Module repeat 

Repeat

Sometimes, an action needs to be applied multiple times.

This mainly happens when cleaning redirect URLs. If you handle t.co after bit.ly, then a bit.ly redirect going to a t.co redirect won’t be fully expanded.

Using Action::Repeat (and, for convenience, maps) allows for handling redirect chains of any length and in any order.

{"Repeat": {
  "actions": [
    {"PartMap": {
      "part": "Host",
      "map": {
        "t.co": "ExpandRedirect",
        "bit.ly": "ExpandRedirect",
        "youtube.com": {"RemoveQueryParam": "si"}
      }
    }}
  ]
}}

Let’s say you give the above example a t.co URL that redirects to a bit.ly URL that redirects to a youtube.com URL with an si query parameter.

1. Repeat makes a backup of the current TaskState to later compare with the TaskState at the end of the loop.

2. Repeat applies its contained actions once. Here it will expand the t.co URL into the intermediate bit.ly URL¹.

3. Once all the contained actions are applied, Repeat will compare the modified TaskState with the backup made in step 1.

4. Because the values are different, Repeat does steps 1 through 3 again.

5. At the start of the second iteration, Repeat once again makes a backup of the current task’s state.

6. The second iteration expands the bit.ly URL into the youtube.com URL with an si query parameter.

7. As with step 4, the task’s state has changed, and thus Repeat will run a third time.

8. The third iteration removes the si query parameter from the youtube.com URL and then runs a fourth time.

9. On the fourth iteration, while the Action::RemoveQueryParam is applied, the end result is identical to the backup made at the start of the loop, and therefore Repeat exits and does not run a fifth time.

By default, Action::Repeat applies its contained actions at most 10 times, but this limit can be increased up to u64::MAX. The limit is that high because I wanted to make a Turing Machine.

Reverted changes

It’s important to note that Repeat won’t loop if a URL is changed then the change is reverted. It only cares if the state at the end of a loop is the same as the state at the start of that loop.

For example, assuming you don’t intentionally give this cleaner a URL with an unused_parameter query parameter to make what I’m about to say wrong, this Repeat will only apply its actions once.

{
  "actions": [
    {"Repeat": {
      "actions": [
        {"SetQueryParam": {
          "query_param": "unused_parameter",
          "value": "whatever"
        }},
        {"RemoveQueryParam": "unused_parameter"}
      ]
    }}
  ]
}

To validate this intuition, see the global debugging tutorial.

---

1. Technically this is only true if the HttpClientConfig::redirect_policy is set to RedirectPolicy::None, which for the default cleaner is usually true as it’s set in the Params::http_client_config used by all HTTP requests. ↩