processReplace

fn processReplace(userdata: ?*anyopaque, options: process.ReplaceOptions) process.ReplaceError {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

    try ev.scanEnviron(); // for PATH
    const PATH = ev.environ.string.PATH orelse default_PATH;

    var arena_allocator = std.heap.ArenaAllocator.init(ev.allocator());
    defer arena_allocator.deinit();
    const arena = arena_allocator.allocator();

    const argv_buf = try arena.allocSentinel(?[*:0]const u8, options.argv.len, null);
    for (options.argv, 0..) |arg, i| argv_buf[i] = (try arena.dupeSentinel(u8, arg, 0)).ptr;

    const env_block = env_block: {
        const prog_fd: i32 = -1;
        if (options.environ_map) |environ_map| break :env_block try environ_map.createPosixBlock(arena, .{
            .zig_progress_fd = prog_fd,
        });
        break :env_block try ev.environ.process_environ.createPosixBlock(arena, .{
            .zig_progress_fd = prog_fd,
        });
    };

    var sync: CancelRegion.Sync = try .init(ev);
    defer sync.deinit(ev);
    return execv(&sync, options.expand_arg0, argv_buf.ptr[0].?, argv_buf.ptr, env_block, PATH);
}