[Blueprints] Support Data Liberation importer in the importWxr step

[adamziel]

Description

Adds the Data Liberation WXR importer as an option in the importWxr step. The new importer is turned by including the "importer": "data-liberation" option:

{
  "steps": [
    {
      "step": "importWxr",
      "file": {
        "resource": "url",
        "url": "https://raw.githubusercontent.com/wpaccessibility/a11y-theme-unit-test/master/a11y-theme-unit-test-data.xml"
      },
      "importer": "data-liberation"
    }
  ]
}

When the importer option is missing or set to "default," nothing changes in the behavior of the step and it continues using the https://github.com/humanmade/WordPress-Importer importer.

The new importer:

• Rewrites links in the imported content
• Downloads assets through Playground's CORS proxy
• Parallelizes the downloads
• Communicates progress

This PR is a part of #1894

Implementation details

This importWxr step fetches and includes the data-liberation-core.phar file. The phar file is built with Box and contains the importer library with its dependencies, which is a subset of the Data Liberation library, a subset of the Blueprints library, and a few vendor libraries.

This, unfortunately, means that any changes in the PHP files require rebuilding the .phar file. Here's how you can do it:

nx build:phar playground-data-liberation

You can also build the entire Data Liberation package as a WordPress plugin complete with a wp-admin page:

nx build:plugin playground-data-liberation

Both commands will output the built files to packages/playground/data-liberation/dist

The progress updates are a first-class feature of the new importer. The updated importer step receives them in real-time via a post_message_to_js() call running after every import step. Then, it passes them on to the progress bar UI.

Other changes

• TLS traffic now goes through the CORS proxy. Since the new importer uses AsyncHTTP\Client which deals with raw sockets, Playground's TLS-based network bridge runs the outbound traffic through a cors proxy. Technically, TCPOverFetchWebsocket gets the corsProxy URL passed to the playground.boot() call.
• A few composer dependencies were forked, downgraded to PHP 7.2 using Rector, and bundled with this PR to keep the Data Liberation importer working.
• CORS proxy – skip rate-limiting when an env variable is present, filter out the HTTP/2 header, don't pass duplicate Access-Control-* headers, add Accept to Access-Control-Allow-Headers. FYI @brandonpayton

Remaining work

• PHP 7.2 compatibility. Done by forking and Rector-downgrading dependencies that were incompatible with PHP 7.2.
• Report the importer's progress on the overall Blueprint progress bar
• Enqueue the data liberation plugin files for downloading at the blueprint compilation stage
• Don't eagerly rewrite attachments URLs in WP_Stream_Importer. Exposing this information to the API consumer requires an explicit decision. Do we rewrite it? Or do we ignore it?
• Fix the TLS errors at the intersection of Playground network transport and the async HTTP client library
• Separate the markdown importer and its dependencies (md parser, frontmatter parser, Symfony libraries) from the core plugin
• Ship the importer and its tree-shaken deps (URL parser) as a minified zip/phar

Follow-up work

• Reconsider the WP_Import_Session API – do we need so many verbosely named methods? Can we achieve the same outcomes with fewer methods?
• Investigate why there's a significant delay before media downloads start on PHP 7.2 – 7.4. It's likely a PHP.wasm issue.

Testing instructions

• Default importer – Open this link and confirm it does what the current importWxr step do, that is it stays at "Importing content" for a moment, fails to fetch media files (CORS issues in network tools), but inserts posts and pages.
• Data Liberation – Open this link, confirm the import progress is visible and that the content and media indeed get imported:

Related issues

• Issue in importing WXR files #1211
• [Data Liberation] Add WXR import CLI script #2012
• WXR Import: Import posts as correct user #1477
• WXR Import: Allow media files from wpthemetestdata.files.wordpress.com #1250
• Rewrite URLs in imported WXR files to avoid broken navigation links (white screen, errors, nested Playground) #1780

[adamziel]

The failures are now tests that are either flaky or disabled on the trunk. This one seems ready; I'm quite excited! With this PR, we can start solving all the import and export problems Playground was affected by since day 1, e.g. rewriting the URLs in imported content, slow assets downloads, mapping author IDs, etc!