RFR: 8341597: ZipFileInflaterInputStream input buffer size uses uncompressed size

[Lance Andersen]

On Sun, 6 Oct 2024 18:02:58 GMT, Eirik Bjørsnøs <eirbjo at openjdk.org> wrote:

> Please review this PR which proposes to change the input buffer size of `ZipFileInflaterInputStream` to be based on the _compressed_ size of a ZIP entry instead of the _uncompressed_ size. This saves allocation since buffers will no longer be oversized:
> 
> * The `size` parameter passed to the `ZipFileInflaterInputStream` constructor is passed on to the superclass `InflaterInputStream` where it determines the size of the input buffer. This buffer is used to read compressed data and pass it on to the `Inflater`.
> * `ZipFile:getInputStream` currently looks at the _uncompressed_ size when determining the input buffer size. It should instead use the _compressed_ size, since this buffer is used for compressed, not uncompressed data.
> * The current implementation somewhat mysteriously adds 2 to the uncompressed size. My guess is that this is to allow fitting a two-byte DEFLATE header for empty files (where the uncompressed size is 0 and the compressed size is 2).
> * There is a check for `size <= 0`. This condition is unreachable in the current code and in the PR as well, since the compressed size will always be `>= 2`. I propose we remove this check.
> 
> Performance: A benchmark which measures the cost of opening and closing input streams using `ZipFile::getInputStream` shows a modest improvement of ~5%, consistent with less allocation of unused buffer space.
> 
> Testing: No tests are added in this PR. The `noreg-cleanup` label is added in JBS. GHA testing is currently pending.

On the surface this looks OK.

I couldn't find any historical reason as to why the change was done originally outside of part of the conversion from leaning on JNI.

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

Marked as reviewed by lancea (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21379#pullrequestreview-2352785217