fill

pub fn fill(mr: *MultiReader, unused_capacity: usize, timeout: Io.Timeout) FillError!void {
    const contexts = mr.streams.contexts();
    const io = contexts[0].fr.io;
    var any_completed = false;

    try mr.batch.awaitConcurrent(io, timeout);

    while (mr.batch.next()) |operation| {
        any_completed = true;
        const context = &contexts[operation.index];
        const n = operation.result.file_read_streaming catch |err| {
            context.err = err;
            continue;
        };
        const r = &context.fr.interface;
        r.end += n;
        if (r.buffer.len - r.end < unused_capacity) {
            rebaseGrowing(mr, context, r.bufferedLen() + unused_capacity) catch |err| {
                context.err = err;
                continue;
            };
            assert(r.seek == 0);
        }
        context.vec[0] = r.buffer[r.end..];
        mr.batch.addAt(operation.index, .{ .file_read_streaming = .{
            .file = context.fr.file,
            .data = &context.vec,
        } });
    }

    if (!any_completed) return error.EndOfStream;
}