When using switch_to_blog() with a persistent object cache that lacks wp_cache_switch_to_blog() support, non-persistent groups are not maintained

[markjaquith]

If you're using a persistent object cache that lacks the new wp_cache_switch_to_blog() support, WordPress core elses into a complete cache clobbering branch. It is smart about grabbing the global groups and re-adding those after wp_cache_init() is called, but it doesn't do the same for non-persistent groups. Those are a hardcoded array. So if you call switch_to_blog(), you would lose any custom-added non-persistent groups.

[nacin]

Curiously, not a regression — existed back when this was just a wp_cache_init() call too.

[ethitter]

@stevenkword and I worked on ​23290.patch during the Boston WP Hack Day.

We introduced a get_non_persistent_groups() method in Core's WP_Object_Cache to handle the core use, and to be implemented by authors of object cache drop ins. Initially we tried to include universal support for getting the non-persistent cache groups, but review of the APC Object Caching plugin revealed that the structure of $global_groups can vary between persistent backends.

The switching fallback that was present in switch_to_blog() and restore_current_blog() was abstracted into _ms_cache_switch_fallback() for consistency, and logic to use the persistent group getter closely mirrors the handling of global groups already in those functions.

[nacin]

I like this; let's address it in 3.7.

[wonderboymusic]

these are all marked 3.7-early

[nacin]

Let's make sure ryan looks this over.

[nunomorgadinho]

Any suggestions on what a unit test for this one should include? Maybe we could even separate it into different unit tests. I'm unsure and would appreciate feedback as I would be interested in working on it.

[wonderboymusic]

No unit tests - no Ryan blessing

[jeremyfelt]

We probably have an opportunity to get some unit tests in for this in 3.9.

@nunomorgadinho, if you are still interested in working on this, please feel free. It's probably worth looking at the current code in phpunit/tests/cache.php for examples of testing switch_to_blog(). We would then want to use additional tests for the pieces that this patch touches. Don't hesitate to provide any first attempts or stop in #wordpress-dev to go over approach.

[jeremyfelt]

This is probably going to miss 3.9 unless we can get some testing and tests in here.

[nacin]

There are a number of Cache API things we'd like to do, so moving this out of 3.9.

It's much, much better for object caches to introduce wp_cache_switch_to_blog(), so this is fairly low priority.

[r1k0]

Patch does not apply cleanly to current trunk. .rej files are generated for wp-includes/cache.php and wp-includes/ms-blogs.php. Patch needs a refresh to match trunk. Removed needs-testing.

Error:

Running "patch:https://core.trac.wordpress.org/attachment/ticket/23290/23290.patch" (patch) task
patching file wp-includes/cache.php
Hunk #1 FAILED at 608.
1 out of 1 hunk FAILED -- saving rejects to file wp-includes/cache.php.rej
patching file wp-includes/ms-blogs.php
Hunk #1 FAILED at 518.
Hunk #2 FAILED at 579.
Hunk #3 FAILED at 626.
3 out of 3 hunks FAILED -- saving rejects to file wp-includes/ms-blogs.php.rej

[johnjamesjacoby]

23290-2.patch does the following:

• Introduces a new wp_cache_switch_to_blog_fallback() function in ms-blogs.php that only gets called when wp_cache_switch_to_blog() does not exist
• Adds multisite tests for wp_cache_switch_to_blog_fallback() for defaults, custom groups, edge cases, and some integration scenarios

[johnjamesjacoby]

23290-3.patch is a shower-thought approach:

• Add a fallback wp_cache_switch_to_blog() function in cache-compat.php
• Move the fallback code out of switch_to_blog() and restore_current_blog() and stop using function_exists()
• Allows us to catch another edge case where wp_cache_switch_to_blog() doesn't exist but $wp_object_cache->switch_to_blog() does

This works because cache.php is included before cache-compat.php, so WordPress can easily identify internally that wp_cache_switch_to_blog() is missing when it should not be and then include its own fallback for it.

I really like this approach, because it also ensures that wp_cache_switch_to_blog() is always callable by plugins.

