Fix single-file classpath JAR resources being extracted onto the shared temp directory

[mohitduhan19]

Closes #9423

Problem

When a single-file classpath resource is loaded from a JAR (e.g. MountableFile.forClasspathResource("nested/inside/jar/Dockerfile")), MountableFile strips the resource's internal JAR path before writing it to the extraction directory. For a single file this collapses the destination name to an empty string, so the file is written directly onto the temp directory's own path (tmpLocation) instead of into it.

Any caller that treats the resolved path's parent as a self-contained context then ends up scanning the shared system temp directory instead of a dedicated one. This is exactly what happens when building a Docker image from a Dockerfile loaded via a classpath resource: docker-java's Dockerfile.parse/withDockerfile(Path) scans the parent directory of the given path, and since the parent is now /tmp itself, the scan can fail (e.g. on files it doesn't have permission to read) with:

DockerClientException: Failed to read build context directory: /tmp

Credit to @ml-james for the original diagnosis and repro in #9423.

Fix

Rather than stripping the resource's internal JAR path (the approach in the issue's suggested patch, which just re-appends a suffix), this PR removes the now-unnecessary fromRoot-stripping entirely:

• copyFromJarToLocation now copies each JAR entry to new File(toRoot, entry.getName()), i.e. it preserves the resource's own path within the JAR underneath the extraction directory.
• extractClassPathResourceToTempLocation returns new File(tmpLocation, internalPath).getCanonicalPath(), pointing at the file/directory inside its own dedicated extraction directory rather than at the extraction directory itself.

This means a single extracted file now always lands inside a directory created specifically for that extraction, with the shared temp directory never used as anything's direct parent.

Tests

Added two tests to MountableFileTest:

• forClasspathResourceFileInJarIsExtractedIntoItsOwnDirectory: builds a synthetic JAR with a single nested file resource, and asserts the extracted file is a real file, has the correct content, and its parent directory is neither the shared system temp directory nor contains anything besides the extracted file itself.
• forClasspathResourceDirectoryInJarPreservesRelativeStructure: a regression guard confirming directory-tree extraction from a JAR still lays out files relative to the resolved path exactly as before this change.

Verification note

I wasn't able to run the full Gradle build/test suite in the environment I used to prepare this PR (no access to Maven Central / the Gradle wrapper distribution). To still get real confidence in the change, I compiled the actual patched MountableFile.java (with Lombok annotations manually expanded, since lombok itself isn't resolvable in that environment) together with the real PathUtils, Base58, Transferable, and UnstableAPI classes from this repo, and exercised it with a standalone harness that:

• reproduces both new test scenarios above (single-file and directory JAR extraction) using javac/java directly against synthetic JARs, and
• runs the same extraction logic against this repo's existing core/testlib fakejar fixture (fakejar-0.jar, containing META-INF/dummy_unique_name.txt and recursive/dir/content.txt) to confirm no regression against the fixture already used by the pre-existing JAR-based tests.

All checks passed in both cases. I'd still appreciate CI running the real test suite here, and I'm happy to make any changes needed based on that.

Summary by CodeRabbit

• Bug Fixes
  • Fixed extraction of classpath resources from JAR files so resolved paths retain their correct internal structure.
  • Ensured extracted files are placed in an isolated temporary directory without unrelated content.
  • Preserved relative paths when extracting directory resources.
  • Prevented JAR entries from being extracted outside the temporary extraction directory.

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Reviewing files that changed from the base of the PR and between ca1f896 and 9716839.

📒 Files selected for processing (1)

• core/src/main/java/org/testcontainers/utility/MountableFile.java

🚧 Files skipped from review as they are similar to previous changes (1)

• core/src/main/java/org/testcontainers/utility/MountableFile.java

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📝 Walkthrough

Walkthrough

JAR-backed classpath extraction preserves each entry’s internal path and returns the selected resource’s path beneath the temporary directory. Extraction rejects entries whose canonical destinations are outside the temporary root. Regression tests cover file and directory resources.

Changes

JAR classpath resource extraction

Layer / File(s)	Summary
Preserve extracted resource paths core/src/main/java/org/testcontainers/utility/MountableFile.java	copyFromJarToLocation writes entries at their full JAR-relative paths and rejects destinations outside the extraction root. extractClassPathResourceToTempLocation returns the selected resource path beneath the temporary directory.
Validate JAR-backed resources core/src/test/java/org/testcontainers/utility/MountableFileTest.java	Added temporary-JAR helpers and regression tests for single-file extraction and directory resources with preserved relative structure.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: eddumelendez

Merge Risk: ⚪ Minimal · up to 97168

JAR-backed resources retain their paths within a dedicated extraction directory. No actionable merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 8.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main fix: preventing single-file classpath resources from JARs from being extracted onto the shared temporary directory.
Description check	✅ Passed	The description is complete. It explains the broken behavior, root cause, fix, tests, linked issue, and verification limits. It also reports the standalone validation that was performed.
Linked Issues check	✅ Passed	The change addresses #9423. extractClassPathResourceToTempLocation copies each selected JAR entry below a dedicated temporary extraction directory and returns the path below that directory. A JAR-ba…
Out of Scope Changes check	✅ Passed	The production changes remain in JAR resource extraction. The canonical-path check prevents JAR entries from escaping the extraction directory. The added tests cover single-file and directory extracti…

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🛠️ Fix failing CI checks 💡

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@core/src/main/java/org/testcontainers/utility/MountableFile.java`:
- Line 293: Update copyFromJarToLocation to resolve entry.getName() against the
canonical extraction root, normalize the destination, and reject it with an
IOException when it does not remain under that root. Perform this validation
before creating parent directories or copying data, then use the validated
destination for the existing extraction flow.

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Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: a9d2eeda-3493-48b3-9f02-6295a926a419

📥 Commits

Reviewing files that changed from the base of the PR and between 8e54951 and ca1f896.

📒 Files selected for processing (2)

• core/src/main/java/org/testcontainers/utility/MountableFile.java
• core/src/test/java/org/testcontainers/utility/MountableFileTest.java

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

[mohitduhan19]

Addressed the CodeRabbit path-traversal note in 9716839: copyFromJarToLocation now resolves each entry's canonical path and throws an IOException if it would land outside the extraction directory, before any directories are created or data is copied.

Could a maintainer approve the CI workflows when you get a chance? I'd like to confirm the full test suite passes. Happy to make any further changes.