Restructure Packages and remove State Variables (#191)

* Move Data sending goroutine

* Refactor Transport State Struct

* Remove state variables

* Pass contexts as separate arguments

* Init logsTransport as part of logs API Subscription method

* Implement WaitRuntimeDone and refactor shutdown

* Refactor flush and APM Data Forwarding

* Fix linting

* Refactor unnecessarily exported fields

* Remove RuntimeDone

* Refactor runtimeDone in main

* Keep waitgroup interactions in data forwarding goroutine

* Additional comments

* Refactor reverse proxy error handler

* Refactor APM server transport-related functions

* Added comments regarding processEvents() signals

* Add test for AgentDone and RuntimeDone race

* Open and Close Agent flush channel for each invocation

* Fix Logs API listener host and handle nil case

* Add relevant unit test

Author: jlvoiseux

apm-lambda-extension/extension/apm_server.go renamed to apm-lambda-extension/extension/apm_server_transport.go

@@ -32,14 +32,10 @@ import (
 	"time"
 )
 
-var bufferPool = sync.Pool{New: func() interface{} {
-	return &bytes.Buffer{}
-}}
-
-type ApmServerTransportStatusType string
-
 // Constants for the state of the transport used in
 // the backoff implementation.
+type ApmServerTransportStatusType string
+
 const (
 	Failing ApmServerTransportStatusType = "Failing"
 	Pending ApmServerTransportStatusType = "Pending"
@@ -48,30 +44,84 @@ const (
 
 // A struct to track the state and status of sending
 // to the APM server. Used in the backoff implementation.
-type ApmServerTransportStateType struct {
+type ApmServerTransport struct {
 	sync.Mutex
-	Status            ApmServerTransportStatusType
-	ReconnectionCount int
-	GracePeriodTimer  *time.Timer
+	bufferPool        sync.Pool
+	config            *extensionConfig
+	AgentDoneSignal   chan struct{}
+	dataChannel       chan AgentData
+	client            *http.Client
+	status            ApmServerTransportStatusType
+	reconnectionCount int
+	gracePeriodTimer  *time.Timer
 }
 
-// The status of transport to the APM server.
-//
-// This instance of the ApmServerTransportStateType is public for use in tests.
-var ApmServerTransportState = ApmServerTransportStateType{
-	Status:            Healthy,
-	ReconnectionCount: -1,
+func InitApmServerTransport(config *extensionConfig) *ApmServerTransport {
+	var transport ApmServerTransport
+	transport.bufferPool = sync.Pool{New: func() interface{} {
+		return &bytes.Buffer{}
+	}}
+	transport.dataChannel = make(chan AgentData, 100)
+	transport.client = &http.Client{
+		Timeout:   time.Duration(config.DataForwarderTimeoutSeconds) * time.Second,
+		Transport: http.DefaultTransport.(*http.Transport).Clone(),
+	}
+	transport.config = config
+	transport.status = Healthy
+	transport.reconnectionCount = -1
+	return &transport
+}
+
+// StartBackgroundApmDataForwarding Receive agent data as it comes in and post it to the APM server.
+// Stop checking for, and sending agent data when the function invocation
+// has completed, signaled via a channel.
+func (transport *ApmServerTransport) ForwardApmData(ctx context.Context) error {
+	if transport.status == Failing {
+		return nil
+	}
+	for {
+		select {
+		case <-ctx.Done():
+			Log.Debug("Invocation context cancelled, not processing any more agent data")
+			return nil
+		case agentData := <-transport.dataChannel:
+			if err := transport.PostToApmServer(ctx, agentData); err != nil {
+				return fmt.Errorf("error sending to APM server, skipping: %v", err)
+			}
+		}
+	}
+}
+
+// FlushAPMData reads all the apm data in the apm data channel and sends it to the APM server.
+func (transport *ApmServerTransport) FlushAPMData(ctx context.Context) {
+	if transport.status == Failing {
+		Log.Debug("Flush skipped - Transport failing")
+		return
+	}
+	Log.Debug("Flush started - Checking for agent data")
+	for {
+		select {
+		case agentData := <-transport.dataChannel:
+			Log.Debug("Flush in progress - Processing agent data")
+			if err := transport.PostToApmServer(ctx, agentData); err != nil {
+				Log.Errorf("Error sending to APM server, skipping: %v", err)
+			}
+		default:
+			Log.Debug("Flush ended - No agent data on buffer")
+			return
+		}
+	}
 }
 
