Fix runtime crash when tracing class iso containing struct val

[SeanTAllen]

Prior to this fix, you could crash the runtime if you sent a val struct wrapper in an iso class. For example:

use "collections"

struct val Foo

class Bar
  let f: Foo = Foo

actor Main
  new create(env: Env) =>
    for i in Range(0, 20000) do
      inspect(recover Bar end)
    end

  be inspect(wrap: Bar iso) =>
    None // Do something with wrap.f

Closes #3840

[SeanTAllen]

@ponylang/committer I'm finding a 0.7% performance degradation from this fix as is on the performance test machine.

Here's the basic command being run in case it means anything to you:

sudo cset proc -s user -e numactl -- -C 1-14,15 ./baseline/3840 --pingers 320 --initial-pings 5 --report-count 1 --report-interval 600 --ponynoblock --ponynoscale --ponypinasio --ponypin --ponymaxthreads=14

Roughly:

• cpu 0 is for kernel and other system processes
• cpu's 1-15 are for the application
• cpu's 1-14 are for scheduler threads
• cpu 15 is for asio

The runs are about 5 minutes (give or take a few nanos) in length.

We are using the following runtime options:

--ponynoblock --ponynoscale --ponypinasio --ponypin --ponymaxthreads=14

I'm going to try some other configurations to see if the same small degradation holds. If pony committers agree this is an acceptable perf lose to fix the bug (I think it is), I'll play around with changing the layout of fields in:

typedef struct object_t
{
  void* address;
  size_t rc;
  uint32_t mark;
  bool immutable;
  pony_type_t* type;
} object_t;

to see if changing the location of type makes a perf difference. I doubt it will, but its worth a try.

[jemc]

The change looks good, and I agree with the small perf loss being acceptable to fix this crash-safety bug.

[SeanTAllen]

I'll look into moving around where type additional field is in object map to see if it has a performance impact. I doubt it will but, it's worth checking and these perf tests are easy to do while watching TV or something.

[ponylang-main]

Hi @SeanTAllen,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3993.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

There's no difference I can see when moving where in objectmap the type is located, so I'm leaving as-is at the end.