Add callback for the soft memory limit handling

[abyss7]

This feature is useful i.e. to handle large heap allocations, pre-OOM state, or just signal about a certain size of heap.

[ckennelly]

Thanks for the pull request.

It's somewhat tricky to support a callback right here, since we're holding pageheap_lock. We read stats about memory usage under that lock, and that plausibly seems like something a zero-argument callee might need to do.

For our internal use cases that need to track memory usage, we've had a lot of success with using callbacks on our sampled allocations: It allows the application to track memory usage with about 2MB of error either way and can be done without holding any locks. While we haven't (yet) open-sourced these APIs, is that something that would work for your use case?

[abyss7]

While we haven't (yet) open-sourced these APIs, is that something that would work for your use case?

Sounds like a good solution, if these APIs get open-sourced.

This pull request emerged from our internal code - we don't collect stats immediately, but fork the process in another thread, stop the parent, and dump any stats inside the child - relying on tcmalloc's heap state shared between processes.

We have another patch to make tcmalloc more fork-friendly to supplement our technique - I'm not sure if it was also open-sourced.