[BUG] Version error and memory leak

[LaurensRietveld]

Do you want to request a feature or report a bug?
Bug

What happens:

• After recently updating mongoose, we've experienced a memory leak.
• When saving a document, a version-error is created and saved to this.$__.$versionError
• When creating this version error it runs a captureStacktrace command. Capturing the stack means references to it's stack (for us that included CLS, requests, etc) are retained. This is related to this nodejs issue
• As a result, the errors and their stack are not garbage collected. With many save operations this would cause our process to run out of memory

Why this didnt happen before

• In this commit the versionerror is stored in this.$__.$versionError instead of options.$versionError
• The options object is cleaned up after a save (see here). The this.$__.$versionError however isnt

Workaround

• Using a post-save hook that deletes this.$__.$versionError

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
mongoose: 5.6.3
node: v10.16.0
mongo: 3.4.1

cc @thombohlk

[vkarpov15]

Fix will be in 5.6.9 within the next 48 hours 👍

[LaurensRietveld]

@vkarpov15 @Fonger : This is still an issue in 5.7.5. We still experience a memory leak when we remove our post-save hook that deletes the versionError manually.
It seems the above fix contains a typo. delete this.$__.versionError; should be delete this.$__.$versionError;

[Fonger]

Sorry it's my fault. A revised PR has been submitted #8326