Irrelevant error when connection fails: busy buffer, commands out of sync

[tdewolff]

I had a whole post written to explain my problem, but while writing I found the solution. The error however is very vague and led me to about 6h lost digging for a bug.

In my case the MySQL server was listening on a socket and not on localhost:3306, however connecting to the latter like below was giving an error only when executing a query, not when connecting. Nor was the error appropriate for the problem.

Example:

	db, err := sql.Open("mysql", "USER:PASSWORD@tcp(localhost:3306)/DBNAME?charset=utf8")
	if err != nil {
		log.Fatalln(err)
	}
	rows, err := db.Query("SELECT domain FROM domains")
	if err != nil {
		log.Fatalln(err) // error here
	}
	// ...

This fails with:

[mysql] 2025/01/27 21:44:02 connection.go:49: busy buffer
commands out of sync. Did you run multiple statements at once?

Using unix(/var/lib/mysql/mysql.sock) as the host fixes the problem. I'm running MariaDB 10.3.28

[methane]

It is database/sql design. sql.Open() creates connection pool, not a connection.
If you want to check connection, call db.Ping().

[tdewolff]

Then why doesn't the error from Query say so? Why does it say busy buffer, commands out of sync, when it should say connection failed? That's not by design I may hope...

[methane]

I don't know. Ask to your localhost:3306.