Embeds: Improve consistency of update and refresh logic for oEmbed caching between oembed_cache and post meta.

* Allow updating oEmbed cache during `parse-embed` requests for non-post editors (such as widgets).
* Update any existing `oembed_cache` post when `usecache` and TTL has passed.
* Do not overwrite a previously valid cache with `{{unknown}}`.

Props dlh.
See #34115.
Fixes #42310.

Built from https://develop.svn.wordpress.org/trunk@42009


git-svn-id: http://core.svn.wordpress.org/trunk@41843 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-embed.php

@@ -284,12 +284,34 @@ class WP_Embed {
 				kses_remove_filters();
 			}
 
-			wp_insert_post( wp_slash( array(
-				'post_name'    => $key_suffix,
-				'post_content' => $html ? $html : '{{unknown}}',
-				'post_status'  => 'publish',
-				'post_type'    => 'oembed_cache',
-			) ) );
+			$insert_post_args = array(
+				'post_name' => $key_suffix,
+				'post_status' => 'publish',
+				'post_type' => 'oembed_cache',
+			);
+
+			if ( $html ) {
+				if ( $cached_post_id ) {
+					wp_update_post( wp_slash( array(
+						'ID' => $cached_post_id,
+						'post_content' => $html,
+					) ) );
+				} else {
+					wp_insert_post( wp_slash( array_merge(
+						$insert_post_args,
+						array(
+							'post_content' => $html,
+						)
+					) ) );
+				}
+			} elseif ( ! $cache ) {
+				wp_insert_post( wp_slash( array_merge(
+					$insert_post_args,
+					array(
+						'post_content' => '{{unknown}}',
+					)
+				) ) );
+			}
 
 			if ( $has_kses ) {
 				kses_init_filters();