Catch stdErr output in golang - go

Question

I have written a go program to monitor some MySQL databases.
In the following example, I simply display "Connection Success!" when the connection is successful or "Connection Error:" followed by the error message in case of failure.
package main
import (
"database/sql"
"fmt"
_ "github.com/go-sql-driver/mysql"
)
func main() {
err := mysqlConnect()
if err != nil {
fmt.Println("Connection Error:", err)
} else {
fmt.Println("Connection Success!")
}
}
func mysqlConnect() error {
db, err := sql.Open("mysql", "myuser:mypwd#tcp(10.2.0.3:3306)/mysql")
if err != nil {
return err
}
defer db.Close()
err = db.Ping()
if err != nil {
return err
}
return nil
}
This works as expected, but in some cases, some error message appears while executing db.Ping() and is displayed to stdErr:
For example:
$ go run main.go
[mysql] 2019/07/22 18:01:59 auth.go:293: unknown auth plugin:dialog
Connection Error: this authentication plugin is not supported
The last line is my expected message, but the previous line is displayed when calling db.Ping().
I would like to catch/hide this message [mysql] 2019/07/22 18:01:59 auth.go:293: unknown auth plugin:dialog, as I already gets a clean error message that I can handle as I wish (displaying it or not).
How can I prevent such an error message from appearing (in code I mean, not at the calling time)?

Answer 1

As the comment by #tkausi suggests, you ought to set a custom logger for the go-sql-driver package with https://godoc.org/github.com/go-sql-driver/mysql#SetLogger
Your custom logger can then do whatever makes sense for your application, including "swallowing" these logging statements and not displaying anything in case of no errors.
To answer the question's title, you can capture os.Stderr with something like this:
func doPrint() {
fmt.Fprintf(os.Stderr, "output")
}
func main() {
old := os.Stderr
r, w, _ := os.Pipe()
os.Stderr = w
doPrint()
w.Close()
var buf bytes.Buffer
io.Copy(&buf, r)
os.Stderr = old
fmt.Println("Captured:", buf.String())
}