# Am I canceling my std.Io.Group incorrectly? **URL:** \ **Category:** Help\ **Tags:** standard-library\ **Created:** [January 6, 2026, 9:57pm UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836 "2026-01-06T21:57:33Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Zambyte](https://ziggit.dev/user_avatar/ziggit.dev/zambyte/32/4977_2.png) [@Zambyte](https://ziggit.dev/u/Zambyte)\ **Post date:** [January 6, 2026, 9:57pm UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836/1 "2026-01-06T21:57:33Z")
I am using `0.16.0-dev.1912+0cbaaa5eb`, and I have this main loop in my code: ```zig pub fn start(server: *Server, io: Io, gpa: Allocator) !void { var tcp_server = try IpAddress.listen(try IpAddress.parse( server.info.host, server.info.port, ), io, .{}); defer tcp_server.deinit(io); log.debug("Server headers: {s}", .{if (server.info.headers) "true" else "false"}); log.debug("Server max payload: {d}", .{server.info.max_payload}); log.info("Server ID: {s}", .{server.info.server_id}); log.info("Server name: {s}", .{server.info.server_name}); log.info("Server listening on {s}:{d}", .{ server.info.host, server.info.port }); var client_group: Group = .init; defer { log.debug("Canceling client group", .{}); client_group.cancel(io); // the panic seems to happen when I call this. } const read_buffer_size, const write_buffer_size = getBufferSizes(io); log.debug("read buf: {d} write buf: {d}", .{ read_buffer_size, write_buffer_size }); var id: usize = 0; while (true) : (id +%= 1) { if (server.clients.contains(id)) continue; log.debug("Accepting next client", .{}); const stream = tcp_server.accept(io) catch |err| { log.debug("error: {s}", .{@errorName(err)}); // prints error: Canceled return err; }; log.debug("Accepted connection {d}", .{id}); _ = client_group.concurrent(io, handleConnectionInfallible, .{ server, gpa, io, id, stream, read_buffer_size, write_buffer_size, }) catch |err| { log.err("Could not start concurrent handler for {d}", .{id}); stream.close(io); return err; }; } } ``` I’m running into this issue when I connect a client to my server, and then cancel this task while the client is connected: ```zig debug: Server headers: true debug: Server max payload: 1048576 info: Server ID: server-id-123 info: Server name: Zits Server info: Server listening on 0.0.0.0:5224 debug: read buf: 212992 write buf: 212992 debug: Accepting next client debug: Accepted connection 0 debug: Accepting next client ^C info: Shutting down... debug: error: Canceled debug: Canceling client group debug: Client 0 disconnected thread 13559 panic: reached unreachable code robby@lambda ~/src/zits$ /home/robby/downloads/zig-x86_64-linux-0.16.0-dev.1912+0cbaaa5eb/lib/std/debug.zig:419:14: 0x1068389 in assert (std.zig) if (!ok) unreachable; // assertion failure ^ /home/robby/downloads/zig-x86_64-linux-0.16.0-dev.1912+0cbaaa5eb/lib/std/Io/Threaded.zig:260:23: 0x10ca181 in start (std.zig) assert(!cancel_acknowledged); // group task acknowledged cancelation but did not return `error.Canceled` ^ /home/robby/downloads/zig-x86_64-linux-0.16.0-dev.1912+0cbaaa5eb/lib/std/Io/Threaded.zig:1317:29: 0x111ac8d in worker (std.zig) runnable.startFn(runnable, &thread, t); ^ /home/robby/downloads/zig-x86_64-linux-0.16.0-dev.1912+0cbaaa5eb/lib/std/Thread.zig:558:13: 0x10f8a50 in callFn__anon_15769 (std.zig) @call(.auto, f, args); ^ /home/robby/downloads/zig-x86_64-linux-0.16.0-dev.1912+0cbaaa5eb/lib/std/Thread.zig:1499:30: 0x10c8e50 in entryFn (std.zig) return callFn(f, self.fn_args); ^ /home/robby/downloads/zig-x86_64-linux-0.16.0-dev.1912+0cbaaa5eb/lib/std/os/linux/x86_64.zig:105:5: 0x10f8b05 in clone (std.zig) asm volatile ( ^ ``` The assertion comment “group task acknowledged cancelation but did not return `error.Canceled`” makes me feel like this is a bug in the `std.Io.Threaded` implementation, since I cannot return `error.Canceled` any more than I already am. But given how new this all is, I’m not confident that I’m using it right. I also tried fiddling with calling `std.Io.recancel`, but it didn’t seem to resolve the issue. Am I doing things wrong or did I run into a bug? ---
