consul: Adding flag to support future incompatible commands. Future self will thank me.

consul/fsm.go

@@ -75,7 +75,18 @@ func (c *consulFSM) State() *StateStore {
 
 func (c *consulFSM) Apply(log *raft.Log) interface{} {
 	buf := log.Data
-	switch structs.MessageType(buf[0]) {
+	msgType := structs.MessageType(buf[0])
+
+	// Check if this message type should be ignored when unknown. This is
+	// used so that new commands can be added with developer control if older
+	// versions can safely ignore the command, or if they should crash.
+	ignoreUnknown := false
+	if msgType&structs.IgnoreUnknownTypeFlag == structs.IgnoreUnknownTypeFlag {
+		msgType &= ^structs.IgnoreUnknownTypeFlag
+		ignoreUnknown = true
+	}
+
+	switch msgType {
 	case structs.RegisterRequestType:
 		return c.decodeRegister(buf[1:], log.Index)
 	case structs.DeregisterRequestType:
@@ -89,8 +100,13 @@ func (c *consulFSM) Apply(log *raft.Log) interface{} {
 	case structs.TombstoneRequestType:
 		return c.applyTombstoneOperation(buf[1:], log.Index)
 	default:
+		if ignoreUnknown {
+			c.logger.Printf("[WARN] consul.fsm: ignoring unknown message type (%d), upgrade to newer version", msgType)
+			return nil
+		} else {
 			panic(fmt.Errorf("failed to apply request: %#v", buf))
 		}
+	}
 }
 
 func (c *consulFSM) decodeRegister(buf []byte, index uint64) interface{} {

consul/fsm_test.go

@@ -1059,3 +1059,33 @@ func TestFSM_TombstoneReap(t *testing.T) {
 		t.Fatalf("bad: %v", res)
 	}
 }
+
+func TestFSM_IgnoreUnknown(t *testing.T) {
+	path, err := ioutil.TempDir("", "fsm")
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	defer os.RemoveAll(path)
+	fsm, err := NewFSM(nil, path, os.Stderr)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	defer fsm.Close()
+
+	// Create a new reap request
+	type UnknownRequest struct {
+		Foo string
+	}
+	req := UnknownRequest{Foo: "bar"}
+	msgType := structs.IgnoreUnknownTypeFlag | 64
+	buf, err := structs.Encode(msgType, req)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Apply should work, even though not supported
+	resp := fsm.Apply(makeLog(buf))
+	if err, ok := resp.(error); ok {
+		t.Fatalf("resp: %v", err)
+	}
+}

consul/structs/structs.go

@@ -26,6 +26,15 @@ const (
 	TombstoneRequestType
 )
 
+const (
+	// IgnoreUnknownTypeFlag is set along with a MessageType
+	// to indicate that the message type can be safely ignored
+	// if it is not recognized. This is for future proofing, so
+	// that new commands can be added in a way that won't cause
+	// old servers to crash when the FSM attempts to process them.
+	IgnoreUnknownTypeFlag MessageType = 128
+)
+
 const (
 	// HealthAny is special, and is used as a wild card,
 	// not as a specific state.