runtime: add debug mode for finalizers and cleanups

[mknyszek]

Finalizers are notoriously difficult to work with due to all the subtleties you need to be aware of. Cleanups have resolved several finalizer issues, but still carry some of the same issues.

To help detect misuse, I'd like to propose a new debug mode (GODEBUG) that runs a partial GC cycle during mark termination to detect reference cycles for finalizers and references from cleanup roots to the objects they're attached to. This debug mode can also include three additional checkers:

• Detect when a finalizer or cleanup function fully blocks the goroutine it's on and report it as a potential issue.
• Detect when a finalizer or cleanup is attached to a tiny alloc block. This will require additional state tracking.

I'll also note that execution traces would be a great place to analyze inefficiencies due to finalizers and/or cleanups.

https://go.dev/cl/634599 is a start.

[gabyhelp]

Related Issues

• runtime: document explicitly that finalizers and cleanups can hold up other finalizers and cleanups #72950

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[gopherbot]

Change https://go.dev/cl/634599 mentions this issue: runtime: add new GODEBUG checkfinalizer

[gopherbot]

Change https://go.dev/cl/662135 mentions this issue: runtime: annotate checkfinalizers reports with source and type info

[gopherbot]

Change https://go.dev/cl/662038 mentions this issue: runtime: add scan trace for checkfinalizers>1

[gopherbot]

Change https://go.dev/cl/662039 mentions this issue: runtime: add package doc for checkfinalizer mode

[gopherbot]

Change https://go.dev/cl/662037 mentions this issue: runtime: mark and identify tiny blocks in checkfinalizers mode