Invalid connection error returned when an unrelated earlier query is cancelled

[polyfloyd]

Issue description

When a query is cancelled, a single subsequent unrelated query returns invalid connection instead of running normally.

Example code

package main

import (
	"context"
	"database/sql"
	"log"

	_ "github.com/go-sql-driver/mysql"
)

func main() {
	db, err := sql.Open("mysql", "root:root@tcp(localhost:3306)/some_db")
	if err != nil {
		log.Fatal(err)
	}

	for {
		ctx, cancel := context.WithCancel(context.Background())
		go cancel()

		row := db.QueryRowContext(ctx, `SELECT 1`)
		var a int
		if err := row.Scan(&a); err != nil && err != context.Canceled {
			log.Fatal(err)
		}
	}
}

Error log

[mysql] 2018/07/17 17:14:03 connection.go:372: invalid connection
...

Configuration

*Driver version (or git SHA): 749ddf1 (latest master at the time of writing)

*Go version:go version go1.10.3 darwin/amd64

*Server version: Percona 5.7

*Server OS: Debian Jessie x64

[yangjueji]

Related issue: #654
When the context is cancelled, the connection will be closed and returned back to conn pool.
Next query may get this closed connection from conn pool and show "invalid connection".
But database/sql will retry and get a non ErrBadConn connection.

[methane]

It doesn't "return". It just "logs".

[sohag1990]

Here are my golang gin server Logs:

[mysql] 2018/09/22 20:12:55 packets.go:36: unexpected EOF
invalid connection
for error happend
[mysql] 2018/09/22 20:13:17 connection.go:374: invalid connection
[mysql] 2018/09/22 20:13:17 connection.go:374: invalid connection

[methane]

fixed via #862