Add Speculative Loading opt-in for authenticated requests

[westonruter]

Speculative Loading was omitted from logged-in users per #1157:

Prefetching really shines with page caching as the server is able to deliver the content almost instantly with near zero CPU usage without waiting for the page to be generated. Logged in visits are predominately dynamic non cacheable requests.

See also #1157 (comment):

The ability to have speculation rules omitted by default for logged-in users came up today in the support forum.

One concern I have about doing this is that a user may activate Speculative Loading and then notice that the plugin doesn't seem to do anything. In this case, maybe it should be enabled for logged-out users and for admin users (who can activate plugins)? Presumably there would not be many admin users navigating around the site while being logged-in, so they wouldn't contribute significantly to the server load. What this means is that logged-in users who are not administrators would not get speculative loading by default, such as in the case of an e-commerce site or membership site.

There could be a filter to customize whether speculation rules are added to the page. For example, the plugin could do:

$should_print = apply_filters( 
    'plsr_print_speculation_rules', 
    ! is_user_logged_in() || current_user_can( 'activate_plugins' ) 
);

In this way, if a site wants to force speculation rules to always be printed, they could add this plugin code:

add_filter( 'plsr_print_speculation_rules', '__return_true' );

Or they could add their own arbitrary conditions for whether to print the speculation rules.

The important thing is to have the right defaults for the 80% of sites.

Nevertheless, if the server is able to handle the load of additional dynamic non-cacheable requests, there should be an opt-in to allow users to enable Speculative Loading for logged-in users. For sites where users are primarily logged-in (for example e-commerce, forums, social, membership) there should already be infrastructure in place to scale for non-cached responses. Users of such sites should be able to benefit from Speculative Loading as well. In fact, Speculative Loading is even more necessary for such sites since uncached requests involve more latency.

Being able to opt in to Speculative Loading would allow the administrators to experience the site more closely to what regular visitors experience, without first having to log out. In order to facilitate this specifically, the opt-in can limit to just administrators in addition to unauthenticated visitors. (Maybe the plugin should even default to enabling Speculative Loading for administrators, so they are able to see the impact of the plugin after it is activated.)

This is related to the No-cache BFCache plugin which allows for instant back/forward navigations to be enabled for authenticated pages. With this PR, authenticated pages also can leverage Speculative Loading. This greatly improves the experience of a logged-in user browsing around a site, perhaps the single user who browses the site the most: the administrator!

This adds a new option to the settings screen:

Adding an such an option to the plugin does not violate the 80% principle since it's not targeting core.

To Do

• Update tests to account for new setting.

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Co-authored-by: westonruter <[email protected]>
Co-authored-by: swissspidy <[email protected]>
Co-authored-by: felixarntz <[email protected]>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.22%. Comparing base (f1e5c20) to head (ac30308).
⚠️ Report is 19 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2097      +/-   ##
==========================================
+ Coverage   67.11%   67.22%   +0.11%     
==========================================
  Files          93       93              
  Lines        7732     7758      +26     
==========================================
+ Hits         5189     5215      +26     
  Misses       2543     2543              

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[westonruter]

• Update README to note the new functionality, but after Update Speculative Loading readme description to note purpose after core merge #2120 has been merged to avoid merge conflicts.

[felixarntz]

@westonruter Looks great! A few notes that ideally should be addressed before merge.

[felixarntz]

For readability, it would be more accessible to have separate conditions and early return true or return false instead of this giant single clause.

[westonruter]

IMO, this seems more readable to me. It would also facilitate eventual filtering per #2097 (comment)

Otherwise, is this what you have in mind?

