Memory leak inside ParticleEmitter

[heretique]

Hey PlayCanvas team, we were doing some memory leaks investigations on our app that uses PlayCanvas engine and we noticed a lot of leaked buffers after the entities containing the particle systems are destroyed. The way we're spawning them is we have some templates with entities with various particle systems and our VFX instantiates them in the scene on demand and then it destroys them when the particle systems have finished playing.
So when doing comparison of heap snapshots before and after spawning those vfx entities a few times we would see something like this:

A lot of JSArrayBufferData objects that are being retained by the meshInstances property of the StandardMaterial for the particular defaultMaterial.
Upon investigation we determined that this was due to the fact that the meshInstance property of the ParticleEmitter is not destroyed when the emitter's destroy method is called (and hence _destroyResources) and when the code reaches the point of destroying the material property it references the meshInstance inside the defaultMaterial where it remains forever, leaking.

Adding these lines of code to the _destroyResources method solves the problem

        this.colorParam?.destroy();
        this.colorParam = null;
// newly added
        this.meshInstance?.destroy();
        this.meshInstance = null;

        this.mesh?.destroy();
        this.mesh = null;
///
        this.vertexBuffer?.destroy();
        this.vertexBuffer = undefined; // we are testing if vb is undefined in some code, no idea why

        this.indexBuffer?.destroy();
        this.indexBuffer = undefined;

        this.material?.destroy();
        this.material = null;

We didn't create a PR because we were not sure if not destroying the meshInstance is intentional or an oversight, but it doesn't seem to have any obvious side effects, except getting rid of the memory leaks

[heretique]

To add more info here, we noticed that for some particle systems the mesh was missing when played the second time and we figured that we should not call

        this.mesh?.destroy();
        this.mesh = null;

only destroy the mesh instance