[wp-trac] [WordPress Trac] #31436: Handle conflicts in concurrent Customizer sessions

[WordPress Trac]

#31436: Handle conflicts in concurrent Customizer sessions
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 Reporter:  westonruter   |      Owner:
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Future Release
Component:  Customize     |    Version:  3.4
 Severity:  normal        |   Keywords:  needs-patch
  Focuses:  javascript    |
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 If two users open the Customizer at the same time and modify the same
 settings, the user who saves their changes last will win out, and the
 person who saves first will have their changes lost. (The frequency of the
 problem was reduced in #29983 since now only dirty settings now get
 POSTed.) The Customizer needs Heartbeat integration to add the “Post
 Locking” functionality. We don't need to lock the entire Customizer,
 however, from concurrent users: we need to add locking for individual
 settings in the Customizer. When a setting becomes dirty, we need to
 broadcast via Heartbeat to other users that the setting has been changed
 and thus any controls for this setting should be marked as "locked", with
 any changes prevented. This will become increasingly important as more and
 more settings are added to the Customizer and users go there more often to
 make changes.

 The locking UI could provide a button to copy the other user's change into
 the other Customizer session, and this could result in the control being
 editable again, with subsequent changes pushed out to other users as well,
 who would then also get the corresponding setting automatically updated if
 it was dirty, but if it was not dirty then it would remain in its clean
 state but with a locking notification added.

 This also should apply when a setting is modified by some means other than
 the Customizer: if someone is in the Customizer and another user changes a
 setting via an admin page or via XML-RPC or REST API, then this setting
 update should also be illustrated in the Customizer to note that the
 settings are stale and should be refreshed. This refresh could be done
 inline, without having to reload the entire page.

 Another special consideration needs to be made for Widgets in the
 Customizer. Each WP_Widget 2.0 “multi-widget” gets an index number
 associated with each instance of a give type. When you add a widget, this
 number gets incremented (`widget.set( 'multi_number', widget.get(
 'multi_number' ) + 1 );`). The initial multi-number is calculated from the
 `next_widget_id_number()` function which takes the max number currently
 used, and increments it by one. It is thus possible for multiple widgets
 to be deleted in one session (from the widgets admin page, since
 Customizer only removes widgets by moving them to the Inactive Widgets
 sidebar) then new ones added back in other sessions and the initial
 `multi_number` will not be consistent. In other words, the `multi_number`
 for each widget type needs to be synced across each Customizer session
 along with the actual setting values.

 Some enhancements for a feature plugin: The Customizer UI would benefit
 from having a list of users currently in the Customizer appearing
 somewhere, with a list of the changes each user has made. If someone left
 their Customizer session open, this list would also allow an administrator
 to sign the user out, using something like the
 [https://wordpress.org/plugins/user-session-control/ User Session Control]
 plugin; or the Customizer UI could provide a way to boot a user from the
 Customizer.

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Ticket URL: <https://core.trac.wordpress.org/ticket/31436>
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