Improve gRPC Plugin when backend is not available (#3966)

* Improve gRPC Plugin when backend is not available
Signed-off-by: Ricardo Pchevuzinske Katz <ricardo.katz@serpro.gov.br>

* Improve gRPC Plugin when backend is not available
Signed-off-by: Ricardo Pchevuzinske Katz <ricardo.katz@serpro.gov.br>

plugin/grpc/grpc.go

@@ -3,6 +3,7 @@ package grpc
 import (
 	"context"
 	"crypto/tls"
+	"errors"
 	"time"
 
 	"github.com/coredns/coredns/plugin"
@@ -38,7 +39,7 @@ func (g *GRPC) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 	var (
 		span, child      ot.Span
 		ret              *dns.Msg
-		err         error
+		upstreamErr, err error
 		i                int
 	)
 	span = ot.SpanFromContext(ctx)
@@ -73,6 +74,8 @@ func (g *GRPC) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 			child.Finish()
 		}
 
+		upstreamErr = err
+
 		// Check if the reply is correct; if not return FormErr.
 		if !state.Match(ret) {
 			debug.Hexdumpf(ret, "Wrong reply for id: %d, %s %d", ret.Id, state.QName(), state.QType())
@@ -87,7 +90,11 @@ func (g *GRPC) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 		return 0, nil
 	}
 
-	return 0, nil
+	if upstreamErr != nil {
+		return dns.RcodeServerFailure, upstreamErr
+	}
+
+	return dns.RcodeServerFailure, ErrNoHealthy
 }
 
 // NewGRPC returns a new GRPC.
@@ -129,3 +136,8 @@ func (g *GRPC) isAllowedDomain(name string) bool {
 func (g *GRPC) list() []*Proxy { return g.p.List(g.proxies) }
 
 const defaultTimeout = 5 * time.Second
+
+var (
+	// ErrNoHealthy means no healthy proxies left.
+	ErrNoHealthy = errors.New("no healthy gRPC proxies")
+)