Database connection lost whenever SQL query string is too long I recently switched from running my Rails app on a single VM to running the database — MariaDB 10.3 — on a separate (Debian Buster) VM. Now that the database is on a separate server, Rails immediately throws Mysql2::Error::ConnectionError: MySQL server has gone away whenever it tries to make a query where the SQL itself is very long. (They query itself isn't necessarily one that would put significant load on the system.)
An example query that causes the problem looks like this:
SELECT `articles`.`id` FROM `articles` WHERE `articles`.`namespace` = 0 AND `articles`.`wiki_id` = 1 AND `articles`.`title` IN ('Abortion', 'American_Civil_Liberties_Union', 'Auschwitz_concentration_camp', 'Agent_Orange', 'Ahimsa')

... except the array of titles is about 5000 items long, and the full query string is ~158kB.
On the database side, this corresponds to warnings like this:
2021-03-25 15:47:13 10 [Warning] Aborted connection 10 to db: 'dashboard' user: 'outreachdashboard' host: 'programs-and-events-dashboard.globaleducation.eqiad1.wikimed' (Got an error reading communication packets)
The problem seems to be with the network layer, but I can't get to the bottom of it. I've tried adjusting many MariaDB config variables (max_allowed_packet, innodb_log_buffer_size, innodb_log_file_size, innodb_buffer_pool_size) but none of those made a difference. The problem seems to be that the connection is aborted while it is attempting to transmit the long SQL query string from the app server to the database server. (There's no corresponding problem with receiving large query results from the database.)
I've tried adjusting several timeout-related settings as well, although that seems unlikely to be the problem because I can replicate the connection error without any significant wait, just by issuing one of the long-SQL-string queries from a Rails console.
I've tried using tcpdump to see what's coming in, but didn't pick up any additional clues from that.
Woo! I finally figured it out!
The problem was solved by upgrading libmariadb-dev on the Rails application server and rebuilding the mysql2 gem.
Previously, the system was running on a single server with a mariadb-10.1 database. After the switch to a separate database server, the database was on mariadb-10.3 but it was still connecting from the same Rails environment as before, with a mysql2 gem built against 10.1.