Feeds: Guard against empty array passed to max() in get_feed_build_date() by chubes4 · Pull Request #11387 · WordPress/wordpress-develop

chubes4 · 2026-03-29T04:29:20Z

Trac Ticket

See https://core.trac.wordpress.org/ticket/59956

Description

get_feed_build_date() passes the result of wp_list_pluck() directly to max() without checking for an empty array. When the query's post objects lack the post_modified_gmt property, wp_list_pluck() returns an empty array and max() throws a ValueError on PHP 8.0+.

This happens in production when bots request RSS feed URLs (e.g. /author/{name}/feed/, /taxonomy/{term}/feed/) for terms or authors with zero published posts. The have_posts() guard on line 722 passes (the query object exists and has a truthy post_count), but the underlying post objects are incomplete — so wp_list_pluck( $wp_query->posts, 'post_modified_gmt' ) returns [].

On PHP 7.x this was a silent warning. On PHP 8.0+ it is a fatal ValueError that returns HTTP 500 to the crawler.

The Fix

Wrap the max() call in if ( ! empty( $modified_times ) ). When the array is empty, $datetime remains false and falls through to the existing get_lastpostmodified( 'GMT' ) fallback on line 741. No behavioral change for feeds that have valid post data.

Testing

Automated: Added test_should_not_error_when_modified_times_is_empty to tests/phpunit/tests/date/getFeedBuildDate.php. The test simulates a query where have_posts() is true but posts lack post_modified_gmt, and verifies the function falls back to get_lastpostmodified() instead of throwing.

Manual (production): Verified on a WordPress 6.9.4 multisite (PHP 8.4) with 1,175 artist taxonomy terms that had zero published posts on the main site. Before the fix, these feed URLs returned HTTP 500 with ValueError: max(): Argument #1 ($value) must contain at least one element. After the fix, all return HTTP 200 with valid empty RSS feeds. Normal feeds with posts are unaffected.

Tested 12 edge cases: empty taxonomy feeds, populated category feeds, author feeds, comment feeds, main site feed, cross-subsite feeds
Monitored debug.log — fatal errors stopped immediately after applying the fix (were firing every 10–20 seconds prior)
Feeds with posts still return correct <item> elements and <lastBuildDate>

AI Disclosure

AI assistance: Yes
Tool(s): Claude Code (Claude Opus 4.6 directly on a VPS via OpenCode)
Used for: Identifying the root cause from production debug logs, drafting the guard condition, and scaffolding the unit test. Final implementation, testing, and PR description were reviewed and edited by a human contributor.

…te(). When `wp_list_pluck()` returns an empty array from posts that lack the `post_modified_gmt` property, `max()` throws a `ValueError` on PHP 8+. This adds an emptiness check before calling `max()`, allowing `$datetime` to remain `false` and fall through to the existing `get_lastpostmodified()` fallback. Includes a unit test that simulates the condition. Fixes #59956.

github-actions · 2026-03-29T04:29:30Z

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github-actions · 2026-03-29T04:29:31Z

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

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

Props extrachill, westonruter.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

github-actions · 2026-03-29T04:32:26Z

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Aligns variable assignments in the test to satisfy the Generic.Formatting.MultipleStatementAlignment sniff.

…_times. The production failure occurs when the posts array contains scalar values (e.g. post IDs) rather than post objects. wp_list_pluck() emits a _doing_it_wrong notice and returns an empty array, which then triggers the max() ValueError. Update the test to use an integer in the posts array and call setExpectedIncorrectUsage() to properly handle the expected notice. See #59956.

westonruter · 2026-04-23T23:33:33Z


 		// Determine the maximum modified time.
-		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
+		if ( ! empty( $modified_times ) ) {


While core uses empty() extensively, we should avoid it if possible. This can be more simply:

Suggested change

if ( ! empty( $modified_times ) ) {

if ( $modified_times ) {

Or more explicitly:

Suggested change

if ( ! empty( $modified_times ) ) {

if ( count( $modified_times ) > 0 ) {

westonruter · 2026-04-23T23:38:49Z

+		global $wp_query;
+


Suggested change

global $wp_query;

westonruter · 2026-04-23T23:39:15Z

+		self::factory()->post->create(
+			array(
+				'post_date' => $datetime->format( 'Y-m-d H:i:s' ),
+			)
+		);
+
+		// Simulate a query where have_posts() is true but wp_list_pluck()
+		// returns an empty array because the posts array contains scalar
+		// values (neither objects nor arrays). This triggers the _doing_it_wrong
+		// notice in WP_List_Util::pluck() and produces an empty result.
+		$wp_query = new WP_Query();
+
+		$wp_query->posts      = array( 1 );
+		$wp_query->post_count = 1;


No need to simulate when we can do the real thing:

Suggested change

self::factory()->post->create(

array(

'post_date' => $datetime->format( 'Y-m-d H:i:s' ),

)

);

// Simulate a query where have_posts() is true but wp_list_pluck()

// returns an empty array because the posts array contains scalar

// values (neither objects nor arrays). This triggers the _doing_it_wrong

// notice in WP_List_Util::pluck() and produces an empty result.

