feat: Allow starting a span on a transaction that has already ended (#…

…2653)

This was a restriction that dated back to the v1 APM Server intake
API that required all a transaction's spans to be sent along with the
transaction. This will allow for cases where transaction.span_count
reported values will *under* count.

The Java APM agent, FWIW, allows this.

CHANGELOG.asciidoc

@@ -43,6 +43,9 @@ Notes:
   child spans from being added to the exit spans.  See issue for a full list of
   spans types that will be treated as exit spans. ({issues}2601[#2601])
 
+* Allow a new span to be created/started even if its transaction has ended.
+  This is expected to be a very rare use case. ({pull}2653[#2653])
+
 [float]
 ===== Bug fixes
 

lib/instrumentation/transaction.js

@@ -133,10 +133,6 @@ Transaction.prototype.createSpan = function (...args) {
   if (!this.sampled) {
     return null
   }
-  if (this.ended) {
-    this._agent.logger.debug('transaction already ended - cannot build new span %o', { trans: this.id, parent: this.parentId, trace: this.traceId }) // TODO: Should this be supported in the new API?
-    return null
-  }
   if (this._builtSpans >= this._agent._conf.transactionMaxSpans) {
     this._droppedSpans++
     return null

test/instrumentation/transaction.test.js

@@ -17,6 +17,7 @@ const agent = require('../..').start({
 var test = require('tape')
 
 var Transaction = require('../../lib/instrumentation/transaction')
+const Span = require('../../lib/instrumentation/span')
 
 test('init', function (t) {
   t.test('name and type', function (t) {
@@ -565,8 +566,10 @@ test('Transaction API on ended transaction', function (t) {
   trans.ensureParentId()
   t.pass('trans.ensureParentId(...) does not blow up')
 
-  const newSpan = trans.startSpan('aNewSpanName')
-  t.equal(newSpan, null, 'trans.startSpan(...) returns null')
+  // Starting a span after the transaction has ended is allowed.
+  const spanStartedAfterTransEnd = trans.startSpan('aNewSpanName')
+  t.ok(spanStartedAfterTransEnd instanceof Span, 'trans.startSpan(...) works')
+  spanStartedAfterTransEnd.end()
 
   // Ending a span that is a child of the transaction uses some of the
   // transaction fields for breakdown metrics calculation. Ensure that works.