**Author:** ![Zambyte](https://ziggit.dev/user_avatar/ziggit.dev/zambyte/32/4977_2.png) [@Zambyte](https://ziggit.dev/u/Zambyte)\ **Post date:** [January 6, 2026, 10:11pm UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836/2 "2026-01-06T22:11:36Z")
I figured it out. I didn’t realize that functions started with `std.Io.Group.concurrent` could actually return `error.Canceled`, I thought they had to return `void`. This basically solved my problem: ```diff diff --git a/src/server/Server.zig b/src/server/Server.zig index e5f199f643..18214ae45c 100644 --- a/src/server/Server.zig +++ b/src/server/Server.zig @@ -126,9 +126,10 @@ stream: Stream, r_buf_size: usize, w_buf_size: usize, -) void { - handleConnection(server, server_allocator, io, id, stream, r_buf_size, w_buf_size) catch |err| { - log.err("Failed processing client {d}: {any}", .{ id, err }); +) !void { + handleConnection(server, server_allocator, io, id, stream, r_buf_size, w_buf_size) catch |err| switch (err) { + error.Canceled => return error.Canceled, + else => log.err("Failed processing client {d}: {any}", .{ id, err }), }; } @@ -229,8 +230,9 @@ }, } } else |err| switch (err) { - error.EndOfStream => { + error.EndOfStream, error.ReadFailed => { log.debug("Client {d} disconnected", .{id}); + return error.Canceled; }, else => { return err; ``` ---
**Author:** ![andrewrk](https://ziggit.dev/user_avatar/ziggit.dev/andrewrk/32/7011_2.png) [@andrewrk](https://ziggit.dev/u/andrewrk)\ **Post date:** [January 7, 2026, 12:35am UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836/3 "2026-01-07T00:35:27Z")
That’s not quite right - you can’t materialize an `error.Canceled` out of thin air, or it will cause an assertion failure in that same place. I think the reason this appears to work is that you are receiving `error.ReadFailed` and the underlying reader err is set to `error.Canceled` which means you’re _not_ materializing it, you’re actually just forwarding it. But the way `error.EndOfStream` is handled is now incorrect, as well as the case when a different read failure occurs. Suggested change: ```zig } else |err| switch (err) { error.EndOfStream => return error.ClientDisconnected, error.ReadFailed => return reader.err.?, // could be `error.Canceled` or something else ``` If you don’t have access to `reader.err` in this context, then you should return `err` directly and let the parent decode the `error.ReadFailed` into the specific error that occurred. In general, on the API boundary where you construct a `*Io.Reader`, you should expect to receive `error.ReadFailed` and then unwrap that into the more specific error from the concrete reader instance that you created. Edit: everything I said above still applies but also there was a recent bug in the std lib regarding this assertion FYI: [#30723 - Io.Threaded: fix bad assertion - ziglang/zig - Codeberg.org](https://codeberg.org/ziglang/zig/pulls/30723) ---
**Author:** ![Zambyte](https://ziggit.dev/user_avatar/ziggit.dev/zambyte/32/4977_2.png) [@Zambyte](https://ziggit.dev/u/Zambyte)\ **Post date:** [January 7, 2026, 2:04am UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836/4 "2026-01-07T02:04:55Z")
Thanks 🙂 I was unaware that the underlying `std.Io.net.Stream.Reader` exposed its errors like that. That makes a lot of sense though. Here is my new diff: ```diff diff --git a/src/server/Server.zig b/src/Server.zig rename from src/server/Server.zig rename to src/Server.zig index e5f199f643..02c539ac16 100644 --- a/src/server/Server.zig +++ b/src/Server.zig @@ -126,9 +128,11 @@ stream: Stream, r_buf_size: usize, w_buf_size: usize, -) void { - handleConnection(server, server_allocator, io, id, stream, r_buf_size, w_buf_size) catch |err| { - log.err("Failed processing client {d}: {any}", .{ id, err }); +) !void { + handleConnection(server, server_allocator, io, id, stream, r_buf_size, w_buf_size) catch |err| switch (err) { + error.Canceled => return error.Canceled, + error.ClientDisconnected => log.debug("Client {d} disconnected", .{id}), + else => log.err("Failed processing client {d}: {any}", .{ id, err }), }; } @@ -229,12 +236,8 @@ }, } } else |err| switch (err) { - error.EndOfStream => { - log.debug("Client {d} disconnected", .{id}); - }, - else => { - return err; - }, + error.EndOfStream => return error.ClientDisconnected, + else => return reader.err.?, } } ``` ---
**Author:** ![andrewrk](https://ziggit.dev/user_avatar/ziggit.dev/andrewrk/32/7011_2.png) [@andrewrk](https://ziggit.dev/u/andrewrk)\ **Post date:** [January 7, 2026, 2:12am UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836/5 "2026-01-07T02:12:25Z")
Some more tips: 1. `"{t}", .{err}` if you just want the “Foo” part of `error.Foo`. (also works for enums) 2. the error handling is still a bit unsafe. I recommend to only assume `reader.err` is non-null in the case you see that exact error code: ```zig error.ReadFailed => return reader.err.?, else => |e| return e, ``` You could also handle `error.EndOfStream` with the `else` prong if that error code unambiguously tells you a client disconnected. Hope that helps! ---
**Author:** ![Zambyte](https://ziggit.dev/user_avatar/ziggit.dev/zambyte/32/4977_2.png) [@Zambyte](https://ziggit.dev/u/Zambyte)\ **Post date:** [January 7, 2026, 3:19am UTC](https://ziggit.dev/t/am-i-canceling-my-std-io-group-incorrectly/13836/6 "2026-01-07T03:19:08Z")
Very helpful indeed, thanks again!