feat(processors.starlark): Allow persistence of global state

agent/agent.go

@@ -106,11 +106,6 @@ func (a *Agent) Run(ctx context.Context) error {
 		time.Duration(a.Config.Agent.Interval), a.Config.Agent.Quiet,
 		a.Config.Agent.Hostname, time.Duration(a.Config.Agent.FlushInterval))
 
-	log.Printf("D! [agent] Initializing plugins")
-	if err := a.initPlugins(); err != nil {
-		return err
-	}
-
 	if a.Config.Persister != nil {
 		log.Printf("D! [agent] Initializing plugin states")
 		if err := a.initPersister(); err != nil {
@@ -124,6 +119,11 @@ func (a *Agent) Run(ctx context.Context) error {
 		}
 	}
 
+	log.Printf("D! [agent] Initializing plugins")
+	if err := a.initPlugins(); err != nil {
+		return err
+	}
+
 	startTime := time.Now()
 
 	log.Printf("D! [agent] Connecting outputs")

plugins/common/starlark/starlark.go

@@ -1,6 +1,8 @@
 package starlark
 
 import (
+	"bytes"
+	"encoding/gob"
 	"errors"
 	"fmt"
 	"strings"
@@ -26,6 +28,77 @@ type Common struct {
 	globals    starlark.StringDict
 	functions  map[string]*starlark.Function
 	parameters map[string]starlark.Tuple
+	state      *starlark.Dict
+}
+
+func (s *Common) GetState() interface{} {
+	// Return the actual byte-type instead of nil allowing the persister
+	// to guess instantiate variable of the appropriate type
+	if s.state == nil {
+		return []byte{}
+	}
+
+	// Convert the starlark dict into a golang dictionary for serialization
+	state := make(map[string]interface{}, s.state.Len())
+	items := s.state.Items()
+	for _, item := range items {
+		if len(item) != 2 {
+			// We do expect key-value pairs in the state so there should be
+			// two items.
+			s.Log.Errorf("state item %+v does not contain a key-value pair", item)
+			continue
+		}
+		k, ok := item.Index(0).(starlark.String)
+		if !ok {
+			s.Log.Errorf("state item %+v has invalid key type %T", item, item.Index(0))
+			continue
+		}
+		v, err := asGoValue(item.Index(1))
+		if err != nil {
+			s.Log.Errorf("state item %+v value cannot be converted: %v", item, err)
+			continue
+		}
+		state[k.GoString()] = v
+	}
+
+	// Do a binary GOB encoding to preserve types
+	var buf bytes.Buffer
+	if err := gob.NewEncoder(&buf).Encode(state); err != nil {
+		s.Log.Errorf("encoding state failed: %v", err)
+		return []byte{}
+	}
+
+	return buf.Bytes()
+}
+
+func (s *Common) SetState(state interface{}) error {
+	data, ok := state.([]byte)
+	if !ok {
+		return fmt.Errorf("unexpected type %T for state", state)
+	}
+	if len(data) == 0 {
+		return nil
+	}
+
+	// Decode the binary GOB encoding
+	var dict map[string]interface{}
+	if err := gob.NewDecoder(bytes.NewBuffer(data)).Decode(&dict); err != nil {
+		return fmt.Errorf("decoding state failed: %w", err)
+	}
+
+	// Convert the golang dict back to starlark types
+	s.state = starlark.NewDict(len(dict))
+	for k, v := range dict {
+		sv, err := asStarlarkValue(v)
+		if err != nil {
+			return fmt.Errorf("value %v of state item %q cannot be set: %w", v, k, err)
+		}
+		if err := s.state.SetKey(starlark.String(k), sv); err != nil {
+			return fmt.Errorf("state item %q cannot be set: %w", k, err)
+		}
+	}
+
+	return nil
 }
 
 func (s *Common) Init() error {
@@ -47,27 +120,46 @@ func (s *Common) Init() error {
 	builtins["Metric"] = starlark.NewBuiltin("Metric", newMetric)
 	builtins["deepcopy"] = starlark.NewBuiltin("deepcopy", deepcopy)
 	builtins["catch"] = starlark.NewBuiltin("catch", catch)
-	err := s.addConstants(&builtins)
-	if err != nil {
+
+	if err := s.addConstants(&builtins); err != nil {
 		return err
 	}
 
+	// Insert the persisted state if any
+	if s.state != nil {
+		builtins["state"] = s.state
+	}
+
+	// Load the program. In case of an error we can try to insert the state
+	// which can be used implicitly e.g. when persisting states
 	program, err := s.sourceProgram(builtins)
 	if err != nil {
-		return err
+		// Try again with a declared state. This might be necessary for
+		// state persistence.
+		s.state = starlark.NewDict(0)
+		builtins["state"] = s.state
+		p, serr := s.sourceProgram(builtins)
+		if serr != nil {
+			return err
+		}
+		program = p
 	}
 
 	// Execute source
 	globals, err := program.Init(s.thread, builtins)
 	if err != nil {
 		return err
 	}
-	// Make available a shared state to the apply function
-	globals["state"] = starlark.NewDict(0)
 
-	// Freeze the global state.  This prevents modifications to the processor
+	// In case the program declares a global "state" we should insert it to
+	// avoid warnings about inserting into a frozen variable
+	if _, found := globals["state"]; found {
+		globals["state"] = starlark.NewDict(0)
+	}
+
+	// Freeze the global state. This prevents modifications to the processor
 	// state and prevents scripts from containing errors storing tracking
-	// metrics.  Tasks that require global state will not be possible due to
+	// metrics. Tasks that require global state will not be possible due to
 	// this, so maybe we should relax this in the future.
 	globals.Freeze()
 
@@ -107,6 +199,9 @@ func (s *Common) AddFunction(name string, params ...starlark.Value) error {
 // Add all the constants defined in the plugin as constants of the script
 func (s *Common) addConstants(builtins *starlark.StringDict) error {
 	for key, val := range s.Constants {
+		if key == "state" {
+			return errors.New("'state' constant uses reserved name")
+		}
 		sVal, err := asStarlarkValue(val)
 		if err != nil {
 			return fmt.Errorf("converting type %T failed: %w", val, err)

plugins/processors/starlark/starlark.go

@@ -27,14 +27,12 @@ func (*Starlark) SampleConfig() string {
 }
 
 func (s *Starlark) Init() error {
-	err := s.Common.Init()
-	if err != nil {
+	if err := s.Common.Init(); err != nil {
 		return err
 	}
 
 	// The source should define an apply function.
-	err = s.AddFunction("apply", &common.Metric{})
-	if err != nil {
+	if err := s.AddFunction("apply", &common.Metric{}); err != nil {
 		return err
 	}
 
@@ -118,6 +116,7 @@ func (s *Starlark) Add(origMetric telegraf.Metric, acc telegraf.Accumulator) err
 	default:
 		return fmt.Errorf("invalid type returned: %T", rv)
 	}
+
 	return nil
 }