Media: Add caching to wp_count_attachments().

This changeset adds caching to `wp_count_attachments()`, for better consistency with `wp_count_posts()`.

Props jeherve, antpb, mukesh27, robinwpdeveloper, costdev.
Fixes #55227.


git-svn-id: https://develop.svn.wordpress.org/trunk@54255 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/post/wpCountAttachments.php

@@ -0,0 +1,33 @@
+<?php
+
+/**
+ * @group post
+ * @group media
+ * @group upload
+ *
+ * @covers ::wp_count_attachments
+ */
+class Tests_Post_wpCountAttachments extends WP_UnitTestCase {
+
+	/**
+	 * Tests that the result is cached.
+	 *
+	 * @ticket 55227
+	 */
+	public function test_wp_count_attachments_should_cache_the_result() {
+		$mime_type = 'image/jpeg';
+		$cache_key = 'attachments:image_jpeg';
+
+		self::factory()->post->create_many(
+			3,
+			array(
+				'post_type'      => 'attachment',
+				'post_mime_type' => $mime_type,
+			)
+		);
+		$expected = wp_count_attachments( $mime_type );
+		$actual   = wp_cache_get( $cache_key, 'counts' );
+
+		$this->assertEquals( $expected, $actual );
+	}
+}