Add new pointer memory management interfaces to structs and unions

[chrisseaton]

Allows you to go from

struct = CStructEntity.malloc([TYPE_INT, TYPE_LONG])
begin
  ...
ensure
  Fiddle.free struct
end

to

CStructEntity.malloc([TYPE_INT, TYPE_LONG], Fiddle::RUBY_FREE) do |struct|
  ..
end

and from

timeval = LIBC::Timeval.malloc()
begin
  timezone = LIBC::Timezone.malloc()
  begin
    LIBC.gettimeofday(timeval, timezone)
  ensure
    Fiddle.free timezone.to_ptr
  end
ensure
  Fiddle.free timeval.to_ptr
end

to

LIBC::Timeval.malloc(Fiddle::RUBY_FREE) do |timeval|
  LIBC::Timezone.malloc(Fiddle::RUBY_FREE) do |timezone|
    LIBC.gettimeofday(timeval, timezone)
  end
end

Both are safer, have better temporal and spatial locality for the memory manager, and reduce the working set size.

[kou]

Thanks.
I'll review this deeply later.

[kou]

If an exception is raised in here, addr isn't freed.

[chrisseaton]

Yeah maybe I should use the pointer constructor rather than using malloc directly as it used to.

[chrisseaton]

Fixed.

[kou]

I understand that we need reconstruct Fiddle::Pointer and lib/fiddle/struct.rb.
It may be better that we merge this as-is and work on it as a follow-up task.

[kou]

Duplicated.

[chrisseaton]

Fixed.

[chrisseaton]

This is the end of this series of PRs tweaking the API.

I do have some ideas about how to improve the implementation. I've got some goals to be able to share code between Fiddle and FFI, in order to make it easier for alternative implementations of Ruby which don't want to have to implement both interfaces. And I've got some goals to allow pointer operations to optimise much better in optimising implementations of Ruby.

[kou]

We can use define_singleton_method for size here because this method will not be called too much.

[chrisseaton]

Done.

[kou]

Could you remove this method?

[chrisseaton]

I didn't add it - but removed as you've asked.

[kou]

Thanks!