Add mechanism to report comment content modifications via sync

sync/class.jetpack-sync-module-comments.php

@@ -24,6 +24,9 @@ public function init_listeners( $callable ) {
 		add_action( 'untrash_post_comments', $callable );
 		add_action( 'comment_approved_to_unapproved', $callable );
 		add_action( 'comment_unapproved_to_approved', $callable );
+		add_action( 'modified_comment_contents', $callable );
+
+		add_filter( 'wp_update_comment_data', array( $this, 'handle_comment_contents_modification' ), 10, 2 );
 
 		// even though it's messy, we implement these hooks because
 		// the edit_comment hook doesn't include the data
@@ -40,6 +43,23 @@ public function init_listeners( $callable ) {
 		$this->init_meta_whitelist_handler( 'comment', array( $this, 'filter_meta' ) );
 	}
 
+	/**
+	 * @param array $data       The new, processed comment data.
+	 * @param array $comment    The old, unslashed comment data.
+	 */
+	public function handle_comment_contents_modification( $data, $comment ) {
+
+		if ( $data['comment_content'] != $comment['comment_content'] ) {
+			$action_args = array(
+				$data['comment_ID'],
+				$data,
+				$comment['comment_content'],
+			);
+			do_action( 'modified_comment_contents', $action_args );
+		}
+		return $data;
+	}
+
 	public function init_full_sync_listeners( $callable ) {
 		add_action( 'jetpack_full_sync_comments', $callable ); // also send comments meta
 	}

tests/php/sync/test_class.jetpack-sync-comments.php

@@ -47,6 +47,27 @@ public function test_update_comment() {
 		$this->assertEquals( "foo bar baz", $remote_comment->comment_content );
 	}
 
+	public function test_modify_comment_contents() {
+
+		//Confirm that 'modify_comment' action is set after changing comment contents
+		$this->comment->comment_content = "foo bar baz";
+		wp_update_comment( (array) $this->comment );
+		$this->sender->do_sync();
+
+		$event = $this->server_event_storage->get_most_recent_event( 'modified_comment_contents' );
+		$this->assertTrue( (bool) $event );
+
+		$this->server_event_storage->reset();
+
+		//Confirm that 'modified_comment_contents' action is not set after unapproving comment
+		$this->comment->comment_approved = 0;
+		wp_update_comment( (array) $this->comment );
+		$this->sender->do_sync();
+
+		$event = $this->server_event_storage->get_most_recent_event( 'modified_comment_contents' );
+		$this->assertFalse( (bool) $event );
+	}
+
 	public function test_unapprove_comment() {
 
 		$this->assertEquals( 1, $this->server_replica_storage->comment_count( 'approve' ) );