Commit 8b69341271db18a03a4aa7400a4c6679c5fbf5c6 | zig

author	Andrew Kelley <andrew@ziglang.org> 2026-03-05 16:34:16-08:00
committer	Andrew Kelley <andrew@ziglang.org> 2026-03-08 19:20:06-07:00
log	8b69341271db18a03a4aa7400a4c6679c5fbf5c6
tree	56a65d8d8da2f43acac12a77a3095429470739a4
parent	be0f188990d8aedc3f5bea5c9ab436a7f778d59d

std.Io.Threaded: optimize batchAwaitConcurrent for net_receive

Eagerly receive messages with MSG_DONTWAIT before polling. This makes
the DNS resolution use case end up doing:

recvmsg (EAGAIN)
poll
recvmsg (success)
recvmsg (success)

rather than:

poll
recvmsg (success)
poll
recvmsg (success)

1 files changed, 32 insertions(+), 7 deletions(-)

lib
- std
  - Io
    Threaded.zig

lib/std/Io/Threaded.zig+32-7

 @@ -2636,8 +2636,9 @@ fn operate(userdata: ?*anyopaque, operation: Io.Operation) Io.Cancelable!Io.Oper
         .net_receive => |*o| return .{ .net_receive = o: {
             if (!have_networking) break :o .{ error.NetworkDown, 0 };
             if (is_windows) break :o netReceiveWindows(t, o.socket_handle, o.message_buffer, o.data_buffer, o.flags);
-            netReceivePosix(o.socket_handle, &o.message_buffer[0], o.data_buffer, o.flags) catch |err| switch (err) {
+            netReceivePosix(o.socket_handle, &o.message_buffer[0], o.data_buffer, o.flags, false) catch |err| switch (err) {
                 error.Canceled => |e| return e,
+                error.WouldBlock => unreachable,
                 else => |e| break :o .{ e, 0 },
             };
             break :o .{ null, 1 };
 @@ -2846,19 +2847,41 @@ fn batchAwaitConcurrent(userdata: ?*anyopaque, b: *Io.Batch, timeout: Io.Timeout
     {
         var index = b.submitted.head;
         while (index != .none) {
-            const submission = &b.storage[index.toIndex()].submission;
+            const storage = &b.storage[index.toIndex()];
+            const submission = storage.submission;
             switch (submission.operation) {
                 .file_read_streaming => |o| try poll_storage.add(o.file.handle, posix.POLL.IN | posix.POLL.ERR),
                 .file_write_streaming => |o| try poll_storage.add(o.file.handle, posix.POLL.OUT | posix.POLL.ERR),
                 .device_io_control => |o| try poll_storage.add(o.file.handle, posix.POLL.IN | posix.POLL.OUT | posix.POLL.ERR),
-                .net_receive => |o| try poll_storage.add(o.socket_handle, posix.POLL.IN | posix.POLL.ERR),
+                .net_receive => |*o| nb: {
+                    var data_i: usize = 0;
+                    const result: Io.Operation.Result = .{ .net_receive = for (o.message_buffer, 0..) |*msg, msg_i| {
+                        const remaining_data_buffer = o.data_buffer[data_i..];
+                        netReceivePosix(o.socket_handle, msg, remaining_data_buffer, o.flags, true) catch |err| switch (err) {
+                            error.Canceled => |e| return e,
+                            error.WouldBlock => {
+                                if (msg_i != 0) break .{ null, msg_i };
+                                try poll_storage.add(o.socket_handle, posix.POLL.IN | posix.POLL.ERR);
+                                break :nb;
+                            },
+                            else => |e| break .{ e, 0 },
+                        };
+                        data_i += msg.data.len;
+                    } else .{ null, o.message_buffer.len } };
+                    switch (b.completed.tail) {
+                        .none => b.completed.head = index,
+                        else => |tail_index| b.storage[tail_index.toIndex()].completion.node.next = index,
+                    }
+                    storage.* = .{ .completion = .{ .node = .{ .next = .none }, .result = result } };
+                    b.completed.tail = index;
+                },
             }
             index = submission.node.next;
         }
     }
     switch (poll_storage.len) {
         0 => return,
-        1 => if (timeout == .none) {
+        1 => if (timeout == .none and b.completed.head == .none) {
             const index = b.submitted.head;
             const storage = &b.storage[index.toIndex()];
             const result = try operate(t, storage.submission.operation);
 @@ -13221,7 +13244,8 @@ fn netReceivePosix(
     message: *net.IncomingMessage,
     data_buffer: []u8,
     flags: net.ReceiveFlags,
-) net.Socket.ReceiveError!void {
+    nonblocking: bool,
+) (net.Socket.ReceiveError || error{WouldBlock})!void {
     // recvmmsg is useless, here's why:
     // * [timeout bug](https://bugzilla.kernel.org/show_bug.cgi?id=75371)
     // * it wants iovecs for each message but we have a better API: one data
 @@ -13232,7 +13256,8 @@ fn netReceivePosix(
         @as(u32, if (flags.oob) posix.MSG.OOB else 0) |
         @as(u32, if (flags.peek) posix.MSG.PEEK else 0) |
         @as(u32, if (flags.trunc) posix.MSG.TRUNC else 0) |
-        posix.MSG.NOSIGNAL;
+        posix.MSG.NOSIGNAL |
+        @as(u32, if (nonblocking) posix.MSG.DONTWAIT else 0);
     var storage: PosixAddress = undefined;
     var iov: posix.iovec = .{ .base = data_buffer.ptr, .len = data_buffer.len };
 @@ -13280,7 +13305,7 @@ fn netReceivePosix(
             .PIPE => return syscall.fail(error.SocketUnconnected),
             .CONNRESET => return syscall.fail(error.ConnectionResetByPeer),
             .NETDOWN => return syscall.fail(error.NetworkDown),
-            .AGAIN => |err| return syscall.errnoBug(err),
+            .AGAIN => return syscall.fail(error.WouldBlock),
             .BADF => |err| return syscall.errnoBug(err),
             .FAULT => |err| return syscall.errnoBug(err),
             .INVAL => |err| return syscall.errnoBug(err),