Clears error collections on transaction end after processing.

[michaelgoin]

Proposed Release Notes

• Fixed issue where capturing axios request errors could result in a memory leak.

  The agent now clears error references on transaction end, which are not used for later processing. Errors returned from 'axios' requests contain a reference to the request object which deeper down has a handle to a promise in handleRequestError. The TraceSegment associated with that promise has a handle to the transaction, which through the error capture ultimately kept the promise in memory and prevented it from being destroyed to free-up the TraceSegment from the segment map. This change also has the benefit of freeing up some memory early for transactions held onto for transaction traces.

• Added active transaction check to wrappedResEnd to prevent unecessary work for ended transactions in the case of multiple Response.prototype.end() invocations.

Links

• Closes: Agent introduces memory leak when axios errors noticed #710

Details

Ideally, I'd like to avoid holding onto user errors passed-in at all. Unfortunately, this requires touching several place and doesn't play well with our duplicate error detection. The duplicate detection uses the object itself as a key to avoid situations such as an end user calling noticeError(error) on the same instance twice on accident.

It may be worth considering if that safety mechanism isn't something we want/need to keep around. I'm not sure if any other agents do similar but I don't remember anything like that from my previous team.

I'm going to submit a separate issue to dig into this further.

While this change has the benefit of also just freeing up some memory sooner... there is some risk in clearing things off the transaction as we might in the future attempt to do something with this information later after transaction end. Unlikely at this point but calling out. More likely would be haphazardly adding something that is used by transaction traces in here to clear out as well.

[carlo-808]

Looks good, we are clearing transaction user errors and exceptions on transaction end. 👍