[wp-trac] [WordPress Trac] #55220: Legacy widgets lack access to some usual server-side globals

[WordPress Trac]

#55220: Legacy widgets lack access to some usual server-side globals
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 Reporter:  noisysocks    |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Future Release
Component:  Widgets       |    Version:
 Severity:  normal        |   Keywords:
  Focuses:                |
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 Copied from https://github.com/WordPress/gutenberg/issues/33404.

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 ## Description

 Legacy widgets whose behavior depends on the `$pagenow` global might not
 render as expected via the new `/wp/v2/widget-types/fm-demo/encode` REST
 API endpoint because `$pagenow` during these requests is `index.php`, not
 `widget.php`.

 Additionally, legacy widgets are no longer able to rely on the
 `$_POST['action']` variable during requests to save widget data, which is
 usually set to `save-widget`, and these widgets might not be able to save
 data as expected.

 Similarly, legacy widgets are no longer able to rely on `DOING_AJAX` or
 `wp_doing_ajax()`, although that is typical of the REST API.

 ## Step-by-step reproduction instructions

 The easiest way to observe this behavior is to observe `$pagenow` and
 `$_POST` using Xdebug with a breakpoint inside of
 `WP_REST_Widget_Types_Controller::encode_form_data()`.

 To see the effect of the behavior on a real-world library:

 1. Clone and include the Fieldmanager library:
 https://github.com/alleyinteractive/wordpress-fieldmanager
 2. Clone and include the Fieldmanager Widgets extension:
 https://github.com/alleyinteractive/fm-widgets/
 3. Load the demo widget included in the README:
 https://github.com/alleyinteractive/fm-
 widgets/blob/296d5aef0585f3ff9bcf223e30867e069c13e2ca/README.md
 4. Step through the `\fm_widgets_calculated_context()` function, which
 relies on all of the signals mentioned in the description.

 ## Expected behaviour

 With respect to legacy widgets, `$pagenow` will be `widgets.php` during
 form rendering, and `$_POST['action']` will be `save-widget` during
 saving.

 ## Actual behaviour

 `$pagenow` is `index.php`, and `$_POST['action']` is unset.

 ## Code snippet (optional)

 See README link above.

 ## WordPress information
 - WordPress version: 5.8-RC2
 - Gutenberg version: Not installed
 - Are all plugins except Gutenberg deactivated? No, see repro steps.
 - Are you using a default theme (e.g. Twenty Twenty-One)? Yes

 ## Device information
 - Device: Desktop
 - Operating system: macOS 10.14
 - Browser: Chrome 91

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Ticket URL: <https://core.trac.wordpress.org/ticket/55220>
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