REST API: Stop rest_send_cors_headers() clobbering Access-Control-Allow-Methods, and make the list filterable.

[moonmeister]

Trac ticket: https://core.trac.wordpress.org/ticket/46992 — background, prior art and the caveat are there.

Approach discussed in person with @adamsilverstein, who closed the ticket in 2019 and has since agreed on option C.

Changes

1. Don't clobber a value set on the response. rest_send_cors_headers() runs on rest_pre_serve_request, after the response's own headers have been sent, and header() replaces by default — so the hardcoded list discarded anything set via rest_post_dispatch.

$response_headers = ( $result instanceof WP_HTTP_Response ) ? $result->get_headers() : array();
$response_headers = array_change_key_case( $response_headers );

if ( ! isset( $response_headers['access-control-allow-methods'] ) ) {
	// ...send the default...
}

get_headers() rather than headers_list(): it reads what the response intends rather than what PHP already emitted, and needs no Xdebug under CLI.

2. Add rest_allowed_cors_methods. Same shape as the sibling CORS list filters rest_exposed_cors_headers and rest_allowed_cors_headers — array in, imploded out, $request in context. rest_pre_serve_request already passes $request, hence add_filter( ..., 10, 3 ).

$allow_methods = array( 'OPTIONS', 'GET', 'POST', 'PUT', 'PATCH', 'DELETE' );
$allow_methods = apply_filters( 'rest_allowed_cors_methods', $allow_methods, $request );
header( 'Access-Control-Allow-Methods: ' . implode( ', ', $allow_methods ) );

The filter runs only when the response has not set the header — a value on the response is the more specific instruction, and there is no default left to shape.

Both are needed: (1) alone forces a plugin to restate the whole list, so two plugins conflict instead of composing; (2) alone still gets discarded when a response sets its own value.

The default output is unchanged for every site.

Tests

rest_send_cors_headers() had no coverage at all. New file, 10 tests / 46 assertions — both changes plus baseline behavior (Origin present and absent, default output, Vary still appending, Access-Control-Allow-Origin and -Credentials not response-settable).

• 6 of 10 fail against stock rest-api.php.
• --group restapi: 3560 / 16285, no new failures against a 3550 / 16239 baseline. The one error in both runs (Test_oEmbed_Controller::test_proxy_with_classic_embed_provider) is pre-existing on trunk.

⚠️ The tests read the wire with xdebug_get_headers() — these headers bypass WP_REST_Server::send_header(), so Spy_REST_Server cannot see them (same approach as tests/phpunit/tests/oembed/headers.php). Without Xdebug they skip silently and the suite reports green.

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