Additional issues with `r2d2-diesel` and MySQL

[kardeiz]

Thanks for PR 727 and the new release (0.11.1)! Your speed and willingness to help is amazing!

Unfortunately, I'm encountering further issues when using r2d2 with MySQL. Whenever I try to run a query, like:

users::table.limit(5).load::<User>(&*conn).expect("Users load failed")

I get

DatabaseError(__Unknown, "Commands out of sync; you can\'t run this command now")

When I get a connection directly using MysqlConnection::establish, everything works fine.

I dug around a little bit, and it is definitely related to the query in the is_valid health check. I put together an alternate impl for ::r2d2::ManageConnection where is_valid always returned Ok(()), and it worked fine. (Of course, that is not a viable solution for the managed connection.)

[sgrif]

I suspect we need to move

diesel/diesel/src/mysql/connection/raw.rs

Lines 98 to 102 in 2d6f6b7

	unsafe {
	while ffi::mysql_next_result(self.0) != -1 {
	self.did_an_error_occur()?;
	}
	}

to

diesel/diesel/src/mysql/connection/raw.rs

Line 82 in 2d6f6b7

self.did_an_error_occur()

. Testing will be a bit of a pain. I will explore it further tomorrow.

[kardeiz]

@sgrif, I think this just has to do with not consuming/flushing the query result. If I change

conn.execute("SELECT 1").map(|_| ()).map_err(Error::QueryError)

to

sql::<::diesel::types::Bool>("SELECT 1")
    .get_result::<bool>(conn)
    .map(|_| ())
    .map_err(|_| Error::QueryError)

it works fine.

I don't know if this is the most efficient way to do a health check. MySQL has a ping mechanism, but I don't know all the considerations around it, and it wouldn't work for a generic Connection in r2d2-diesel.

Or maybe there is just some way to flush/drop the row/value without using it?

[Ameobea]

I'm getting this same issue; unable to perform any queries using r2d2-diesel and MySQL. Is there a fix on the radar for this? I've tried to look into a solution on my own but I'm very unfamiliar with diesel and databases in general.