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

[Chen Liang]

On Sun, 23 Mar 2025 09:28:38 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

>> 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 mainta...
>
> 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

I think the root cause is the stateless nature of `ZipFile.Source` and the stateful nature of `ZipCoder`. Exposing the UTF8 coder as stateless in class-level docs IMO increases complexity; it is just an implementation artifact that allows `ZipCoder.get` to return the same singleton instead of recreating a new instance to carry states. We shouldn't promote the storage of any `ZipCoder` in `Source`.

src/java.base/share/classes/java/util/zip/ZipCoder.java line 44:

> 42:  * <p>
> 43:  * The {@code ZipCoder} for UTF-8 charset is thread safe, {@code ZipCoder}
> 44:  * for other charsets require external synchronization.

I think the "thread safe" feature is already implied by the comment on UTF8: "Encoding/decoding is stateless". So I recommend just mentioning that a ZipCoder may carry states, and a ZipCoder obtained from `ZipCoder.get` should only be used locally.

src/java.base/share/classes/java/util/zip/ZipFile.java line 1145:

> 1143:     static record EntryPos(String name, int pos) {}
> 1144: 
> 1145:     // Implementation note: This class is be thread safe.

Should we comment that this class has no observable state in addition to being thread safe?

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PR Review: https://git.openjdk.org/jdk/pull/23986#pullrequestreview-2708904844
PR Review Comment: https://git.openjdk.org/jdk/pull/23986#discussion_r2009327538
PR Review Comment: https://git.openjdk.org/jdk/pull/23986#discussion_r2009328334