middleware/debug: add (#735)

* middleware/debug: add

Add a debug "middleware" that disables the recover() and just lets
CoreDNS crash; very useful for testing.

Fixes ##563

* fix test

* Feedback: check the value of Debug

core/dnsserver/config.go

@@ -23,6 +23,9 @@ type Config struct {
 	// First consumer is the file middleware to looks for zone files in this place.
 	Root string
 
+	// Debug controls the panic/recover mechanism that is enabled by default.
+	Debug bool
+
 	// The transport we implement, normally just "dns" over TCP/UDP, but could be
 	// DNS-over-TLS or DNS-over-gRPC.
 	Transport string

core/dnsserver/server.go

@@ -36,6 +36,7 @@ type Server struct {
 	dnsWg       sync.WaitGroup     // used to wait on outstanding connections
 	connTimeout time.Duration      // the maximum duration of a graceful shutdown
 	trace       trace.Trace        // the trace middleware for the server
+	debug       bool               // disable recover()
 }
 
 // NewServer returns a new CoreDNS server and compiles all middleware in to it.
@@ -56,6 +57,9 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 	s.dnsWg.Add(1)
 
 	for _, site := range group {
+		if site.Debug {
+			s.debug = true
+		}
 		// set the config per zone
 		s.zones[site.Zone] = site
 		// compile custom middleware for everything
@@ -166,13 +170,15 @@ func (s *Server) Address() string { return s.Addr }
 // defined in the request so that the correct zone
 // (configuration and middleware stack) will handle the request.
 func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) {
-	defer func() {
-		// In case the user doesn't enable error middleware, we still
-		// need to make sure that we stay alive up here
-		if rec := recover(); rec != nil {
-			DefaultErrorFunc(w, r, dns.RcodeServerFailure)
-		}
-	}()
+	if !s.debug {
+		defer func() {
+			// In case the user doesn't enable error middleware, we still
+			// need to make sure that we stay alive up here
+			if rec := recover(); rec != nil {
+				DefaultErrorFunc(w, r, dns.RcodeServerFailure)
+			}
+		}()
+	}
 
 	if m, err := edns.Version(r); err != nil { // Wrong EDNS version, return at once.
 		w.WriteMsg(m)

core/dnsserver/zdirectives.go

@@ -14,6 +14,7 @@ var directives = []string{
 	"tls",
 	"root",
 	"bind",
+	"debug",
 	"trace",
 	"health",
 	"pprof",

core/zmiddleware.go

@@ -8,6 +8,7 @@ import (
 	_ "github.com/coredns/coredns/middleware/bind"
 	_ "github.com/coredns/coredns/middleware/cache"
 	_ "github.com/coredns/coredns/middleware/chaos"
+	_ "github.com/coredns/coredns/middleware/debug"
 	_ "github.com/coredns/coredns/middleware/dnssec"
 	_ "github.com/coredns/coredns/middleware/erratic"
 	_ "github.com/coredns/coredns/middleware/errors"