$wp_query = new WP_Query();

$wp_query->posts = array( 1 );

$wp_query->post_count = 1;

$id = self::factory()->post->create(

array(

'post_date' => $datetime->format( 'Y-m-d H:i:s' ),

)

);

// Use a query where have_posts() is true but wp_list_pluck()

// returns an empty array because the posts array contains scalar

// values (neither objects nor arrays). This triggers the _doing_it_wrong

// notice in WP_List_Util::pluck() and produces an empty result.

query_posts(

array(

'posts__in' => array( $id ),

'fields' => 'ids',

)

);

@westonruter

- Replace `! empty( $modified_times )` with truthy check `if ( $modified_times )`, aligning with the ongoing effort to avoid empty() in new core code. - Rewrite the test setup to use the real query codepath: self::factory()->post->create() + query_posts( [ 'posts__in' => [ $id ], 'fields' => 'ids' ] ). 'fields' => 'ids' makes WP_Query->posts an array of integer IDs, which causes wp_list_pluck() to emit _doing_it_wrong and return an empty array naturally — no more manual $wp_query property mutation. - Add $this->assertIsString( $result ) to narrow the string|false return type before it flows into strtotime(), keeping PHPStan happy at higher analysis levels. Addresses review feedback from @westonruter. Props westonruter. See #59956.

chubes4 · 2026-04-24T12:34:44Z

Thanks so much for the thorough review @westonruter — really appreciate you showing up for this one. All five suggestions applied in 6b4c4c3:

! empty( $modified_times ) → if ( $modified_times ) (will bookmark Add checks to disallow use of empty() performance#1219 for future contributions)
Replaced the manual $wp_query property mutation with query_posts( [ 'posts__in' => [ $id ], 'fields' => 'ids' ] ) — much cleaner, and the 'fields' => 'ids' trick to make wp_list_pluck bail naturally is a nice pattern I hadn't seen before
Added $this->assertIsString( $result ) before the strtotime() calls to narrow the string|false return type for PHPStan

Ready for another look whenever you have a minute.

westonruter · 2026-04-24T17:23:19Z

+		// Use a query where have_posts() is true but wp_list_pluck()
+		// returns an empty array because the posts array contains scalar
+		// values (neither objects nor arrays). This triggers the _doing_it_wrong
+		// notice in WP_List_Util::pluck() and produces an empty result.


Minor, per https://developer.wordpress.org/coding-standards/inline-documentation-standards/php/#5-2-multi-line-comments

Suggested change

// Use a query where have_posts() is true but wp_list_pluck()

// returns an empty array because the posts array contains scalar

// values (neither objects nor arrays). This triggers the _doing_it_wrong

// notice in WP_List_Util::pluck() and produces an empty result.

/*

* Use a query where have_posts() is true but wp_list_pluck()

* returns an empty array because the posts array contains scalar

* values (neither objects nor arrays). This triggers the _doing_it_wrong

* notice in WP_List_Util::pluck() and produces an empty result.

*/

Copilot

Pull request overview

This PR hardens feed date generation by preventing a PHP 8+ fatal error in get_feed_build_date() when the list of modified times is empty, ensuring RSS/Atom feeds still render successfully and fall back to get_lastpostmodified( 'GMT' ).

Changes:

Add a guard to avoid calling max() with an empty array in get_feed_build_date().
Add a PHPUnit test that exercises the empty-$modified_times path and verifies the fallback behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
`src/wp-includes/feed.php`	Guards the `max( $modified_times )` call to avoid PHP 8+ `ValueError` on empty arrays and allow existing fallback logic to run.
`tests/phpunit/tests/date/getFeedBuildDate.php`	Adds coverage to ensure `get_feed_build_date()` does not error when plucked modified times are empty and instead falls back to last-modified time.

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Copilot · 2026-04-24T17:27:38Z

+		$datetime     = new DateTimeImmutable( 'now', wp_timezone() );
+		$datetime_utc = $datetime->setTimezone( new DateTimeZone( 'UTC' ) );
+
+		$id = self::factory()->post->create(
+			array(
+				'post_date' => $datetime->format( 'Y-m-d H:i:s' ),
+			)
+		);
+
+		// Use a query where have_posts() is true but wp_list_pluck()
+		// returns an empty array because the posts array contains scalar
+		// values (neither objects nor arrays). This triggers the _doing_it_wrong
+		// notice in WP_List_Util::pluck() and produces an empty result.
+		query_posts(
+			array(
+				'posts__in' => array( $id ),
+				'fields'    => 'ids',
+			)
+		);
+
+		$this->setExpectedIncorrectUsage( 'WP_List_Util::pluck' );
+


The test currently uses query_posts( [ 'fields' => 'ids' ] ) to force wp_list_pluck() to return an empty array, which requires setExpectedIncorrectUsage( 'WP_List_Util::pluck' ) and depends on a DB query plus an unrelated _doing_it_wrong() path. You can simulate the reported inconsistency more directly (and avoid the incorrect-usage expectation) by setting global $wp_query to a fresh WP_Query with post_count = 1 (so have_posts() is true) but an empty posts array (so wp_list_pluck() returns []). This keeps the test focused on guarding max() against empty input.

