childWait

fn childWait(userdata: ?*anyopaque, child: *process.Child) process.Child.WaitError!process.Child.Term {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

    var maybe_sync: CancelRegion.Sync.Maybe = .{ .cancel_region = .init() };
    defer maybe_sync.deinit(ev);
    defer ev.childCleanup(child);

    const pid = child.id.?;
    var info: linux.siginfo_t = undefined;
    while (true) {
        const thread = try maybe_sync.cancel_region.awaitIoUring();
        thread.enqueue().* = .{
            .opcode = .WAITID,
            .flags = 0,
            .ioprio = 0,
            .fd = pid,
            .off = @intFromPtr(&info),
            .addr = 0,
            .len = @backingInt(linux.P.PID),
            .rw_flags = 0,
            .user_data = @intFromPtr(maybe_sync.cancel_region.fiber),
            .buf_index = 0,
            .personality = 0,
            .splice_fd_in = linux.W.EXITED |
                @as(i32, if (child.request_resource_usage_statistics) linux.W.NOWAIT else 0),
            .addr3 = 0,
            .resv = 0,
        };
        ev.yield(null, .nothing);
        switch (maybe_sync.cancel_region.errno()) {
            .SUCCESS => {
                if (child.request_resource_usage_statistics) {
                    const sync = try maybe_sync.enterSync(ev);
                    while (true) {
                        try sync.cancel_region.await(.nothing);
                        var rusage: linux.rusage = undefined;
                        switch (linux.errno(linux.waitid(
                            .PID,
                            pid,
                            &info,
                            linux.W.EXITED | linux.W.NOHANG,
                            &rusage,
                        ))) {
                            .SUCCESS => {
                                child.resource_usage_statistics.rusage = rusage;
                                break;
                            },
                            .INTR, .CANCELED => {},
                            .CHILD => |err| return errnoBug(err), // Double-free.
                            else => |err| return unexpectedErrno(err),
                        }
                    }
                }
                const status: u32 = @bitCast(info.fields.common.second.sigchld.status);
                const code: linux.CLD = @fromBackingInt(@intCast(info.code));
                return switch (code) {
                    .EXITED => .{ .exited = @truncate(status) },
                    .KILLED, .DUMPED => .{ .signal = @fromBackingInt(@intCast(status)) },
                    .TRAPPED, .STOPPED => .{ .stopped = @fromBackingInt(@intCast(status)) },
                    _, .CONTINUED => .{ .unknown = status },
                };
            },
            .INTR, .CANCELED => {},
            .CHILD => |err| return errnoBug(err), // Double-free.
            else => |err| return unexpectedErrno(err),
        }
    }
}