RFR: 8294696 - BufferedInputStream.transferTo should drain buffer when mark set [v6]

[Markus KARG]

On Wed, 29 Nov 2023 22:49:17 GMT, Markus KARG <duke at openjdk.org> wrote:

>> Buffer copy was not there before, and defensive copy was never present in `ByteArrayInputStream` as well: https://github.com/openjdk/jdk/blob/9a6ca233c7e91ffa2ce9451568b3be88ccd04504/src/java.base/share/classes/java/io/ByteArrayInputStream.java#L213
>
> Alan asked for this, and for good reason: While we implicitly trust subclasses as the buffer is "theirs" as part of the inheritance, we do not know where target stream comes from. It could be an injected verhicle to perform (at least) the following:
> * Leaking data. The target stream could access data beyond the intended region.
> * Poisoning. The target stream could write into the buffer.

As Alan pointed out, it is a bug (actually even a security risk), so BAIS should get fixed, too.

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PR Review Comment: https://git.openjdk.org/jdk/pull/10525#discussion_r1409958304