Upload Media: Enable concurrent sideload uploads

lib/media/class-gutenberg-rest-attachments-controller.php

@@ -476,6 +476,24 @@ public function sideload_item( WP_REST_Request $request ) {
 
 		$image_size = $request['image_size'];
 
+		// Acquire a per-attachment transient lock to prevent concurrent sideloads
+		// from overwriting each other's metadata changes (read-modify-write race).
+		// Uses the same transient lock pattern as wp_maybe_generate_attachment_metadata().
+		$lock_key = 'wp_sideload_att_' . $attachment_id;
+
+		if ( ! $this->acquire_metadata_lock( $lock_key ) ) {
+			// Clean up the uploaded file since we cannot update metadata.
+			wp_delete_file( $path );
+			return new WP_Error(
+				'rest_upload_lock_timeout',
+				__( 'Could not acquire metadata lock for this attachment. Please try again.', 'gutenberg' ),
+				array( 'status' => 503 )
+			);
+		}
+
+		// Invalidate the object cache to read the latest metadata from the database.
+		wp_cache_delete( $attachment_id, 'post_meta' );
+
 		$metadata = wp_get_attachment_metadata( $attachment_id, true );
 
 		if ( ! $metadata ) {
@@ -514,6 +532,8 @@ public function sideload_item( WP_REST_Request $request ) {
 
 		wp_update_attachment_metadata( $attachment_id, $metadata );
 
+		delete_transient( $lock_key );
+
 		$response_request = new WP_REST_Request(
 			WP_REST_Server::READABLE,
 			rest_get_route_for_post( $attachment_id )
@@ -531,4 +551,29 @@ public function sideload_item( WP_REST_Request $request ) {
 
 		return $response;
 	}
+
+	/**
+	 * Acquires a transient-based lock for attachment metadata updates.
+	 *
+	 * Uses the same transient lock pattern as wp_maybe_generate_attachment_metadata()
+	 * in WordPress core. Spins with brief pauses until the lock is available or
+	 * the timeout is reached.
+	 *
+	 * @param string $lock_key Transient key for the lock.
+	 * @param int    $timeout  Maximum wait time in seconds. Default 5.
+	 * @return bool True if the lock was acquired, false on timeout.
+	 */
+	private function acquire_metadata_lock( string $lock_key, int $timeout = 5 ): bool {
+		$start = time();
+
+		while ( get_transient( $lock_key ) ) {
+			if ( ( time() - $start ) >= $timeout ) {
+				return false;
+			}
+			usleep( 50000 ); // 50ms.
+		}
+
+		set_transient( $lock_key, time(), $timeout );
+		return true;
+	}
 }

packages/upload-media/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Enhancement
+
+- Remove sideload upload serialization: thumbnail uploads now run concurrently, governed by `maxConcurrentUploads` instead of being queued one-at-a-time per attachment ([#75257](https://github.com/WordPress/gutenberg/pull/75257)).
+
 ## 0.28.0 (2026-04-01)
 
 ## 0.27.0 (2026-03-18)

packages/upload-media/src/store/private-actions.ts

@@ -43,7 +43,6 @@ import type {
 	PauseQueueAction,
 	QueueItem,
 	QueueItemId,
-	ResumeItemAction,
 	ResumeQueueAction,
 	RevokeBlobUrlsAction,
 	SideloadAdditionalData,
@@ -62,8 +61,6 @@ type ActionCreators = {
 	addItem: typeof addItem;
 	addSideloadItem: typeof addSideloadItem;
 	removeItem: typeof removeItem;
-	pauseItem: typeof pauseItem;
-	resumeItemByPostId: typeof resumeItemByPostId;
 	prepareItem: typeof prepareItem;
 	processItem: typeof processItem;
 	finishOperation: typeof finishOperation;
@@ -94,32 +91,6 @@ type ThunkArgs = {
 	registry: WPDataRegistry;
 };
 
