Ensure completions are executed on the currently connected client

For the time being, given that we only allow 1 client at a time, I took a
shortcut to implement this. The server has an incrementing "current_client_id"
which is part of every completion. On completion callback, we just check if
its client_id is still equal to the server's current_client_id.

src/server.zig

@@ -52,6 +52,7 @@ pub const Client = ClientT(*Server, CDP);
 const Server = struct {
     allocator: Allocator,
     loop: *jsruntime.Loop,
+    current_client_id: usize = 0,
 
     // internal fields
     listener: posix.socket_t,
@@ -64,10 +65,9 @@ const Server = struct {
     // a memory poor for our Clietns
     client_pool: std.heap.MemoryPool(Client),
 
-    timeout_completion_pool: std.heap.MemoryPool(Completion),
+    completion_state_pool: std.heap.MemoryPool(CompletionState),
 
     // I/O fields
-    conn_completion: Completion,
     close_completion: Completion,
     accept_completion: Completion,
 
@@ -77,7 +77,7 @@ const Server = struct {
     fn deinit(self: *Server) void {
         self.send_pool.deinit();
         self.client_pool.deinit();
-        self.timeout_completion_pool.deinit();
+        self.completion_state_pool.deinit();
         self.allocator.free(self.json_version_response);
     }
 
@@ -99,40 +99,37 @@ const Server = struct {
     ) void {
         std.debug.assert(self.client == null);
         std.debug.assert(completion == &self.accept_completion);
-
-        const socket = result catch |err| {
+        self.doCallbackAccept(result) catch |err| {
             log.err("accept error: {any}", .{err});
             self.queueAccept();
-            return;
         };
+    }
 
-        const client = self.client_pool.create() catch |err| {
-            log.err("failed to create client: {any}", .{err});
-            posix.close(socket);
-            return;
-        };
+    fn doCallbackAccept(
+        self: *Server,
+        result: AcceptError!posix.socket_t,
+    ) !void {
+        const socket = try result;
+        const client = try self.client_pool.create();
         errdefer self.client_pool.destroy(client);
 
+        self.current_client_id += 1;
         client.* = Client.init(socket, self);
 
         self.client = client;
 
         log.info("client connected", .{});
-        self.queueRead();
-        self.queueTimeout();
+        try self.queueRead();
+        try self.queueTimeout();
     }
 
-    fn queueTimeout(self: *Server) void {
-        const completion = self.timeout_completion_pool.create() catch |err| {
-            log.err("failed to create timeout completion: {any}", .{err});
-            return;
-        };
-
+    fn queueTimeout(self: *Server) !void {
+        const cs = try self.createCompletionState();
         self.loop.io.timeout(
             *Server,
             self,
             callbackTimeout,
-            completion,
+            &cs.completion,
             TimeoutCheck,
         );
     }
@@ -142,7 +139,16 @@ const Server = struct {
         completion: *Completion,
         result: TimeoutError!void,
     ) void {
-        self.timeout_completion_pool.destroy(completion);
+        const cs: *CompletionState = @alignCast(
+            @fieldParentPtr("completion", completion),
+        );
+        defer self.completion_state_pool.destroy(cs);
+
+        if (cs.client_id != self.current_client_id) {
+            // completion for a previously-connected client
+            return;
+        }
+
         const client = self.client orelse return;
 
         if (result) |_| {
@@ -160,20 +166,23 @@ const Server = struct {
         // very unlikely IO timeout error.
         // AKA: we don't requeue this if the connection timed out and we
         // closed the connection.s
-        self.queueTimeout();
+        self.queueTimeout() catch |err| {
+            log.err("queueTimeout error: {any}", .{err});
+        };
     }
 
