Added dynamic plugins_loaded hook priority.

[zinigor]

Before we were using priority 1 always, but some plugins want to use the hook even earlier than that, so we are watching the filters being added on each plugin load and reassigning our own configure call to an earlier priority.

Fixes a bug with authenticated REST API calls with plugins like Give or WPMU enabled.

Changes proposed in this Pull Request:

• Added dynamic plugins_loaded hook priority determining mechanism.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This is a bug fix.

Testing instructions:

• Use a snippet like this to trigger the problem:

function break_jetpack(){
	wp_get_current_user();
}
add_action( 'plugins_loaded', 'break_jetpack', 0 );

• Make an authenticated REST API call to your site using a mobile application or CLI.
• Observe the fatal error.
• Use this PR, observe no fatal errors.

Proposed changelog entry for your changes:

• Fixed a compatibility issue with several plugins on REST API calls.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: March 3, 2020.
Scheduled code freeze: February 25, 2020

Generated by 🚫 dangerJS against 56fb5a3

[kbrown9]

This looks good to me! I manually tested this change as described in the testing instructions and verified that the PR fixes the error.

[kraftbj]

It's pretty clever. Works as advertised, but definitely want more eyes on it since it is a rather big change that has a point release potential.

[astralbodies]

I've tested the branch on my WooCommerce test site with one of the problematic plugins activated and have confirmed that this fixes it. Switching back to stable breaks the API again.

[kraftbj]

Added to branch-8.2 via a6ac28a and bumped version to 8.2.2-beta1 via 4bf2c62f2 to enable beta channel folks to get this update.

[peterfabian]

Does this mean that the autoloader check for plugins_loaded is no longer necessary https://github.com/Automattic/jetpack/blob/master/packages/autoloader/src/autoload.php#L144?

[zinigor]

@peterfabian no, this check applies to other consumers as well that may be using the Autoloader incorrectly. Otherwise we may end up in a situation where a consumer unknowingly forces usage of a library before every plugin has had a chance to load their dependencies.