Chore: picker support get first error

common/picker/picker.go

@@ -15,8 +15,10 @@ type Picker struct {
 
 	wg sync.WaitGroup
 
-	once   sync.Once
-	result interface{}
+	once    sync.Once
+	errOnce sync.Once
+	result  interface{}
+	err     error
 }
 
 func newPicker(ctx context.Context, cancel func()) *Picker {
@@ -49,6 +51,11 @@ func (p *Picker) Wait() interface{} {
 	return p.result
 }
 
+// Error return the first error (if all success return nil)
+func (p *Picker) Error() error {
+	return p.err
+}
+
 // Go calls the given function in a new goroutine.
 // The first call to return a nil error cancels the group; its result will be returned by Wait.
 func (p *Picker) Go(f func() (interface{}, error)) {
@@ -64,6 +71,10 @@ func (p *Picker) Go(f func() (interface{}, error)) {
 					p.cancel()
 				}
 			})
+		} else {
+			p.errOnce.Do(func() {
+				p.err = err
+			})
 		}
 	}()
 }

common/picker/picker_test.go

@@ -36,4 +36,5 @@ func TestPicker_Timeout(t *testing.T) {
 
 	number := picker.Wait()
 	assert.Nil(t, number)
+	assert.NotNil(t, picker.Error())
 }

dns/resolver.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"math/rand"
 	"net"
 	"strings"
@@ -182,7 +183,11 @@ func (r *Resolver) batchExchange(clients []dnsClient, m *D.Msg) (msg *D.Msg, err
 
 	elm := fast.Wait()
 	if elm == nil {
-		return nil, errors.New("All DNS requests failed")
+		err := errors.New("All DNS requests failed")
+		if fErr := fast.Error(); fErr != nil {
+			err = fmt.Errorf("%w, first error: %s", err, fErr.Error())
+		}
+		return nil, err
 	}
 
 	msg = elm.(*D.Msg)