-/**
- * Determines if an upload should be paused to avoid race conditions.
- *
- * When sideloading thumbnails, we need to pause uploads if another
- * upload to the same post is already in progress.
- *
- * @param item      Queue item to check.
- * @param operation Current operation type.
- * @param select    Store selectors.
- * @return Whether the upload should be paused.
- */
-function shouldPauseForSideload(
-	item: QueueItem,
-	operation: OperationType | undefined,
-	select: Selectors
-): boolean {
-	if (
-		operation !== OperationType.Upload ||
-		! item.parentId ||
-		! item.additionalData.post
-	) {
-		return false;
-	}
-	return select.isUploadingToPost( item.additionalData.post as number );
-}
-
 interface AddItemArgs {
 	// It should always be a File, but some consumers might still pass Blobs only.
 	file: File | Blob;
@@ -306,16 +277,6 @@ export function processItem( id: QueueItemId ) {
 			? item.operations[ 0 ][ 1 ]
 			: undefined;
 
-		// If we're sideloading a thumbnail, pause upload to avoid race conditions.
-		// It will be resumed after the previous upload finishes.
-		if ( shouldPauseForSideload( item, operation, select ) ) {
-			dispatch< PauseItemAction >( {
-				type: Type.PauseItem,
-				id,
-			} );
-			return;
-		}
-
 		/*
 		 * If the next operation is an upload, check concurrency limit.
 		 * If at capacity, the item remains queued and will be processed
@@ -516,28 +477,6 @@ export function pauseItem( id: QueueItemId ) {
 	};
 }
 
-/**
- * Resumes processing for a given post/attachment ID.
- *
- * This function looks up paused uploads by post ID and resumes them.
- * It's typically called after a sideload completes to resume paused
- * thumbnail uploads.
- *
- * @param postOrAttachmentId Post or attachment ID.
- */
-export function resumeItemByPostId( postOrAttachmentId: number ) {
-	return async ( { select, dispatch }: ThunkArgs ) => {
-		const item = select.getPausedUploadForPost( postOrAttachmentId );
-		if ( item ) {
-			dispatch< ResumeItemAction >( {
-				type: Type.ResumeItem,
-				id: item.id,
-			} );
-			dispatch.processItem( item.id );
-		}
-	};
-}
-
 /**
  * Removes a specific item from the queue.
  *
@@ -850,11 +789,9 @@ export function sideloadItem( id: QueueItemId ) {
 			signal: item.abortController?.signal,
 			onFileChange: ( [ attachment ] ) => {
 				dispatch.finishOperation( id, { attachment } );
-				dispatch.resumeItemByPostId( post as number );
 			},
 			onError: ( error ) => {
 				dispatch.cancelItem( id, error );
-				dispatch.resumeItemByPostId( post as number );
 			},
 		} );
 	};

packages/upload-media/src/store/private-selectors.ts

@@ -3,7 +3,6 @@
  */
 import {
 	type BatchId,
-	ItemStatus,
 	OperationType,
 	type QueueItem,
 	type QueueItemId,
@@ -51,44 +50,6 @@ export function isBatchUploaded( state: State, batchId: BatchId ): boolean {
 	return batchItems.length === 0;
 }
 
-/**
- * Determines whether an upload is currently in progress given a post or attachment ID.
- *
- * @param state              Upload state.
- * @param postOrAttachmentId Post ID or attachment ID.
- *
- * @return Whether upload is currently in progress for the given post or attachment.
- */
-export function isUploadingToPost(
-	state: State,
-	postOrAttachmentId: number
-): boolean {
-	return state.queue.some(
-		( item ) =>
-			item.currentOperation === OperationType.Upload &&
-			item.additionalData.post === postOrAttachmentId
-	);
-}
-
-/**
- * Returns the next paused upload for a given post or attachment ID.
- *
- * @param state              Upload state.
- * @param postOrAttachmentId Post ID or attachment ID.
- *
- * @return Paused item.
- */
-export function getPausedUploadForPost(
-	state: State,
-	postOrAttachmentId: number
-): QueueItem | undefined {
-	return state.queue.find(
-		( item ) =>
-			item.status === ItemStatus.Paused &&
-			item.additionalData.post === postOrAttachmentId
-	);
-}
-
 /**
  * Determines whether uploading is currently paused.
  *