unit-state.db grows apparently without bound

Hi,

Ubuntu 20.04, charm version 206.

After running for a while, unit-state.db grows quite big - mine was 50MB when I noticed. This is all in the "reactive.state_watch" key. It was basically a 50MB dict.

This is leading to longer and longer hook duration, from :

2020-07-23 01:49:59 unit-postgresql-2: uniter (run update-status hook), waited 0s, held 4m0s

to

2020-08-12 14:22:08 unit-postgresql-2: uniter (run update-status hook), waited 0s, held 23m34s

(full report at https://pastebin.canonical.com/p/gXwk5cwSwH/)

23 minutes to run update-status just doesn't work.

I had to clean up reactive.state_watch to get hooks back to a sane duration :
$ cd /var/lib/juju/agents/unit-postgresql-*/charm
$ sudo sqlite3 .unit-state.db
sqlite> UPDATE kv SET data='{"iteration": 0, "changes": [], "pending": []}' WHERE key='reactive.state_watch';
sqlite> VACUUM;

Thanks