	// Disabled if the user is logged in, unless the setting explicitly allows the current user's role.
	if (
		is_user_logged_in()
		&&
		'any' !== $option['authentication']
		&&
		( ! current_user_can( 'manage_options' ) || 'logged_out_and_admins' !== $option['authentication'] )
	) {
		return false;
	}

	// Disabled if pretty permalinks are not enabled, unless explicitly overridden by the filter.
	if (
		! (bool) get_option( 'permalink_structure' )
		&&
		/**
		 * Filters whether speculative loading should be enabled even though the site does not use pretty permalinks.
		 *
		 * Since query parameters are commonly used by plugins for dynamic behavior that can change state, ideally any
		 * such URLs are excluded from speculative loading. If the site does not use pretty permalinks though, they are
		 * impossible to recognize. Therefore, speculative loading is disabled by default for those sites.
		 *
		 * For site owners of sites without pretty permalinks that are certain their site is not using such a pattern,
		 * this filter can be used to still enable speculative loading at their own risk.
		 *
		 * @since 1.4.0
		 *
		 * @param bool $enabled Whether speculative loading is enabled even without pretty permalinks.
		 */
		! apply_filters( 'plsr_enabled_without_pretty_permalinks', false )
	) {
		return false;
	}

[felixarntz]

Yeah. Separate clauses are easier to follow I think, as you don't have to look at a large multi-nested clause and figure out where the || and && are :)

[westonruter]

Done in 48b9016

[felixarntz]

We may want to explicitly include 'logged_out' === $option['authentication'] next to this. Makes it easier to understand why the ! is_user_logged_in(), plus it also makes it safer e.g. in case we add further possible options in the future there would be no chance to mess up this check here by accident.

[westonruter]

@felixarntz I'm not entirely clear on this. Can you provide a suggestion, especially after 48b9016?

[felixarntz]

This actually looks okay as is now.

[felixarntz]

Worth clarifying here that this would still only affect the frontend I think. Enabling something for logged-in users could easily be misinterpreted as enabling it in WP Admin, which is not what this does.

Also for WP Admin there would be the problems of "action links" (activating plugins by accident etc.), so this would give confidence that it's not a relevant problem for opting in.

[westonruter]

Like so?

diff --git a/plugins/speculation-rules/settings.php b/plugins/speculation-rules/settings.php
index a9789b75..39fbd48f 100644
--- a/plugins/speculation-rules/settings.php
+++ b/plugins/speculation-rules/settings.php
@@ -163,7 +163,7 @@ function plsr_register_setting(): void {
 							'enum'        => array_keys( plsr_get_eagerness_labels() ),
 						),
 						'authentication' => array(
-							'description' => __( 'Only unauthenticated pages are typically served from cache. So in order to reduce load on the server, speculative loading is not enabled by default for logged-in users. If your server can handle the additional load, you can opt in to speculative loading for all logged-in users or just administrator users only.', 'speculation-rules' ),
+							'description' => __( 'Only unauthenticated pages are typically served from cache. So in order to reduce load on the server, speculative loading is not enabled by default for logged-in users. If your server can handle the additional load, you can opt in to speculative loading for all logged-in users or just administrator users only. This only applies to pages on frontend; admin screens are excluded.', 'speculation-rules' ),
 							'type'        => 'string',
 							'enum'        => array_keys( plsr_get_authentication_labels() ),
 						),
@@ -211,7 +211,7 @@ function plsr_add_setting_ui(): void {
 		),
 		'authentication' => array(
 			'title'       => __( 'User Authentication Status', 'speculation-rules' ),
-			'description' => __( 'Only unauthenticated pages are typically served from cache. So in order to reduce load on the server, speculative loading is not enabled by default for logged-in users. If your server can handle the additional load, you can opt in to speculative loading for all logged-in users or just administrator users only.', 'speculation-rules' ),
+			'description' => __( 'Only unauthenticated pages are typically served from cache. So in order to reduce load on the server, speculative loading is not enabled by default for logged-in users. If your server can handle the additional load, you can opt in to speculative loading for all logged-in users or just administrator users only. This only applies to pages on frontend; admin screens are excluded.', 'speculation-rules' ),
 		),
 	);
 	foreach ( $fields as $slug => $args ) {

[westonruter]

See also e1f3c18

[felixarntz]

This sounds right. Maybe say "admin screens remain excluded"? To clarify that this only working on the frontend is already the default behavior, and also (currently) the only behavior. But not sure about that, either way works.

[westonruter]

Done in 4add0f4