 // PostToApmServer takes a chunk of APM agent data and posts it to the APM server.
 //
 // The function compresses the APM agent data, if it's not already compressed.
 // It sets the APM transport status to failing upon errors, as part of the backoff
 // strategy.
-func PostToApmServer(client *http.Client, agentData AgentData, config *extensionConfig, ctx context.Context) error {
+func (transport *ApmServerTransport) PostToApmServer(ctx context.Context, agentData AgentData) error {
 	// todo: can this be a streaming or streaming style call that keeps the
 	//       connection open across invocations?
-	if !IsTransportStatusHealthyOrPending() {
+	if transport.status == Failing {
 		return errors.New("transport status is unhealthy")
 	}
 
@@ -83,10 +133,10 @@ func PostToApmServer(client *http.Client, agentData AgentData, config *extension
 		r = bytes.NewReader(agentData.Data)
 	} else {
 		encoding = "gzip"
-		buf := bufferPool.Get().(*bytes.Buffer)
+		buf := transport.bufferPool.Get().(*bytes.Buffer)
 		defer func() {
 			buf.Reset()
-			bufferPool.Put(buf)
+			transport.bufferPool.Put(buf)
 		}()
 		gw, err := gzip.NewWriterLevel(buf, gzip.BestSpeed)
 		if err != nil {
@@ -101,98 +151,90 @@ func PostToApmServer(client *http.Client, agentData AgentData, config *extension
 		r = buf
 	}
 
-	req, err := http.NewRequest("POST", config.apmServerUrl+endpointURI, r)
+	req, err := http.NewRequest("POST", transport.config.apmServerUrl+endpointURI, r)
 	if err != nil {
 		return fmt.Errorf("failed to create a new request when posting to APM server: %v", err)
 	}
 	req.Header.Add("Content-Encoding", encoding)
 	req.Header.Add("Content-Type", "application/x-ndjson")
-	if config.apmServerApiKey != "" {
-		req.Header.Add("Authorization", "ApiKey "+config.apmServerApiKey)
-	} else if config.apmServerSecretToken != "" {
-		req.Header.Add("Authorization", "Bearer "+config.apmServerSecretToken)
+	if transport.config.apmServerApiKey != "" {
+		req.Header.Add("Authorization", "ApiKey "+transport.config.apmServerApiKey)
+	} else if transport.config.apmServerSecretToken != "" {
+		req.Header.Add("Authorization", "Bearer "+transport.config.apmServerSecretToken)
 	}
 
-	Log.Debug("Sending data chunk to APM Server")
-	resp, err := client.Do(req)
+	Log.Debug("Sending data chunk to APM server")
+	resp, err := transport.client.Do(req)
 	if err != nil {
-		SetApmServerTransportState(Failing, ctx)
+		transport.SetApmServerTransportState(ctx, Failing)
 		return fmt.Errorf("failed to post to APM server: %v", err)
 	}
 
 	//Read the response body
 	defer resp.Body.Close()
 	body, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
-		SetApmServerTransportState(Failing, ctx)
+		transport.SetApmServerTransportState(ctx, Failing)
 		return fmt.Errorf("failed to read the response body after posting to the APM server")
 	}
 
-	SetApmServerTransportState(Healthy, ctx)
+	transport.SetApmServerTransportState(ctx, Healthy)
 	Log.Debug("Transport status set to healthy")
 	Log.Debugf("APM server response body: %v", string(body))
 	Log.Debugf("APM server response status code: %v", resp.StatusCode)
 	return nil
 }
 
