -407,7 +409,6 @@ pub const Loop = struct {
                 noasync os.close(self.os_data.final_eventfd);
                 while (self.available_eventfd_resume_nodes.pop()) |node| noasync os.close(node.data.eventfd);
                 noasync os.close(self.os_data.epollfd);
-                self.allocator.free(self.eventfd_resume_nodes);
             },
             .macosx, .freebsd, .netbsd, .dragonfly => {
                 noasync os.close(self.os_data.kqfd);