The only compromise with this patch iteration is: I could not see how to retroactively introduce a new switch_to_blog feature inside of wp_cache_supports() - it's a catch-22 because existing drop-in plugins would not have needed to announce/register their supporting this, its default return value is false, and it's already confusing enough without added logic inside of the compat wp_cache_supports() function.

[johnjamesjacoby]

23290-4.patch is a riskier side quest - same as 23290-3.patch but:

• Removes the hard-coded calls to wp_cache_switch_to_blog() inside of switch_to_blog() and restore_current_blog()
• Adds wp_cache_switch_to_blog to the switch_blog hook, inside of ms-default-filters.php

This works because wp_cache_switch_to_blog() will always exist now, which allows us to decouple the Cache API from the Blog Switching API, similar to how the Roles API and wp_switch_roles_and_user() is.

I consider this risky because it hides an important function call that has always been an obvious one. But it is neat so I thought I'd suggest it too.

[ozgursar]

I only tested one the patches (23290-3.patch) that @johnjamesjacoby submitted.

Patch Testing Report

Patch Tested: https://core.trac.wordpress.org/attachment/ticket/23290/23290-3.patch

Environment

• WordPress: 7.0-alpha-61215-src
• PHP: 8.2.29
• Server: nginx/1.29.4
• Database: mysqli (Server: 8.4.7 / Client: mysqlnd 8.2.29)
• Browser: Chrome 145.0.0.0
• OS: macOS
• Theme: Twenty Twenty-Five 1.4
• MU Plugins: None activated
• Plugins:
  • Code Snippets 3.9.5
  • SQLite Object Cache 1.6.1
  • Test Reports 1.2.1

Steps taken

1. Enable multisite by adding the following values to the wp-config.php and add a second site to the network.

define( 'WP_ALLOW_MULTISITE', true );
define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );
define( 'DOMAIN_CURRENT_SITE', 'localhost:8889' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );

2. Install and activate SQLite Object Cache plugin
3. Edit /wp-content/object-cache.php and comment out the function wp_cache_switch_to_blog() between the lines 3084-3088 which implements the switch_to_blog()

function wp_cache_switch_to_blog( $blog_id ) {
   global $wp_object_cache;

   $wp_object_cache->switch_to_blog( $blog_id );
}

4. Add the test snippet below via Code Snippets plugin or via functions.php
5. Visit any page of Dashboard to view the test results
6. ✅ Patch is solving the problem

Expected result

After applying the patch, when switch_to_blog() is called on a multisite installation using a persistent object cache drop-in that does not implement wp_cache_switch_to_blog(), custom non-persistent cache groups should be preserved.

Specifically, a value stored in a custom non-persistent group (my_test_group) before the blog switch should still be retrievable after switch_to_blog() and restore_current_blog() are called. The admin notice should display a green success message confirming that the value test_value survived the switch, matching the value stored before the switch.

Additional Notes

Without the patch, the value is lost (returns false) because wp_cache_init() wipes the cache and non-persistent groups are not restored. With the patch, the fallback wp_cache_switch_to_blog() function added to cache-compat.php correctly saves and restores non-persistent groups, so the value survives.

Screenshots/Screencast with results

Before patch:

After patch:

Support Content

• This test snippet is created with the help of Claude.ai

add_action( 'admin_notices', function () {
    if ( ! is_multisite() ) {
        echo '<div class="notice notice-warning"><p><strong>Trac #23290 Test:</strong> Multisite is required.</p></div>';
        return;
    }

    // Check a second site exists
    $sites = get_sites( [ 'number' => 2 ] );
    if ( count( $sites ) < 2 ) {
        echo '<div class="notice notice-warning"><p><strong>Trac #23290 Test:</strong> A second site is required. Please create one under My Sites → Network Admin → Sites → Add New.</p></div>';
        return;
    }

    $other_blog_id = (int) $sites[1]->blog_id;

    wp_cache_add_non_persistent_groups( [ 'my_test_group' ] );
    wp_cache_set( 'test_key', 'test_value', 'my_test_group' );

