Moving a post fails in unknown circumstances

When trying to move a single post into a new topic, it fails with:

[db8237] Processing by TopicsController#move_posts as */*
[db8237]   Parameters: {"title"=>"importer 3 fichiers", "post_ids"=>["4455"], "category_id"=>"6", "topic_id"=>"1521"}
[db8237] Completed 500 Internal Server Error in 336ms (ActiveRecord: 108.5ms)
[db8237] ActiveRecord::RecordNotUnique (PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_posts_on_topic_id_and_post_number"
DETAIL:  Key (topic_id, post_number)=(1521, 3) already exists.

However, that is the post I’m trying to move.

db8237=# select id,post_number from posts where topic_id=1521;
  id  | post_number 
------+-------------
 4085 |           1
 4151 |           2
 4455 |           3

reindex index index_posts_on_topic_id_and_post_number; did not help.

Does anyone have any idea where to look? Or did we trigger an obscure bug here?

Can you repro this @jomaxro?

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Hmm…I’ve moved many posts around in the last 2 weeks without issue. @RGJ, is this site on tests-passed or stable? Is this just a single post with issues, or global? Can you still repro it?

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Sorry for the late response.
It’s on stable, and it’s just a single post (maybe more, but it’s not global). I can still repro it.

If it’s not global I’m not going to be able to reproduce this easily. Perhaps one of the engineers can tell more from the error above.

We need a repro @RGJ

I understand - but we can only repro this on one post on one forum so far.
I wasn’t expecting an immediate solution, just reporting it for documentation purposes, and maybe getting some advice, or hearing from other people who’re seeing the same issue allowing us to maybe see a common pattern.

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I’ve noticed the same problem with imported posts. They do not always conform to requirements like min_post_length. Moving posts triggered those validations and silently prevented the posts from being moved to the target topic. It should be fixed.

https://github.com/discourse/discourse/commit/0ecdf900230eee21beaf54b80428bd18768127ae

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