atproto relay implementation in zig
zlay.waow.tech
fix evented GPF: ReleaseSafe optimizer bug, build with ReleaseFast
Io.Uring fiber context-switch GPFs under ReleaseSafe on x86_64-linux.
Reproduced with a 30-line minimal program (repro_evented.zig):
- Debug (safety ON, opts OFF): passes
- ReleaseFast (safety OFF, opts ON): passes
- ReleaseSmall (safety OFF, opts ON): passes
- ReleaseSafe (safety ON, opts ON): GPF in fiber.zig contextSwitch
This is a zig 0.16-dev stdlib bug (confirmed in both dev.3059 and
dev.3066) — the LLVM optimizer miscompiles safety-checked code in
the fiber context switch path.
Fix: build with ReleaseFast instead of ReleaseSafe. Backend stays
on Io.Evented (fibers on io_uring), eliminating thread-per-PDS.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Dockerfile
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Dockerfile
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# then copy source and build
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COPY src/ src/
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RUN zig build -Doptimize=ReleaseSafe -Dcpu=baseline -Dtarget=x86_64-linux-gnu
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# ReleaseFast (not ReleaseSafe): Io.Uring fiber context-switch GPFs under ReleaseSafe
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# due to a zig 0.16-dev optimizer+safety interaction bug. See repro_evented.zig.
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RUN zig build -Doptimize=ReleaseFast -Dcpu=baseline -Dtarget=x86_64-linux-gnu
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FROM --platform=linux/amd64 debian:bookworm-slim
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RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates && rm -rf /var/lib/apt/lists/*
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repro_evented.zig
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repro_evented.zig
···
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///! minimal reproduction: does Io.Evented fiber context-switch work?
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///! tests several variants to isolate the failure mode
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const std = @import("std");
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const Io = std.Io;
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var evented: Io.Evented = undefined;
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var debug_threaded_io: Io.Threaded = undefined;
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pub const std_options_debug_threaded_io: ?*Io.Threaded = &debug_threaded_io;
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fn fiberNoSleep(_: Io) void {
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std.debug.print(" fiber-nosleep: entered and returning\n", .{});
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}
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fn fiberSleep(io: Io) void {
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std.debug.print(" fiber-sleep: entered, sleeping 50ms\n", .{});
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io.sleep(Io.Duration.fromMilliseconds(50), .awake) catch {
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std.debug.print(" fiber-sleep: cancelled\n", .{});
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return;
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};
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std.debug.print(" fiber-sleep: woke up\n", .{});
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}
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fn fiberYieldOnly(io: Io) void {
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std.debug.print(" fiber-yield: entered, yielding\n", .{});
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io.sleep(Io.Duration.fromMilliseconds(0), .awake) catch {
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std.debug.print(" fiber-yield: cancelled\n", .{});
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return;
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};
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std.debug.print(" fiber-yield: resumed\n", .{});
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}
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pub fn main() !void {
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const allocator = std.heap.c_allocator;
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debug_threaded_io = Io.Threaded.init(allocator, .{});
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try Io.Evented.init(&evented, allocator, .{});
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const io = evented.io();
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std.debug.print("evented init: ok\n", .{});
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// test 1: fiber that returns immediately (no yield)
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std.debug.print("test 1: fiber returns immediately\n", .{});
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{
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var f = try io.concurrent(fiberNoSleep, .{io});
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io.sleep(Io.Duration.fromMilliseconds(10), .awake) catch {};
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f.cancel(io);
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}
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std.debug.print("test 1: PASSED\n\n", .{});
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// test 2: fiber that yields once (0ms sleep)
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std.debug.print("test 2: fiber yields once\n", .{});
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{
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var f = try io.concurrent(fiberYieldOnly, .{io});
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io.sleep(Io.Duration.fromMilliseconds(50), .awake) catch {};
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f.cancel(io);
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}
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std.debug.print("test 2: PASSED\n\n", .{});
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// test 3: fiber that sleeps 50ms
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std.debug.print("test 3: fiber sleeps 50ms\n", .{});
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{
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var f = try io.concurrent(fiberSleep, .{io});
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io.sleep(Io.Duration.fromMilliseconds(200), .awake) catch {};
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f.cancel(io);
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}
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std.debug.print("test 3: PASSED\n\n", .{});
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std.debug.print("all tests passed\n", .{});
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}
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src/main.zig
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src/main.zig
···
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// Evented (fibers): network orchestration, subscriber connections, WS server, broadcasting.
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// Worker threads use a dedicated pool_io (Threaded) for their sync — they never touch Evented io.
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// The broadcast queue bridges workers → broadcaster fiber (atomics only, no Io dependency).
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//
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// NOTE: Io.Uring has a fiber context-switch GPF under ReleaseSafe (optimizer + safety checks).
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// Debug and ReleaseFast both work fine. Build with ReleaseFast until the stdlib bug is fixed.
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// See repro_evented.zig for the minimal reproduction case.
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const Backend = Io.Evented;
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var backend: Backend = undefined;