    $before = wp_cache_get( 'test_key', 'my_test_group' );

    switch_to_blog( $other_blog_id ); // switch to a DIFFERENT blog
    restore_current_blog();

    $after = wp_cache_get( 'test_key', 'my_test_group' );

    $survived = ( 'test_value' === $after );
    $type     = $survived ? 'notice-success' : 'notice-error';
    $status   = $survived
        ? '✅ PASS — Non-persistent group survived switch_to_blog() (patch working).'
        : '❌ FAIL — Non-persistent group was LOST after switch_to_blog() (bug confirmed).';

    echo '<div class="notice ' . $type . '"><p>'
        . '<strong>Trac #23290 — Non-Persistent Cache Groups Test</strong><br>'
        . $status . '<br>'
        . '<small>'
        . 'Switched to blog ID: <code>' . $other_blog_id . '</code><br>'
        . 'Value before switch: <code>' . var_export( $before, true ) . '</code><br>'
        . 'Value after switch: <code>' . var_export( $after, true ) . '</code>'
        . '</small>'
        . '</p></div>';
} );

[johnjamesjacoby]

Unit test file for patches 3 & 4

[johnjamesjacoby]

23290-5.patch is a combo of 3.patch, 4.patch, and phpunit.patch:

• Adds wp_cache_switch_to_blog_fallback() to ms-blogs.php so it can be independently unit tested
• Adds wp_cache_switch_to_blog() to cache-compat.php.php so it always exists
• Adds Tests_Multisite_WpCacheSwitchToBlogFallback with around 25 new tests that the fallback is working
• Moves wp_cache_switch_to_blog() to the switch_blog action

---

Critical eyes may notice that wp_cache_switch_to_blog_fallback() does not have a $blog_id parameter, even though one is passed into it inside of Tests_Multisite_WpCacheSwitchToBlogFallback::call_cache_switch_fallback(). This is intentional, because: the function doesn't currently need the parameter, if we add one later the tests are already covered, and PHP doesn't complain about it.

Tangentially, while manually stress testing this, I discovered that if switch_to_blog() gets called via sunrise.php bad things happen, because ms-default-filters.php isn't included until later, and wp_switch_roles_and_user() (and now wp_cache_switch_to_blog()) are hooked to the switch_blog action. Not certain if this is sanctioned or not, but my guess is not.

---

IMO, this is ready for 2nd opinion & commit. Maybe @jeremyfelt or @spacedmonkey?

[johnjamesjacoby]

In 61760:

Cache API: Introduce fallback for when the wp_cache_switch_to_blog() function is missing.

This commit protects against an edge-case where a persistent object-cache drop-in plugin not including its own wp_cache_switch_to_blog() function would cause non-persistent cache groups to go missing when switching between sites, and includes the following changes:

• A new wp_cache_switch_to_blog_fallback() function in ms-blogs.php which abstracts duplicated code from switch_to_blog() and restore_current_blog() for easier unit testing
• A new wpCacheSwitchToBlogFallback.php file with approximately 25 new unit tests
• Conditionally declares wp_cache_switch_to_blog() in cache-compat.php only if it does not already exist, either via core's cache.php or a drop-in plugin

With this change, WordPress no longer needs to check if the wp_cache_switch_to_blog() function exists (because it always will) so those checks have been removed.

Props ethitter, jeremyfelt, johnjamesjacoby, markjaquith, nacin, ozgursar, r1k0.

Fixes #23290.

[johnjamesjacoby]

In 61772:

Cache API: Handle numerically-keyed global_groups when switching sites.

This commit protects against another multisite cache edge-case where a persistent object-cache drop-in plugin (namely memcached) may use a numerically-keyed global_groups array instead of 'key' => true like the default object-cache class, and includes the following changes:

• Use wp_is_numeric_array() inside of wp_cache_switch_to_blog_fallback() so that the global group names array are always properly formatted regardless of the caching back-end in use
• Add private helper methods to Tests_Multisite_WpCacheSwitchToBlogFallback to properly format global group names, and tweak a few tests to make them more resilient to different caching back-ends

Follow up to r61760.

See #23290.