feat(iocp): prefer async connect

src/backend/iocp.zig

@@ -538,13 +538,58 @@ pub const Loop = struct {
             .close => |v| .{ .result = .{ .close = windows.CloseHandle(v.fd) } },
 
             .connect => |*v| action: {
-                const result = windows.ws2_32.connect(asSocket(v.socket), &v.addr.any, @as(i32, @intCast(v.addr.getOsSockLen())));
-                if (result != 0) {
-                    const err = windows.ws2_32.WSAGetLastError();
-                    break :action switch (err) {
-                        else => .{ .result = .{ .connect = windows.unexpectedWSAError(err) } },
+                const as_socket = asSocket(v.socket);
+                // Associate our socket with loop's completion port.
+                self.associate_fd(v.socket) catch unreachable;
+
+                // ConnectEx requires socket to be initially bound.
+                // https://github.com/tigerbeetle/tigerbeetle/blob/main/src/io/windows.zig#L467
+                {
+                    const inaddr_any = std.mem.zeroes([4]u8);
+                    const bind_addr = std.net.Address.initIp4(inaddr_any, 0);
+                    // NOTE: This may return many other errors; we should extend `ConnectError` set.
+                    posix.bind(as_socket, &bind_addr.any, bind_addr.getOsSockLen()) catch unreachable;
+                }
+
+                // NOTE: This can be declared in somewhere else; it all happens in comptime though so no issue putting it here.
+                const LPFN_CONNECTEX = *const fn (
+                    Socket: windows.ws2_32.SOCKET,
+                    SockAddr: *const windows.ws2_32.sockaddr,
+                    SockLen: posix.socklen_t,
+                    SendBuf: ?*const anyopaque,
+                    SendBufLen: windows.DWORD,
+                    BytesSent: *windows.DWORD,
+                    Overlapped: *windows.OVERLAPPED,
+                ) callconv(.winapi) windows.BOOL;
+
+                // Dynamically load the ConnectEx function.
+                const ConnectEx = windows.loadWinsockExtensionFunction(LPFN_CONNECTEX, as_socket, windows.ws2_32.WSAID_CONNECTEX) catch |err| switch (err) {
+                    error.OperationNotSupported => unreachable, // Something other than sockets has given.
+                    error.FileDescriptorNotASocket => unreachable, // Must be preferred on a socket.
+                    error.ShortRead => unreachable,
+                    error.Unexpected => break :action .{ .result = .{ .connect = error.Unexpected } },
+                };
+
+                // Connect attempt.
+                var bytes_transferred: windows.DWORD = 0;
+                const result = ConnectEx(as_socket, &v.addr.any, v.addr.getOsSockLen(), null, 0, &bytes_transferred, &completion.overlapped);
+
+                // If ConnectEx returns `windows.TRUE`, it means operation completed immediately.
+                // Which is most of the time not the case; we should check it anyways though!
+                if (result == windows.FALSE) {
+                    // NOTE: This may return many other errors; we should extend `ConnectError` set.
+                    break :action switch (windows.ws2_32.WSAGetLastError()) {
+                        .WSA_IO_PENDING, .WSAEWOULDBLOCK, .WSA_IO_INCOMPLETE => .{ .submitted = {} }, // Operation will be completed in the future.
+                        else => |err| .{ .result = .{ .connect = windows.unexpectedWSAError(err) } },
                     };
                 }
+
+                // Surprisingly, we connected immediately.
+                // The following is necessary for various functions (shutdown, getsockopt, setsockopt, getsockname, getpeername) to work.
+                // https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nc-mswsock-lpfn_connectex#remarks
+                // https://stackoverflow.com/questions/13598530/connectex-requires-the-socket-to-be-initially-bound-but-to-what
+                _ = windows.ws2_32.setsockopt(asSocket(v.socket), windows.ws2_32.SOL.SOCKET, windows.ws2_32.SO.UPDATE_CONNECT_CONTEXT, null, 0);
+
                 break :action .{ .result = .{ .connect = {} } };
             },
 
@@ -961,7 +1006,7 @@ pub const Completion = struct {
     /// operation for the completion.
     pub fn perform(self: *Completion) Result {
         return switch (self.op) {
-            .noop, .close, .connect, .shutdown, .timer, .cancel => {
+            .noop, .close, .shutdown, .timer, .cancel => {
                 std.log.warn("perform op={s}", .{@tagName(self.op)});
                 unreachable;
             },
@@ -986,6 +1031,26 @@ pub const Completion = struct {
                 break :r .{ .accept = self.op.accept.internal_accept_socket.? };
             },
 
+            .connect => |*v| r: {
+                const as_socket = asSocket(v.socket);
+                var transferred: windows.DWORD = 0;
+                var flags: windows.DWORD = 0;
+                const result = windows.ws2_32.WSAGetOverlappedResult(as_socket, &self.overlapped, &transferred, windows.FALSE, &flags);
+
+                // Connected successfully.
+                if (result == windows.TRUE) {
+                    // The following is necessary for various functions (shutdown, getsockopt, setsockopt, getsockname, getpeername) to work.
+                    // https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nc-mswsock-lpfn_connectex#remarks
+                    // https://stackoverflow.com/questions/13598530/connectex-requires-the-socket-to-be-initially-bound-but-to-what
+                    _ = windows.ws2_32.setsockopt(as_socket, windows.ws2_32.SOL.SOCKET, windows.ws2_32.SO.UPDATE_CONNECT_CONTEXT, null, 0);
+
+                    break :r .{ .connect = {} };
+                }
+
+                // We got an error.
+                break :r .{ .connect = windows.unexpectedWSAError(windows.ws2_32.WSAGetLastError()) };
+            },
+
             .read => |*v| r: {
                 var bytes_transferred: windows.DWORD = 0;
                 const result = windows.kernel32.GetOverlappedResult(v.fd, &self.overlapped, &bytes_transferred, windows.FALSE);