Add support for fallthrough to the grpc plugin (#7359)

Fixes: https://github.com/coredns/coredns/issues/7358

Signed-off-by: Blake Barnett <bbarnett@groq.com>

plugin/grpc/grpc_test.go

@@ -3,10 +3,12 @@ package grpc
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 
 	"github.com/coredns/coredns/pb"
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/pkg/fall"
 	"github.com/coredns/coredns/plugin/test"
 
 	"github.com/miekg/dns"
@@ -73,3 +75,30 @@ func TestGRPC(t *testing.T) {
 		})
 	}
 }
+
+// Test that fallthrough works correctly when there's no next plugin
+func TestGRPCFallthroughNoNext(t *testing.T) {
+	g := newGRPC()     // Use the constructor to properly initialize
+	g.Fall = fall.Root // Enable fallthrough for all zones
+	g.Next = nil       // No next plugin
+	g.from = "."
+
+	// Create a test request
+	r := new(dns.Msg)
+	r.SetQuestion("test.example.org.", dns.TypeA)
+
+	w := &test.ResponseWriter{}
+
+	// Should return SERVFAIL since no backends are configured and no next plugin
+	rcode, err := g.ServeDNS(context.Background(), w, r)
+
+	// Should not return the "no next plugin found" error
+	if err != nil && strings.Contains(err.Error(), "no next plugin found") {
+		t.Errorf("Expected no 'no next plugin found' error, got: %v", err)
+	}
+
+	// Should return SERVFAIL
+	if rcode != dns.RcodeServerFailure {
+		t.Errorf("Expected SERVFAIL when no backends and no next plugin, got: %d", rcode)
+	}
+}