testcontrol: replace panic with error

I have seen this once in the VM test (caused by an EOF, I believe on
shutdown) that didn't need to cause the test to fail.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -36,6 +36,8 @@ import (
 	"tailscale.com/types/wgkey"
 )
 
+const msgLimit = 1 << 20 // encrypted message length limit
+
 // Server is a control plane server. Its zero value is ready for use.
 // Everything is stored in-memory in one tailnet.
 type Server struct {
@@ -397,15 +399,23 @@ func (s *Server) CompleteAuth(authPathOrURL string) bool {
 }
 
 func (s *Server) serveRegister(w http.ResponseWriter, r *http.Request, mkey tailcfg.MachineKey) {
+	msg, err := ioutil.ReadAll(io.LimitReader(r.Body, msgLimit))
+	if err != nil {
+		r.Body.Close()
+		http.Error(w, fmt.Sprintf("bad map request read: %v", err), 400)
+		return
+	}
+	r.Body.Close()
+
 	var req tailcfg.RegisterRequest
-	if err := s.decode(mkey, r.Body, &req); err != nil {
-		panic(fmt.Sprintf("serveRegister: decode: %v", err))
+	if err := s.decode(mkey, msg, &req); err != nil {
+		go panic(fmt.Sprintf("serveRegister: decode: %v", err))
 	}
 	if req.Version != 1 {
-		panic(fmt.Sprintf("serveRegister: unsupported version: %d", req.Version))
+		go panic(fmt.Sprintf("serveRegister: unsupported version: %d", req.Version))
 	}
 	if req.NodeKey.IsZero() {
-		panic("serveRegister: request has zero node key")
+		go panic("serveRegister: request has zero node key")
 	}
 	if s.Verbose {
 		j, _ := json.MarshalIndent(req, "", "\t")
@@ -558,8 +568,16 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey tailcfg.M
 	defer s.incrInServeMap(-1)
 	ctx := r.Context()
 
+	msg, err := ioutil.ReadAll(io.LimitReader(r.Body, msgLimit))
+	if err != nil {
+		r.Body.Close()
+		http.Error(w, fmt.Sprintf("bad map request read: %v", err), 400)
+		return
+	}
+	r.Body.Close()
+
 	req := new(tailcfg.MapRequest)
-	if err := s.decode(mkey, r.Body, req); err != nil {
+	if err := s.decode(mkey, msg, req); err != nil {
 		go panic(fmt.Sprintf("bad map request: %v", err))
 	}
 
@@ -747,15 +765,7 @@ func (s *Server) sendMapMsg(w http.ResponseWriter, mkey tailcfg.MachineKey, comp
 	return nil
 }
 
-func (s *Server) decode(mkey tailcfg.MachineKey, r io.Reader, v interface{}) error {
-	if c, _ := r.(io.Closer); c != nil {
-		defer c.Close()
-	}
-	const msgLimit = 1 << 20
-	msg, err := ioutil.ReadAll(io.LimitReader(r, msgLimit))
-	if err != nil {
-		return err
-	}
+func (s *Server) decode(mkey tailcfg.MachineKey, msg []byte, v interface{}) error {
 	if len(msg) == msgLimit {
 		return errors.New("encrypted message too long")
 	}