RFR: 8129776: The optimized Stream returned from Files.lines should unmap the mapped byte buffer (if created) when closed [v2]

[Brian Burkhalter]

On Tue, 26 Jan 2021 19:53:45 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Marked as reviewed by rriggs (Reviewer).
>
> Looks good. There is some complexity because the `Spliterator` can be obtained from the `Stream` returned by `Files.lines()`. This is not a common case but unfortunately we need to deal with it, otherwise we could just unmap when the `Stream` is closed.  There are edge cases where by splits may occur but they are not traversed, but i don't think we should be concerned about. This is on a good best-effort basis to free up the resource.
> 
> Is unmapping via `nioAccess.unmapper(b).unmap();` wired up?
> 
> In `Buffer`:
>                 @Override
>                 public UnmapperProxy unmapper(ByteBuffer bb) {
>                     if (bb instanceof MappedByteBuffer) {
>                         return ((MappedByteBuffer)bb).unmapper();
>                     } else {
>                         return null;
>                     }
>                 }
> 
> In MappedByteBuffer:
>     UnmapperProxy unmapper() {
>         return fd != null ?
>                 new UnmapperProxy() {
>                     @Override
>                     public long address() {
>                         return address;
>                     }
> 
>                     @Override
>                     public FileDescriptor fileDescriptor() {
>                         return fd;
>                     }
> 
>                     @Override
>                     public boolean isSync() {
>                         return isSync;
>                     }
> 
>                     @Override
>                     public void unmap() {
>                         throw new UnsupportedOperationException();
>                     }
>                 } : null;
>     }
> I don't see where `unmapper` is overridden.

`unmapper` looks to be overridden in `sun.nio.ch.FileChannelImpl`. Specifically, `map()` obtains the `Unmapper` instance from `mapInternal()`.

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PR: https://git.openjdk.java.net/jdk/pull/2229