This was reported first on https://github.com/catalyst/moodle-search_elastic/issues/27 as a bug of the search_elastic plugin, but was identified as a Moodle core bug by mattporritt.

Steps to reproduce:

• Create a Moodle instance with two users in the same course
• Make sure that global search is properly set up and that the "Messages - received" and "Messages - sent" search areas are active
• Have the first user send the second a message
• Have the second user respond to the first user message
• Delete the first user
• Run the site index task (/search/cli/indexer.php)

Expected result:

• The message exchange with the deleted user shouldn't be indexed.

Actual result:

• Indexing the message fails quite ungracefully with a debug trace:

  ++ Error retrieving core_message-message_received 1 document, not all required data is available: Invalid user ++

  * line 59 of /message/classes/search/base_message.php: call to debugging()

  * line 62 of /message/classes/search/message_received.php: call to core_message\search\base_message->get_document()

  * line ? of unknownfile: call to core_message\search\message_received->get_document()

  * line 103 of /lib/classes/dml/recordset_walk.php: call to call_user_func()

  * line 382 of /search/engine/elastic/classes/engine.php: call to core\dml\recordset_walk->current()

  * line 666 of /search/classes/manager.php: call to search_elastic\engine->add_documents()

  * line 93 of /search/cli/indexer.php: call to core_search\manager->index()

  Processed 1 records containing 1 documents, in 0.028 seconds.

mattporritt's guess was that \core_message\message_sent get_document() needs to be modified to deal with missing users.