db prepare return invalid connection

[richmonkey]

Issue description

when net conn write error, why ErrInvalidConn , but not driver.ErrBadConn?

Example code

//setup two connection(equal const maxBadConnRetries = 2)
conn1, _ := db.Conn(context.Background())
conn2, _:= db.Conn(context.Background())
conn1.Close()
conn2.Close()

Then restart mysql server, now the connections in the pool all bad.

db.prepare()

Error log

If you have an error log, please paste it here.

Configuration

Driver version (or git SHA):

Go version: run go version in your console

Server version: E.g. MySQL 5.6, MariaDB 10.0.20

Server OS: E.g. Debian 8.1 (Jessie), Windows 10

[richmonkey]

To fix the problem, I need prepare again:
s, err := db.prepare()
if err == mysql.ErrInvalidConn {
s, err = db.prepare()
}

[methane]

It make sense that return ErrBadConn when we know it safe to retry.
But you shoud note that database/sql retries only limited (two) times.

https://github.com/golang/go/blob/a38a917aee626a9b9d5ce2b93964f586bf759ea0/src/database/sql/sql.go#L1390

You should do retry manually if your DB connectivity is unstable.