RFR: 8338445: jdk.internal.loader.URLClassPath may leak JarFile instance when dealing with unexpected Class-Path entry in manifest

[Jaikiran Pai]

Can I please get a review of this change which proposes to fix the issue noted in https://bugs.openjdk.org/browse/JDK-8338445?

The JDK internal class `jdk.internal.loader.URLClassPath` is used by classloader implementations in the JDK (for example by the `java.net.URLClassLoader` and the `jdk.internal.loader.ClassLoaders$AppClassLoader`). The implementation in `URLClassPath` constructs internal `Loader`s for each URL that is in the classpath of that instance. `Loader` implementations hold on to underlying resources from which the classpath resources are served by the `URLClassPath`.

When constructing a `Loader`, the `URLClassPath` allows the loader implementation to parse a local classpath for that specific `Loader`. For example, the `JarLoader` (an internal class of the JDK) will parse `Class-Path` attribute in the jar file's manifest to construct additional URLs that will be included in the classpath through which resources will be served by `URLClassPath`. While parsing the local classpath, if the `Loader` instance runs into any `IOException` or a `SecurityException`, the `URLClassPath` will ignore that specific instance of the `Loader` and will not hold on to it as a classpath element.

As noted earlier, `Loader` instances may hold onto underlying resources. When the `URLClassPath` ignores such `Loader`(s) and doesn't call `close()` on them, then that results in potential resource leaks. The linked JBS issue demonstrates a case where the `JarFile` held by the `JarLoader` doesn't get closed (until GC garbage collects the `JarLoader` and thus the `JarFile`), when the `URLClassPath` ignores that `JarLoader` due to an `IOException` when the `JarLoader` is parsing the `Class-Path` value from the jar's manifest.

The commit in this PR addresses the issue by calling `close()` on such `Loader`s that get rejected by the `URLClassPath` when either an `IOException` or a `SecurityException` gets thrown when constructing the `Loader` or parsing the local classpath.

A new jtreg test has been introduced which consistently reproduces this issue (on Windows) and verifies the fix. Additionally tier1, tier2 and tier3 testing has completed with this change without any failures.

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Commit messages:
 - 8338445: close loaders that are not used by the URLClassPath
 - 8338445: add test

Changes: https://git.openjdk.org/jdk/pull/20691/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20691&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338445
  Stats: 195 lines in 2 files changed: 183 ins; 3 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/20691.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20691/head:pull/20691

PR: https://git.openjdk.org/jdk/pull/20691