RFR: 8347712: IllegalStateException on multithreaded ZipFile access with non-UTF8 charset [v6]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to fix an issue `java.util.zip.ZipFile` which would cause failures when multiple instances of `ZipFile` using non-UTF8 `Charset` were operating against the same underlying ZIP file? This addresses https://bugs.openjdk.org/browse/JDK-8347712.
> 
> ZIP file specification allows for ZIP entries to mark a `UTF-8` flag to indicate that the entry name and comment are encoded using UTF8. A `java.util.zip.ZipFile` can be constructed by passing it a `Charset`. This `Charset` (which defaults to UTF-8) gets used for decoding entry names and comments for non-UTF8 entries.
> 
> The internal implementation of `ZipFile` uses a `ZipCoder` (backed by `java.nio.charset.CharsetEncoder/CharsetDecoder` instance) for the given `Charset`. Except for UTF8 `ZipCoder`, other `ZipCoder`s are not thread safe.
> 
> The internal implementation of `ZipFile` maintains a cache of `ZipFile$Source`. A `Source` corresponds to the underlying ZIP file and during construction, uses a `ZipCoder` for parsing the ZIP entries and once constructed holds on to the parsed ZIP structure. Multiple instances of a `ZipFile` which all correspond to the same ZIP file on the filesystem, share a single instance of `Source` (after the `Source` has been constructed and cached). Although `ZipFile` instances aren't expected to be thread-safe, the fact that multiple different instances of `ZipFile` could be sharing the same instance of `Source` in concurrent threads, mandates that the `Source` must be thread-safe.
> 
> In Java 15, we did a performance optimization through https://bugs.openjdk.org/browse/JDK-8243469. As part of that change, we started holding on to the `ZipCoder` instance (corresponding to the `Charset` provided during `ZipFile` construction) in the `Source`. This stored `ZipCoder` was then used for `ZipFile` operations when working with the ZIP entries. As noted previously, any non-UTF8 `ZipCoder` is not thread-safe and as a result, any usages of `ZipCoder` in the `Source` makes `Source` not thread-safe too. That effectively violates the requirement that `Source` must be thread-safe to allow for its usage in multiple different `ZipFile` instances concurrently. This then causes `ZipFile` usages to fail in unexpected ways like the one shown in the linked https://bugs.openjdk.org/browse/JDK-8347712.
> 
> The commit in this PR addresses the issue by not maintaining `ZipCoder` as a instance field of `Source`. Instead the `ZipCoder` is now maintained in the `ZipFile`,...

Jaikiran Pai has updated the pull request incrementally with three additional commits since the last revision:

 - improve code comment for ZipFile.zipCoder
 - Alan's suggestion - change code comment about Source class being thread safe
 - Alan's suggestion - trim the javadoc of (internal) ZipCoder class

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23986/files
  - new: https://git.openjdk.org/jdk/pull/23986/files/83ac59fc..4e3f8e70

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23986&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23986&range=04-05

  Stats: 11 lines in 2 files changed: 0 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23986.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23986/head:pull/23986

PR: https://git.openjdk.org/jdk/pull/23986