[foreign-memaccess] RFR: 8253243: Investigate ways to make MemorySegment::ofNativeRestricted more composable

[Athijegannathan Sundararajan]

On Wed, 16 Sep 2020 14:51:24 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This patch simplifies the MemorySegment::ofNativeRestricted API, based on a number of observations:
> 
> * now that we have shared segment support, passing an explicit owner thread isn't that useful (e.g. this is no longer an
>   escape hatch, and can - and should - be done with public, safe API)
> * if we had a public safe API to add cleanup action to existing segments, we could also get rid of the cleanup parameter
> * if a custom cleanup action embedded some other object, that object will be kept reachable, so no attachment API is
>   needed
> * if we moved the method to MemoryAddress, we would streamline uses of this API quite a bit, as in
>   `address.asSegmentRestricted(100)` - e.g. only size is needed
> 
> Adding the new API for attaching cleanup action is relatively straightforward - as usual we have to dup the segment and
> the scope, and create a new segment, with a new scope whose cleanup action calls the custom runnable before the
> original action (that original action should still be called, since otherwise we would fail to free/unmap memory). Any
> exception thrown by the cleanup action is caught and ignored, so as not to mess with the critical cleanup which has to
> occur.  A nice consequence of this work is that the surface for unsafety is much reduced, and the various methods
> compose much better. For instance, the "everything" segment can be expressed as follows:  MemoryAddress.NULL
>     .asSegmentRestricted(Long.MAX_VALUE)
>     .withOwnerThread(null)
>     .withAccessModes(READ | WRITE);
> 
> Which is, I think, much nicer. The changes in tests also suggest that the new API is easier on clients, and in one case
> (TestCleaner) the need for opting into unsafe operation actually vanished.
> I also did a bit of cleaning on the javadoc, which appeared to be off-sync when it comes to confinement.

Marked as reviewed by sundar (Committer).

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PR: https://git.openjdk.java.net/panama-foreign/pull/328