fix: do not back-off on intake req errors in Lambda env (#180)

This ensures that the APM agent instrumentation in a Lambda
function will not cause a `null` response from the user's handler
if the agent cannot talk to the extension.

Note that the spec at elastic/apm#613 related to this might
yet change.

Refs: elastic/apm-agent-nodejs#1831
Refs: elastic/apm#613

CHANGELOG.md

@@ -1,5 +1,14 @@
 # elastic-apm-http-client changelog
 
+## Unreleased
+
+- Fix an issue when running in a Lambda function, where a missing or erroring
+  APM Lambda extension could result in apmclient back-off such that (a) the
+  end-of-lambda-invocation signaling (`?flushed=true`) would not happen and
+  (b) premature "beforeExit" event could result in the Lambda Runtime
+  responding `null` before the Lambda function could respond
+  (https://github.com/elastic/apm-agent-nodejs/issues/1831).
+
 ## v11.0.0
 
 - Add support for coordinating data flushing in an AWS Lambda environment. The

index.js

@@ -781,9 +781,18 @@ Client.prototype._destroy = function (err, cb) {
 // Return the appropriate backoff delay (in milliseconds) before a next possible
 // request to APM server.
 // Spec: https://github.com/elastic/apm/blob/main/specs/agents/transport.md#transport-errors
+//
+// In a Lambda environment, a backoff delay can be harmful: The backoff
+// setTimeout is unref'd, to not hold the process open. A subsequent Lambda
+// function invocation during that timer will result in no active handles and
+// a process "beforeExit" event. That event is interpreted by the Lambda Runtime
+// as "the Lambda function callback was never called", and it terminates the
+// function and responds with `null`.  The solution is to never backoff in a
+// Lambda environment -- we expect and assume the Lambda extension is working,
+// and pass responsibility for backoff to the extension.
 Client.prototype._getBackoffDelay = function (isErr) {
   let reconnectCount = this._backoffReconnectCount
-  if (isErr) {
+  if (isErr && !isLambdaExecutionEnvironment) {
     this._backoffReconnectCount++
   } else {
     this._backoffReconnectCount = 0