The [https://make.wordpress.org/core/2021/02/19/feature-plugin-rollback-update-failure/ Rollback Update Failure feature project] creates a temporary backup of plugins and themes before updating. This aims to make the update process more reliable and ensure that if a plugin or theme update fails, the previous version can be safely restored.
If the [https://wordpress.org/plugins/rollback-update-failure/ Rollback Update Failure plugin] is installed, `WP_Upgrader::install_package()` will use the `move_dir()` function from there for better performance. Instead of copying a directory from one location to another, it uses the `rename()` PHP function to speed up the process, which is instrumental in creating a temporary backup without a delay. If the renaming failed, it falls back to `copy_dir()` WP function.
This conditional aims to facilitate broader testing of the feature. It is temporary, until the plugin is merged into core.
Props afragen, pbiron, costdev, davidbaumwald, audrasjb, jrf, SergeyBiryukov.
Fixes#56057. See #51857, #54166.
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