registry: return store.New error instead of panicking

sessions.go

@@ -137,9 +137,18 @@ func (s *Registry) Get(store Store, name string) (session *Session, err error) {
 		session, err = info.s, info.e
 	} else {
 		session, err = store.New(s.request, name)
+		if session == nil {
+			if err == nil {
+				err = fmt.Errorf("sessions: store.New returned a nil session for %s", name)
+			}
+			return nil, err
+		}
 		session.name = name
 		s.sessions[name] = sessionInfo{s: session, e: err}
 	}
+	if session == nil {
+		return nil, err
+	}
 	session.store = store
 	return
 }

sessions_test.go

@@ -7,6 +7,7 @@ package sessions
 import (
 	"bytes"
 	"encoding/gob"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -189,6 +190,34 @@ func TestFlashes(t *testing.T) {
 	}
 }
 
+type errNilStore struct{}
+
+func (errNilStore) Get(r *http.Request, name string) (*Session, error) {
+	return GetRegistry(r).Get(errNilStore{}, name)
+}
+
+func (errNilStore) New(*http.Request, string) (*Session, error) {
+	return nil, errors.New("store boom")
+}
+
+func (errNilStore) Save(*http.Request, http.ResponseWriter, *Session) error {
+	return nil
+}
+
+func TestRegistryGetNilSessionFromStore(t *testing.T) {
+	req, err := http.NewRequest("GET", "http://localhost/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	session, err := GetRegistry(req).Get(errNilStore{}, "session-key")
+	if session != nil {
+		t.Fatalf("expected nil session, got %#v", session)
+	}
+	if err == nil || err.Error() != "store boom" {
+		t.Fatalf("expected store boom, got %v", err)
+	}
+}
+
 func TestCookieStoreMapPanic(t *testing.T) {
 	defer func() {
 		err := recover()