[crac] Integrated: 8364737: [CRaC] Support checkpointing with --patch-module by treating patch JARs as persistent

[mazhen]

On Thu, 10 Jul 2025 03:11:03 GMT, mazhen <duke at openjdk.org> wrote:

> #### Summary
> 
> This change enhances CRaC's checkpointing capabilities to correctly handle applications launched with the `--patch-module` option. Previously, using a JAR file with `--patch-module` would cause checkpointing to fail due to unmanaged file descriptors. This patch resolves the issue by treating these JARs as persistent resources, similar to how classpath JARs are handled.
> 
> #### Problem
> 
> When a JAR file is specified via `--patch-module`, it is opened by two distinct mechanisms within the JVM:
> 
> 1.  **Native `ClassLoader`**: The C++ `ClassLoader` implementation opens the JAR file to build its internal class path structures. This results in a file descriptor that is not managed by CRaC's Java-level resource tracking, leading to it being flagged as a "BAD: opened by application" file during `check_fds`.
> 2.  **Java `ModulePatcher`**: The Java-level `jdk.internal.module.ModulePatcher` also opens the same JAR to scan for packages and resources. While this could be made CRaC-aware, the native-level file descriptor remains an issue.
> 
> This dual-opening mechanism leads to `CheckpointOpenFileException` being thrown for both file descriptors during a checkpoint, preventing a successful image creation. The log output clearly illustrates this problem:
> 
> 
> $ jcmd 3403046 JDK.checkpoint
> 3403046:
> ...
> JVM: FD fd=4 type=regular path="/home/mazhen/works/patch-demo/legacy-patch.jar" BAD: opened by application
> JVM: FD fd=5 type=regular path="/home/mazhen/works/patch-demo/legacy-patch.jar" OK: claimed by java code
> ...
> An exception during a checkpoint operation:
> jdk.internal.crac.mirror.CheckpointException
>     Suppressed: jdk.internal.crac.mirror.impl.CheckpointOpenFileException: legacy-patch.jar
>         at java.base/jdk.internal.crac.JDKFileResource.lambda$beforeCheckpoint$0(JDKFileResource.java:90)
>         ...
>     Caused by: java.lang.Exception: This file descriptor was created by main at epoch:1752027938791 here
>         at java.base/jdk.internal.crac.JDKFdResource.<init>(JDKFdResource.java:40)
>         ...
>         at java.base/jdk.internal.module.ModulePatcher$JarResourceFinder.<init>(ModulePatcher.java:433)
>         ...
>     Suppressed: jdk.internal.crac.mirror.impl.CheckpointOpenFileException: FD fd=4 type=regular path=/home/mazhen/works/patch-demo/legacy-patch.jar
>         at java.base/jdk.internal.crac.mirror.Core.translateJVMExceptions(Core.java:115)
>         ...
> 
> 
> #### Solution
> 
> The fundamental assumption behind this solution is that JARs provided via `--patch-module` ...

This pull request has now been integrated.

Changeset: 329cbcc7
Author:    mazhen <mz1999 at gmail.com>
Committer: Radim Vansa <rvansa at openjdk.org>
URL:       https://git.openjdk.org/crac/commit/329cbcc7318a15db6e7d2ec62b7cc379c8dd9f54
Stats:     168 lines in 5 files changed: 167 ins; 0 del; 1 mod

8364737: [CRaC] Support checkpointing with --patch-module by treating patch JARs as persistent

Reviewed-by: rvansa

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

PR: https://git.openjdk.org/crac/pull/241