Ensure collected data is sent to the agent even if flushInterval is negative (#5923)

If the tracer was configured with a negative `flushInterval`, it would never
flush its internal state to the agent and would keep consuming memory
indefinitely until either the process exited naturally, or ran out of memory.

Other minor code changes:
- Once a timer has fired, there's no need to call `clearTimeout` on it.
- Additionally, the fact that `clearTimeout` returns `undefined`, is just
  because it doesn't return anything. So if we want to set the reference to the
  timer to `undefined`, just do so directly instead of relying on the return
  value of `clearTimeout`.

Author: watson

packages/dd-trace/src/exporters/agent/index.js

@@ -5,6 +5,8 @@ const log = require('../../log')
 const Writer = require('./writer')
 
 class AgentExporter {
+  #timer
+
   constructor (config, prioritySampler) {
     this._config = config
     const { url, hostname, port, lookup, protocolVersion, stats = {}, apmTracingEnabled } = config
@@ -28,8 +30,11 @@ class AgentExporter {
       config
     })
 
-    this._timer = undefined
-    process.once('beforeExit', () => this._writer.flush())
+    process.once('beforeExit', () => {
+      clearTimeout(this.#timer)
+      this.#timer = undefined
+      this._writer.flush()
+    })
   }
 
   setUrl (url) {
@@ -49,10 +54,10 @@ class AgentExporter {
 
     if (flushInterval === 0) {
       this._writer.flush()
-    } else if (flushInterval > 0 && !this._timer) {
-      this._timer = setTimeout(() => {
+    } else if (this.#timer === undefined) {
+      this.#timer = setTimeout(() => {
         this._writer.flush()
-        this._timer = clearTimeout(this._timer)
+        this.#timer = undefined
       }, flushInterval).unref()
     }
   }

packages/dd-trace/src/exporters/common/agent-info-exporter.js

@@ -59,10 +59,10 @@ class AgentInfoExporter {
 
     if (flushInterval === 0) {
       writer.flush()
-    } else if (flushInterval > 0 && !this[timerKey]) {
+    } else if (this[timerKey] === undefined) {
       this[timerKey] = setTimeout(() => {
         writer.flush()
-        this[timerKey] = clearTimeout(this[timerKey])
+        this[timerKey] = undefined
       }, flushInterval).unref()
     }
   }