-    fn queueRead(self: *Server) void {
-        if (self.client) |client| {
-            self.loop.io.recv(
-                *Server,
-                self,
-                callbackRead,
-                &self.conn_completion,
-                client.socket,
-                client.readBuf(),
-            );
-        }
+    fn queueRead(self: *Server) !void {
+        var client = self.client orelse return;
+
+        const cs = try self.createCompletionState();
+        self.loop.io.recv(
+            *Server,
+            self,
+            callbackRead,
+            &cs.completion,
+            client.socket,
+            client.readBuf(),
+        );
     }
 
     fn callbackRead(
@@ -181,7 +190,15 @@ const Server = struct {
         completion: *Completion,
         result: RecvError!usize,
     ) void {
-        std.debug.assert(completion == &self.conn_completion);
+        const cs: *CompletionState = @alignCast(
+            @fieldParentPtr("completion", completion),
+        );
+        defer self.completion_state_pool.destroy(cs);
+
+        if (cs.client_id != self.current_client_id) {
+            // completion for a previously-connected client
+            return;
+        }
 
         var client = self.client orelse return;
 
@@ -205,7 +222,10 @@ const Server = struct {
 
         // if more == false, the client is disconnecting
         if (more) {
-            self.queueRead();
+            self.queueRead() catch |err| {
+                log.err("queueRead error: {any}", .{err});
+                client.close(null);
+            };
         }
     }
 
@@ -218,12 +238,15 @@ const Server = struct {
         const sd = try self.send_pool.create();
         errdefer self.send_pool.destroy(sd);
 
+        const cs = try self.createCompletionState();
+        errdefer self.completion_state_pool.destroy(cs);
+
         sd.* = .{
             .unsent = data,
             .server = self,
             .socket = socket,
-            .completion = undefined,
             .arena = arena,
+            .completion_state = cs,
         };
         sd.queueSend();
     }
@@ -246,6 +269,18 @@ const Server = struct {
         std.debug.assert(completion == &self.close_completion);
         self.queueAccept();
     }
+
+    fn createCompletionState(self: *Server) !*CompletionState {
+        var cs = try self.completion_state_pool.create();
+        cs.client_id = self.current_client_id;
+        cs.completion = undefined;
+        return cs;
+    }
+};
+
+const CompletionState = struct {
+    client_id: usize,
+    completion: Completion,
 };
 
 // I/O Send
@@ -259,17 +294,19 @@ const Send = struct { // Any unsent data we have.
     unsent: []const u8,
 
     server: *Server,
-    completion: Completion,
     socket: posix.socket_t,
+    completion_state: *CompletionState,
 
     // If we need to free anything when we're done
     arena: ?ArenaAllocator,
 
     fn deinit(self: *Send) void {
-        var server = self.server;
         if (self.arena) |arena| {
             arena.deinit();
         }
+
+        var server = self.server;
+        server.completion_state_pool.destroy(self.completion_state);
         server.send_pool.destroy(self);
     }
 
@@ -278,16 +315,25 @@ const Send = struct { // Any unsent data we have.
             *Send,
             self,
             sendCallback,
-            &self.completion,
+            &self.completion_state.completion,
             self.socket,
             self.unsent,
         );
     }
 
     fn sendCallback(self: *Send, _: *Completion, result: SendError!usize) void {
+        const server = self.server;
+        const cs = self.completion_state;
+
+        if (cs.client_id != server.current_client_id) {
+            // completion for a previously-connected client
+            self.deinit();
+            return;
+        }
+
         const sent = result catch |err| {
             log.info("send error: {any}", .{err});
-            if (self.server.client) |client| {
+            if (server.client) |client| {
                 client.close(null);
             }
             self.deinit();
@@ -1011,13 +1057,12 @@ pub fn run(
         .timeout = timeout,
         .listener = listener,
         .allocator = allocator,
-        .conn_completion = undefined,
         .close_completion = undefined,
         .accept_completion = undefined,
         .json_version_response = json_version_response,
         .send_pool = std.heap.MemoryPool(Send).init(allocator),
         .client_pool = std.heap.MemoryPool(Client).init(allocator),
-        .timeout_completion_pool = std.heap.MemoryPool(Completion).init(allocator),
+        .completion_state_pool = std.heap.MemoryPool(CompletionState).init(allocator),
     };
     defer server.deinit();