RFR: 8321387: SegmentAllocator:allocateFrom(AddressLayout, MemorySegment) does not throw stated UnsupportedOperationException [v3]

[Per Minborg]

On Wed, 6 Dec 2023 15:36:57 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update throws docs fror SegmentAllocator
>
> src/java.base/share/classes/java/lang/foreign/SegmentAllocator.java line 95:
> 
>> 93:      * @param str the Java string to be converted into a C string
>> 94:      * @return a new native segment containing the converted C string
>> 95:      * @throws IllegalArgumentException if the allocated segment is
> 
> I don't think the changes here are useful. What does it mean for an allocated segment to be read-only? I think all these conditions are tied to `prefixAllocator` blindly accepting read-only segments, which should NOT be the case. I suggest to revert all the chnages here and document (and throw) a new exception for when a prefix allocator is created from a read-only segment.

I have updated the PR according to this. It should be noted that, theoretically, it is still possible to write a bespoke `SegmentAllocator` that returns read-only segments. I think then, the user made a deliberate choice and perhaps understands the consequences.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16993#discussion_r1418543624