[foreign-abi] [Rev 01] RFR: 8247439: NativeAllocationScope should have a way to register existing segments onto it

[Athijegannathan Sundararajan]

On Fri, 12 Jun 2020 10:54:32 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> In extracted code, there are two cases where segments can be created outside of an allocation scope:
>> 
>> * structs passed returned by value
>> * callback segments
>> 
>> In these cases it is necessary for clients to manually handle the life-cycle of these segments with a separate (nested)
>> try with resource, which makes the code harder to follow.
>> Jextract is currently bypassing this limitation by providing a "register" mechanism, but the provided mechanism is not
>> very safe, since it is up to the user not to prematurely close the registered segment.
>> This patch add a safer routine (called `NativeAllocationScope::claim`) which can be used to attach a segment onto an
>> existing scope. This returns a _new_ segment with a brand _new_ memory scope (and the old segment is killed). The new
>> segment only features mininal access modes (READ and WRITE). As such, it cannot be closed directly - it can only be
>> closed when the allocation scope is.  Furthermore, there are some important restrictions on which segments can be
>> attached to a scope:
>> * the thread owner of the segment must match that of the allocation scope (yes, now allocation scopes are confined too)
>> * the segment to be attached must feature the CLOSE access mode
>> 
>> The latter restriction is important for a number of reasons:
>> 
>> 1. it prevents an attached segment to be re-attached somewhere else (since attached segment cannot feature CLOSE mode
>> by definition) 2. it prevents acquired views to be attached - the lifecycle of acquired views is closely tied to that
>> of the parent segment, and the spliterator they come from - so it doesn't seem a good idea to attach them somewhere
>> else 3. it prevents, more generally, NativeAllocationScope::close to throw because some attached segment cannot be
>> closed  Under the hood, NativeAllocationScope::claim uses the same logic as MemorySegment::withOwnerThread, so we know
>> that logic is safe.
>> I'm open to suggestion for the method name (e.g. maybe `claimOwnership` could be more descriptive?); I'm also open to
>> consider renaming `NativeAllocationScope` to just `NativeScope`.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   * Renamed "claim" to "register"
>   * Renamed "NativeAllocationScope" to "NativeScope"

Marked as reviewed by sundar (Committer).

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PR: https://git.openjdk.java.net/panama-foreign/pull/199