Endpoint conns default emits a multi‑hundred‑MB zero blob into the binary

[Daeren]

Description

Endpoint(cfg) declares:

conns: [max]SessionT = [_]SessionT{.{}} ** max,

With game-sized limits (e.g. max_connections = 64, channel_cap = 256, max_payload ≈ 1200, fragmentation on), each Session is on the order of ~2 MB. Zig materializes the field default as a constant, so the linked executable’s .rdata contains roughly:

max_connections × sizeof(Session)

of zeros (often ~150–200 MB), while .text stays well under 1 MB.

This is unrelated to optimize mode (Debug / ReleaseSafe / ReleaseFast all bloat). Runtime still needs a large Endpoint in memory by design (zero steady-state allocation); the bug is that the same table is also baked into the on-disk binary via the array default.

Live slots are already initialized in findOrAdd:

self.conns[i] = .{};
self.conns[i].setup();

Unused slots are never read (used[]). The [_]SessionT{.{}} ** max default is redundant for correctness and only hurts binary size.

Minimal reproduction

const std = @import("std");
const magnet = @import("magnet");

const Schema = magnet.proto.channels(.{
    .a = .{ .mode = .reliable_ordered, .Message = void },
    .b = .{ .mode = .unreliable, .Message = void },
    .c = .{ .mode = .reliable_ordered, .Message = void },
});

const Cfg = magnet.Config{
    .channels = Schema,
    .limits = .{
        .max_connections = 64,
        .channel_cap = 256,
        .max_payload = 1200,
    },
    .delivery = .{
        .fragmentation = true,
        .max_fragments = 64,
    },
    .mtu = 1400,
    .max_datagram = 1400,
};

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer _ = gpa.deinit();
    const ep = try magnet.open(Cfg, gpa.allocator());
    defer magnet.close(Cfg, gpa.allocator(), ep);
    std.debug.print("Endpoint sizeof = {d} bytes\n", .{@sizeOf(magnet.Endpoint(Cfg))});
}

Build with zig build -Doptimize=ReleaseFast (or any optimize mode) and inspect the exe:

• file size ≈ 100–200+ MB
• .rdata ≈ that size, ~99% zero bytes
• .text remains small

(Exact size depends on Config; the pattern appears as soon as sizeof(Session) × max_connections is large.)

Solution

In src/proto/endpoint.zig, change the conns field default:

// before
conns: [max]SessionT = [_]SessionT{.{}} ** max,

// after
conns: [max]SessionT = undefined,

No other call-site changes are required: findOrAdd / connectTo already assign conns[i] = .{} and call setup() before use. Endpoint{} / magnet.open leave idle slots uninitialized, which is fine while used[i] == false.

*Same pattern on Server.baselines, much smaller; optional follow-up (baselines: [max_clients]Inner = undefined)

[schphe]

good catch, thanks for the fix!