-// IsTransportStatusHealthyOrPending returns true if the APM server transport status is 
-// healthy or pending, and false otherwise.
-//
-// This function is public for use in tests.
-func IsTransportStatusHealthyOrPending() bool {
-	return ApmServerTransportState.Status != Failing
-}
-
 // SetApmServerTransportState takes a state of the APM server transport and updates
 // the current state of the transport. For a change to a failing state, the grace period
 // is calculated and a go routine is started that waits for that period to complete
 // before changing the status to "pending". This would allow a subsequent send attempt
 // to the APM server.
 //
 // This function is public for use in tests.
-func SetApmServerTransportState(status ApmServerTransportStatusType, ctx context.Context) {
+func (transport *ApmServerTransport) SetApmServerTransportState(ctx context.Context, status ApmServerTransportStatusType) {
 	switch status {
 	case Healthy:
-		ApmServerTransportState.Lock()
-		ApmServerTransportState.Status = status
-		Log.Debugf("APM Server Transport status set to %s", status)
-		ApmServerTransportState.ReconnectionCount = -1
-		ApmServerTransportState.Unlock()
+		transport.Lock()
+		transport.status = status
+		Log.Debugf("APM server Transport status set to %s", transport.status)
+		transport.reconnectionCount = -1
+		transport.Unlock()
 	case Failing:
-		ApmServerTransportState.Lock()
-		ApmServerTransportState.Status = status
-		Log.Debugf("APM Server Transport status set to %s", status)
-		ApmServerTransportState.ReconnectionCount++
-		ApmServerTransportState.GracePeriodTimer = time.NewTimer(computeGracePeriod())
-		Log.Debugf("Grace period entered, reconnection count : %d", ApmServerTransportState.ReconnectionCount)
+		transport.Lock()
+		transport.status = status
+		Log.Debugf("APM server Transport status set to %s", transport.status)
+		transport.reconnectionCount++
+		transport.gracePeriodTimer = time.NewTimer(transport.computeGracePeriod())
+		Log.Debugf("Grace period entered, reconnection count : %d", transport.reconnectionCount)
 		go func() {
 			select {
-			case <-ApmServerTransportState.GracePeriodTimer.C:
+			case <-transport.gracePeriodTimer.C:
 				Log.Debug("Grace period over - timer timed out")
 			case <-ctx.Done():
 				Log.Debug("Grace period over - context done")
 			}
-			ApmServerTransportState.Status = Pending
-			Log.Debugf("APM Server Transport status set to %s", status)
-			ApmServerTransportState.Unlock()
+			transport.status = Pending
+			Log.Debugf("APM server Transport status set to %s", transport.status)
+			transport.Unlock()
 		}()
 	default:
-		Log.Errorf("Cannot set APM Server Transport status to %s", status)
+		Log.Errorf("Cannot set APM server Transport status to %s", status)
 	}
 }
 
 // ComputeGracePeriod https://github.com/elastic/apm/blob/main/specs/agents/transport.md#transport-errors
-func computeGracePeriod() time.Duration {
-	gracePeriodWithoutJitter := math.Pow(math.Min(float64(ApmServerTransportState.ReconnectionCount), 6), 2)
+func (transport *ApmServerTransport) computeGracePeriod() time.Duration {
+	gracePeriodWithoutJitter := math.Pow(math.Min(float64(transport.reconnectionCount), 6), 2)
 	jitter := rand.Float64()/5 - 0.1
 	return time.Duration((gracePeriodWithoutJitter + jitter*gracePeriodWithoutJitter) * float64(time.Second))
 }
 
 // EnqueueAPMData adds a AgentData struct to the agent data channel, effectively queueing for a send
 // to the APM server.
-func EnqueueAPMData(agentDataChannel chan AgentData, agentData AgentData) {
+func (transport *ApmServerTransport) EnqueueAPMData(agentData AgentData) {
 	select {
-	case agentDataChannel <- agentData:
+	case transport.dataChannel <- agentData:
 		Log.Debug("Adding agent data to buffer to be sent to apm server")
 	default:
 		Log.Warn("Channel full: dropping a subset of agent data")