REST API: Restore the global post after preparing a revision

[MicahelE]

Trac: https://core.trac.wordpress.org/ticket/65495

WP_REST_Revisions_Controller::prepare_item_for_response() sets the global $post and calls setup_postdata(), but never restores them, so the change leaks for the rest of the request.

The autosaves controller delegates here, and the block editor preloads /autosaves on every load. When the post has a pending autosave, the global $post is left pointing at the autosave, so the editor can initialize with the wrong post id and redirect to it. That presents as the editor randomly opening a different post.

This captures the previous global $post and restores it on every return path (including the HEAD early return), which also covers the autosaves endpoint.

Reproduction (no browser):

wp eval '
$pid = wp_insert_post( array( "post_status" => "publish" ) );
wp_create_post_autosave( array( "post_ID" => $pid, "post_content" => "x", "post_type" => "post" ) );
global $post; $post = get_post( $pid ); setup_postdata( $post );
rest_get_server()->dispatch( new WP_REST_Request( "GET", "/wp/v2/posts/$pid/autosaves" ) );
echo $GLOBALS["post"]->ID === $pid ? "OK" : "LEAKED";
'

Before: LEAKED. After: OK.

Tests: Adds coverage for the restore (GET, HEAD, and the no-global-post case). The existing "sets up postdata" tests were asserting the leak, so they now confirm rendered fields still reflect the revision while the global post is restored.

[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props micahele, dhrupo.

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[github-actions]

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[dhrupo]

Test Report

Patch tested: this PR (#12248), applied on top of trunk.

Environment

• WordPress: 7.1-alpha (trunk, 62161-src)
• PHP: 8.3.30
• Web server: nginx 1.29.8
• Database: MySQL 9.7.1
• OS: Debian GNU/Linux 13 (trixie) — the official wordpress-develop Docker environment
• Browser: n/a (verified server-side via WP-CLI)
• Theme: Twenty Twenty-Five 1.5
• MU Plugins: none
• Plugins: none

Steps to reproduce

1. Clean single-site install, default theme, no plugins.
2. Create a published post, then create a pending autosave for it.
3. Set the global $post to the published post and call setup_postdata() (simulating normal page context).
4. Dispatch GET /wp/v2/posts/<id>/autosaves with context=edit — this is exactly what block_editor_rest_api_preload() runs on every block-editor load.
5. Inspect the global $post ID after the request returns.

Reproduction script (WP-CLI), taken from the ticket:

wp_set_current_user( 1 );
$pid = wp_insert_post( array( 'post_title' => 'Repro 65495', 'post_content' => 'original body', 'post_status' => 'publish' ) );
wp_create_post_autosave( array( 'post_ID' => $pid, 'post_title' => 'Repro 65495', 'post_content' => 'autosaved body', 'post_type' => 'post' ) );
global $post; $post = get_post( $pid ); setup_postdata( $post );
$before = (int) $GLOBALS['post']->ID;
$req = new WP_REST_Request( 'GET', "/wp/v2/posts/$pid/autosaves" );
$req->set_param( 'context', 'edit' );
rest_get_server()->dispatch( $req );
$after = (int) $GLOBALS['post']->ID;
printf( "before=%d after=%d => %s\n", $before, $after, $after !== $before ? "LEAKED to $after" : "OK" );

Expected result

After the request, the global $post should still point at the post being edited.

Actual result — trunk (before the patch)

post ID=18, global $post before=18, after=19  =>  LEAKED (now points at autosave revision 19)

The global $post is left pointing at the autosave revision. In the block editor this means edit-form-blocks.php can build wp.editPost.initializeEditor() from the wrong post id, and the editor's history sync then "redirects" to the revision — an intermittent bug that only shows up when a pending autosave exists.

Result — with this PR applied

post ID=20, global $post before=20, after=20  =>  OK (restored)

The previous global $post is restored. Verified as consistent across repeated runs by reverting and re-applying the patch on the same install.

Automated tests

phpunit --filter 'WP_Test_REST_Autosaves_Controller|WP_Test_REST_Revisions_Controller'
OK (105 tests, 549 assertions)

All green, including the new coverage added by this PR:

• WP_Test_REST_Revisions_Controller::test_get_items_restores_global_post
• WP_Test_REST_Revisions_Controller::test_get_items_head_request_restores_global_post — the HEAD early-return path
• WP_Test_REST_Revisions_Controller::test_get_items_without_global_post_leaves_it_unset
• WP_Test_REST_Autosaves_Controller::test_get_item_sets_up_postdata_without_leaking_global_post

Notes

The fix captures the previous global $post before setup_postdata() and restores it on every return path, including the HEAD early-return. It correctly leaves the global unset when there was none beforehand rather than clobbering it, which matches the root cause and the wp_reset_postdata() finding noted in the related #43502. Reproduced the regression, confirmed the fix, and the suite is green — looks good to me. 👍