[foreign-memaccess] RFR: Preserve memory scope for buffer segments

[Jorn Vernee]

On Thu, 16 Apr 2020 10:48:17 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> Not 100% sure about this, but wanted to share this PR for evaluation. Basically, I've realized that we are also not
> preserving the memory scope of the original segment when we do a segment -> buffer -> segment transition.
> This means that, e.g. code like this:
> 
> MemorySegment segment = MemorySegment.allocateNative(16);
> segment = MemorySegment.ofByteBuffer(segment.asByteBuffer());
> segment.close(); // no cleanup of native memory
> 
> will not really 'close' the original segment. Similarly, if the original segment is being worked upon by some other
> thread (through a spliterator) it is possible for the morphed segment to lose this info, and to allow a close - if the
> scope is created afresh (as is now).  This patch fixes this, but I'm torn as to whether it makes things better;
> preserving access modes and thread ownership is mostly about not allowing clients to use the byte buffer escape hatch
> as a way to defy the segment restrictions. I don't think preserving scope falls into the same bucket - e.g. this is a
> separate question as to whether a segment S2 obtained through S1 -> BB -> S2 is just a *view* of S1 (with same temporal
> bounds), or is just an independent segment, with new temporal bounds. I think both answers are legitimate, and it's
> mostly a question of making up our mind.  My take is that inferring scope like this can be seen a bit too magic - and
> you are never sure of what happens when you do a MemorySegment::close, so perhaps what we have w/o this patch is
> preferrable. What do you think?

Marked as reviewed by jvernee (Committer).

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PR: https://git.openjdk.java.net/panama-foreign/pull/113