[foreign-memaccess] RFR: JDK-8243284: Remove Foreign class

[Henry Jen]

On Tue, 21 Apr 2020 16:15:46 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> The Foreign class is currently used for 3 methods:
> 
> * withSize, which takes an unchecked address and gives it a size
> * asUnconfined, which takes an existing segment and turns it into an unconfined segment
> * asMallocSegment, which takes an unchecked address and converts that into a segment which can be closed, and where
>   closed is mapped into "free"
> 
> This patch replaces all these with this method:
> 
> static MemorySegment ofNativeUnsafe(MemoryAddress addr, long bytesSize, Thread owner, AutoCloseable cleanup, Object
> attachment)
> Which is then moved directly into MemorySegment - so that we can garbage-collect the Foreign class.
> 
> I've also made some changes on the JDK property - new name is simply "jdk.foreign" and I now made sure that it cannot
> be bypassed with a simple static initializer.

Mostly looks good to me, and I like the customizable cleanup routine. Given ability to derive a MS from another raises
the usual question of what to expect on calling close() method.

src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/Utils.java line 89:

> 88:                     s
> 89:                             .forEach(f -> sb.append(System.lineSeparator()).append("\tat ").append(f));
> 90:                     return null;

Is this format intentional? Also it seems to me this can be replaced with

StackWalker().getInstance().forEach()?

src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemorySegment.java line 494:

> 493:                                               segment,
> 494:                                               null);
> 495:      * }</pre></blockquote>

In such use case, close() called on either segment will affect the other as well, correct?
I am assuming in use case like this, the new segment close is preferred to the old segment and developer most likely
should not keep reference to the old segment?

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

Marked as reviewed by henryjen (Committer).

PR: https://git.openjdk.java.net/panama-foreign/pull/122