[foreign-memaccess] [Rev 02] RFR: JDK-8241154: Clarify the role of MemorySegments

[Maurizio Cimadamore]

> This patch proposes a restacking of the memory access API. Currently, memory segments are playing a dual role: they are
> both view of a resource, and they impersonate the resource itself. This creates confusion when thinking about
> operationssuch as close() and acquire().  The idea put forward by this patch is to put all segments on equal footing;
> e.g. remove the distinction between normal segments and *acquired* segments. Now *all* segments are, in a sense,
> acquired from some memory *source*. A memory source, in other words, model the actual memory that the segment is a view
> of.  Memory sources are unconfined, which makes them ideal to support operation such as registration with cleaners (to
> allow for automatic cleanup, where needed). Moreover, since we can support many kinds of memory sources, this patch
> also adds a MappedMemorySource which is specific to mapped segments; such memory source contains methods for syncing
> contents of memory against the mapped file (e.g. force()).   This split between memory segment and memory source allows
> us to keep the memory segment API sane, while at the same time providing us room to expand the API in the future to add
> more memory sources. And it makes the API cleaner too, as we can put methods where they belong (e.g. see difference
> between MemorySegment::isAlive vs. MemorySource::isReleased).  A javadoc for this refactoring is available here:
> http://cr.openjdk.java.net/~mcimadamore/panama/8241154_javadoc/javadoc/jdk/incubator/foreign/package-summary.html  Many
> thanks to Brian, John, Jorn, Paul and Stuart for the feedback which led to this iteration.

Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:

 - Forgotr to add files
 - * addressed review comments
   * added raw address/object base accessors to memory source - remove unused ForeignUnsafe

-------------

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/54/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/54/files/3873032c..3b83ef95

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/54/webrev.02
 - incr: https://webrevs.openjdk.java.net/panama-foreign/54/webrev.01-02

  Stats: 184 lines in 7 files changed: 95 ins; 79 del; 10 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/54.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/54/head:pull/54

PR: https://git.openjdk.java.net/panama-foreign/pull/54