go: database/sql: DB full resetterCh causes driver.ErrBadConn error

What version of Go are you using (go version)?

$ go version
go version go1.12 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

$ go env

What did you do?

I am doing stress test for mysql server using golang. I create a sql.DB and set

db.SetMaxOpenConns(128)
db.SetMaxIdleConns(32) 

Then I create 500 go-routines (500 clients) and send 1000000 queries to mysql server. After I run the program, it sometimes pops up error “driver: bad connection” (driver.ErrBadConn).

I found that in sql.OpenDB, it creates a *sql.DB struct with: resetterCh: make(chan *driverConn, 50)

In func (db *DB) putConn(dc *driverConn, err error, resetSession bool), if db.resetterCh is full, it marks connection as bad

select {
default:
	// If the resetterCh is blocking then mark the connection
	// as bad and continue on.
	dc.lastErr = driver.ErrBadConn
	dc.Unlock()
case db.resetterCh <- dc:
}

and if number of connections exceeds max connection, it reuses old connection which is marked as bad and return driver.ErrBadConn.

I can solve it by set max connection less than 50 (which is size of db.resetterCh). Why did you hardcoded size of db.resetterCh to 50? Should it be set to max connections?

https://play.golang.org/p/phUILuRV3hJ

What did you expect to see?

What did you see instead?