Comment and term meta lazyloading for `WP_Query` loops, introduced in 4.4,
depended on filter callback methods belonging to `WP_Query` objects. This meant
storing `WP_Query` objects in the `$wp_filter` global (via `add_filter()`),
requiring that PHP retain the objects in memory, even when the local variables
would typically be expunged during normal garbage collection. In cases where a
large number of `WP_Query` objects were instantiated on a single pageload,
and/or where the contents of the `WP_Query` objects were quite large, serious
performance issues could result.
We skirt this problem by moving metadata lazyloading out of `WP_Query`. The
new `WP_Metadata_Lazyloader` class acts as a lazyload queue. Query instances
register items whose metadata should be lazyloaded - such as post terms, or
comments - and a `WP_Metadata_Lazyloader` method will intercept comment and
term meta requests to perform the cache priming. Since `WP_Metadata_Lazyloader`
instances are far smaller than `WP_Query` (containing only object IDs), and
clean up after themselves far better than the previous `WP_Query` methods (bp
only running their callbacks a single time for a given set of queued objects),
the resource use is decreased dramatically.
See [36525] for an earlier step in this direction.
Props lpawlik, stevegrunwell, boonebgorges.
Fixes#35816.
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- Cache the output in non-persistent cache.
- Cache the result from `wp_mkdir_p()` in persistent cache (when present).
- Introduce `wp_get_upload_dir()` for use when not uploading files. It is equivalent to `wp_upload_dir()` but does not check for the existence or create the upload directory.
- Change tests to use the non-cached `_wp_upload_dir()`. They change options on the fly (should never be used in production) to simulate different environments.
- Introduce `_upload_dir_no_subdir()` and `_upload_dir_https()` to facilitate testing. These use the proper `upload_dir` filter to simulate different environments.
Props kovshenin, azaozz.
See #34359.
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Backbone, from 1.1.2 to 1.2.3. Underscore, from 1.6.0 to 1.8.3.
The new versions of Backbone and Underscore offer numerous small bug fixes and some optimizations and other improvements. Check the [http://backbonejs.org/#changelog Backbone changelog] and [http://underscorejs.org/#changelog Underscore changelog] for the full details.
The new versions include some significant changes that may break existing code. Plugins or themes that rely on the bundled Backbone and/or Underscore libraries should carefully check functionality with the latest versions and run any available unit tests to ensure compatibility.
Some changes of note that were addressed in core as part of this upgrade:
* `_.flatten` no longer works with objects since Underscore.js 1.7. `_.flatten()` working with objects was an unintended side-affect of the implementation, see [https://github.com/jashkenas/underscore/issues/1904#issuecomment-60241576 underscore#1904]. Check any `_flatten` usage and only flatten arrays.
* As of Backbone 1.2.0, you can no longer modify the `events` hash or your view's `el` property in `initialize`, so don't try to modify them there.
* Since Underscore 1.7, Underscore templates no longer accept an initial data object. `_.template` always returns a function now so make sure you use it that way.
Props adamsilverstein.
Fixes#34350.
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Introduces `WP_Customize_Manager::get_previewable_devices()` with a `customize_previewable_devices` filter to change the default device and which devices are available for previewing. This is a feature that was first pioneered on WordPress.com.
Props celloexpressions, folletto, valendesigns, westonruter, welcher, adamsilverstein, michaelarestad, Fab1en.
Fixes#31195.
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Lazyloading for comment meta and term meta, introduced into `WP_Query` in
4.4, used flags - `updated_term_meta_cache` and `updated_comment_meta_cache` -
in an attempt to prevent cache priming from happening more than once per query
object. This technique was mostly effective, but not entirely efficient, since
the flag didn't prevent the `lazyload_*_meta` callbacks from running. The
obvious solution - removing the filter callback after it'd be run once - was
dismissed for 4.4 because of concerns that `remove_filter()` could disable
lazyloading too generally in the context of nested queries, due to the way
`_wp_filter_build_unique_id()` doesn't always build sufficiently unique IDs for
similar objects. However, further testing shows that this concern is only valid
in a very small subset of cases, while the cost of keeping the query objects in
memory, via the `$wp_filter` global, is quite significant. As such, this
changeset removes the flags in favor of the `remove_filter()` technique.
See #35454, #35816.
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