Suggested change

$datetime = new DateTimeImmutable( 'now', wp_timezone() );

$datetime_utc = $datetime->setTimezone( new DateTimeZone( 'UTC' ) );

$id = self::factory()->post->create(

array(

'post_date' => $datetime->format( 'Y-m-d H:i:s' ),

)

);

// Use a query where have_posts() is true but wp_list_pluck()

// returns an empty array because the posts array contains scalar

// values (neither objects nor arrays). This triggers the _doing_it_wrong

// notice in WP_List_Util::pluck() and produces an empty result.

query_posts(

array(

'posts__in' => array( $id ),

'fields' => 'ids',

)

);

$this->setExpectedIncorrectUsage( 'WP_List_Util::pluck' );

global $wp_query;

$datetime = new DateTimeImmutable( 'now', wp_timezone() );

$datetime_utc = $datetime->setTimezone( new DateTimeZone( 'UTC' ) );

self::factory()->post->create(

array(

'post_date' => $datetime->format( 'Y-m-d H:i:s' ),

)

);

// Simulate a query where have_posts() is true but wp_list_pluck()

// receives an empty posts array, so modified_times is empty.

$wp_query = new WP_Query();

$wp_query->post_count = 1;

$wp_query->posts = array();

@chubes4 Would this replicate the original scenario that you've encountered? Curious what your WP_Query has in it which caused this issue in the first place.

No, that does not replicate the original scenario.

The specific query that triggers it can be observed here: https://github.com/Extra-Chill/data-machine-events/blob/main/inc/Core/Event_Post_Type.php#L153-L180

We use a custom artist taxonomy, and a custom data_machine_events post type.

When there are existing data_machine_events posts using a given artist term, but 0 regular posts, and we query a feed URL with fields => 'ids' set, WP_Query returns an array of scalar ids instead of WP_Post objects.

wp_list_pluck() requires each element to be an object or array, so it hits _doing_it_wrong and returns an empty array. max([]) then fatals.

Is this masking maybe a lower level issue? Shouldn't this also ensure that get_feed_build_date() works when posts is a list of post IDs and not WP_Post objects? This would fix the underlying issue.

Now, it would sense still to include the check added in this PR to prevent passing an empty array to max(), but I see this as additional hardening on top of a more fundamental issue that can be fixed.

I think this may be needed in both of these places:

wordpress-develop/src/wp-includes/feed.php

Line 724 in 59a9905

$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );

wordpress-develop/src/wp-includes/feed.php

Line 729 in 59a9905

$comment_times = wp_list_pluck( $wp_query->comments, 'comment_date_gmt' );

Instead of using wp_list_pluck(), it should be using array_map() and just pass through the value if it is an integer. Otherwise, it can return the ID property of the respective object.

You're right. The original fix only stopped the fatal, but it returned the global last modified time instead of the expected post. The bug was deeper down, and now it returns the timestamp for the correct post.

@westonruter

Follow WordPress inline documentation standards section 5.2 — multi-line comments use /* */ with leading asterisks, not repeated //. Addresses follow-up feedback from @westonruter. Props westonruter. See #59956.

…jects. Replace wp_list_pluck() with array_map() + array_filter() that resolves each entry via get_post() / get_comment(), supporting WP_Query results where $posts contains integer IDs (fields => 'ids') instead of WP_Post objects. Previously, a feed query using fields => 'ids' produced scalar IDs that wp_list_pluck() could not pluck post_modified_gmt from. The function would silently fall through to get_lastpostmodified() and return the site-wide latest modified time instead of the modified time of the posts actually in the feed. Adds a regression test asserting the build date matches the modified time of the post in the feed when fields => 'ids' is used. Follow-up to [60079].

chubes4 and others added 4 commits March 29, 2026 04:35

Fix PHPCS: align equals signs in test assignments.

eeb7bd5

Aligns variable assignments in the test to satisfy the Generic.Formatting.MultipleStatementAlignment sniff.

Merge branch 'trunk' into fix/59956-feed-build-date-empty-array

cdfa9f5

Merge branch 'trunk' into fix/59956-feed-build-date-empty-array

c0fe548

westonruter reviewed Apr 23, 2026

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westonruter reviewed Apr 24, 2026

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westonruter requested a review from Copilot April 24, 2026 17:23

Copilot started reviewing on behalf of westonruter April 24, 2026 17:24 View session

Copilot AI reviewed Apr 24, 2026

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chubes4 added 2 commits April 24, 2026 13:34

Tests: Use multi-line block comment style in get_feed_build_date test.

7962e11

Follow WordPress inline documentation standards section 5.2 — multi-line comments use /* */ with leading asterisks, not repeated //. Addresses follow-up feedback from @westonruter. Props westonruter. See #59956.

	if ( ! empty( $modified_times ) ) {
	if ( count( $modified_times ) > 0 ) {

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