by pcrosby:

In database/sql/sql.go, all the calls to db.putConn that use a defer will always pass
nil for the error.

For example, assuming the following exec function gets to the "defer db.putConn(ci,
err)" line, err will always be nil, no matter what happens to err after the
"defer..." line (since the arguments to the deferred function are evaluated
when the defer executes, not when the call executes).

I know putConn only cares if err is a driver.ErrBadConn, but it seems to me like that is
a possible error in many of the functions in sql.go that use this defer db.putConn(ci,
err) pattern.

func (db *DB) exec(query string, sargs []driver.Value) (res Result, err error) {
    ci, err := db.conn()
    if err != nil {
        return nil, err
    }
    defer db.putConn(ci, err)

    if execer, ok := ci.(driver.Execer); ok {
        resi, err := execer.Exec(query, sargs)
        if err != driver.ErrSkip {
            if err != nil {
                return nil, err
            }
            return result{resi}, nil
        }
    }

    sti, err := ci.Prepare(query)
    if err != nil {
        return nil, err
    }
    defer sti.Close()

    resi, err := sti.Exec(sargs)
    if err != nil {
        return nil, err
    }
    return result{resi}, nil
}