Fix issue where default go logger is not discarded when either * or s…

…tdout is selected. Fix elastic#10251.

libbeat/logp/core.go

@@ -37,7 +37,8 @@ import (
 )
 
 var (
-	_log unsafe.Pointer // Pointer to a coreLogger. Access via atomic.LoadPointer.
+	_log          unsafe.Pointer // Pointer to a coreLogger. Access via atomic.LoadPointer.
+	_defaultGoLog = golog.Writer()
 )
 
 func init() {
@@ -104,6 +105,8 @@ func Configure(cfg Config) error {
 			// Disable standard logging by default (this is sometimes used by
 			// libraries and we don't want their spam).
 			golog.SetOutput(ioutil.Discard)
+		} else {
+			golog.SetOutput(_defaultGoLog)
 		}
 
 		sink = selectiveWrapper(sink, selectors)

libbeat/logp/core_test.go

@@ -18,6 +18,8 @@
 package logp
 
 import (
+	"io/ioutil"
+	golog "log"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -116,3 +118,14 @@ func TestL(t *testing.T) {
 		assert.Equal(t, "warning 1", log.Message)
 	}
 }
+
+func TestAllStdoutDisablesDefaultGoLogger(t *testing.T) {
+	DevelopmentSetup(WithSelectors("*"))
+	assert.Equal(t, ioutil.Discard, golog.Writer())
+
+	DevelopmentSetup(WithSelectors("stdlog"))
+	assert.Equal(t, ioutil.Discard, golog.Writer())
+
+	DevelopmentSetup(WithSelectors("*", "stdlog"))
+	assert.NotEqual(t, ioutil.Discard, golog.Writer())
+}