MEDIUM: Race condition between close() and loadClassData() in MavenRepositoryClassSource

[sfloess]

close() synchronizes on perArtifactLocks map but loadClassData() only checks volatile 'closed' flag.

Location: src/main/java/org/flossware/classloader/MavenRepositoryClassSource.java:392
Severity: MEDIUM - Resource leak
Impact: Temp files and JarFile handles leaked after close()

🤖 AI Attribution: opus (85% confidence, 1/1 consensus)

[sfloess]

✅ Fixed in commit aa7edc2

Solution

Replace the broken synchronization strategy in MavenRepositoryClassSource with a ReentrantReadWriteLock that coordinates close() with loadClassData(). Currently, close() synchronizes on perArtifactLocks (the ConcurrentHashMap itself) while loadClassData() only checks the volatile 'closed' flag -- this allows a thread in loadClassData() to obtain a JarFile reference, then have close() close that JarFile and delete its temp file out from under it, or allows a thread to download and cache a new JAR after close() has already cleaned up, leaking the temp file and JarFile handle.

The fix introduces a ReentrantReadWriteLock (closeLock):

• loadClassData() acquires the READ lock, allowing concurrent class loading.
• close() acquires the WRITE lock, ensuring exclusive access so no loadClassData() call is mid-flight when resources are cleaned up.
• The per-artifact lock mechanism inside ensureJarCached() is retained for fine-grained download coordination.
• A re-check of the 'closed' flag inside ensureJarCached() (under the per-artifact lock) prevents new downloads after close() sets the flag.

Details

Commit: aa7edc2
Files Modified: src/main/java/org/flossware/classloader/MavenRepositoryClassSource.java
Confidence: 92%
Consensus: undefined% agreement across 3 AI models

Rationale

The race condition has three distinct failure scenarios, all caused by the lack of coordination between close() and loadClassData():

Scenario 1 - JarFile closed mid-read: Thread A calls loadClassData(), passes the volatile check (closed==false), gets a JarFile from ensureJarCached(), and begins extractClassFromCachedJar(). Before extractClassFromCachedJar() finishes reading from the JarFile, Thread B calls close(), which closes the JarFile and deletes the temp file. Thread A then gets an IOException reading from the closed JarFile (or worse, reads corrupt data).

Scenario 2 - Resource leak after close: Thread A calls loadClassData(), passes the volatile check (closed==false), enters ensureJarCached(). Thread B calls close(), sets closed=true, clears jarFileCache, jarPathCache, and perArtifactLocks. Thread A then proceeds to download a new JAR, creates a temp file, opens a JarFile, and puts both in the now-cleared caches. Since close() already ran, nothing will ever clean up these resources.

Scenario 3 - ConcurrentModificationException on perArtifactLocks: close() calls perArtifactLocks.clear() while synchronized on perArtifactLocks (the map itself). Meanwhile, loadClassData() calls perArtifactLocks.computeIfAbsent() without any coordination. While ConcurrentHashMap handles this safely at the map level, the semantic inconsistency means the lock object retrieved by computeIfAbsent() could be cleared immediately by close().

The ReentrantReadWriteLock fix addresses all three scenarios:

• The read lock in loadClassData() guarantees that the entire operation (from closed-flag check through JarFile usage) runs atomically with respect to close().
• The write lock in close() waits for all in-flight loadClassData() operations to complete before destroying resources.
• Multiple concurrent loadClassData() calls can still run in parallel (read locks are shared), so there is no loss of concurrency for the normal case.
• The additional closed check inside ensureJarCached() is a defense-in-depth measure for edge cases where close() could interleave with per-artifact lock acquisition.

This pattern is consistent with MavenNexusClassSource which uses synchronized(this) for the same purpose, but ReentrantReadWriteLock is superior here because it allows concurrent reads (class loads) while only blocking for the exclusive close() operation.

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🤖 AI Attribution

Worker AI (Selected Solution)

• Model: opus
• Confidence: 92%
• Approach: Replace the broken synchronization strategy in MavenRepositoryClassSource with a ReentrantReadWriteLock that coordinates close() with loadClassData(). Currently, close() synchronizes on perArtifactLocks (the ConcurrentHashMap itself) while loadClassData() only checks the volatile 'closed' flag -- this allows a thread in loadClassData() to obtain a JarFile reference, then have close() close that JarFile and delete its temp file out from under it, or allows a thread to download and cache a new JAR after close() has already cleaned up, leaking the temp file and JarFile handle.

The fix introduces a ReentrantReadWriteLock (closeLock):

• loadClassData() acquires the READ lock, allowing concurrent class loading.
• close() acquires the WRITE lock, ensuring exclusive access so no loadClassData() call is mid-flight when resources are cleaned up.
• The per-artifact lock mechanism inside ensureJarCached() is retained for fine-grained download coordination.
• A re-check of the 'closed' flag inside ensureJarCached() (under the per-artifact lock) prevents new downloads after close() sets the flag.
• Rationale: The race condition has three distinct failure scenarios, all caused by the lack of coordination between close() and loadClassData():

Scenario 1 - JarFile closed mid-read: Thread A calls loadClassData(), passes the volatile check (closed==false), gets a JarFile from ensureJarCached(), and begins extractClassFromCachedJar(). Before extractClassFromCachedJar() finishes reading from the JarFile, Thread B calls close(), which closes the JarFile and deletes the temp file. Thread A then gets an IOException reading from the closed JarFile (or worse, reads corrupt data).

Scenario 2 - Resource leak after close: Thread A calls loadClassData(), passes the volatile check (closed==false), enters ensureJarCached(). Thread B calls close(), sets closed=true, clears jarFileCache, jarPathCache, and perArtifactLocks. Thread A then proceeds to download a new JAR, creates a temp file, opens a JarFile, and puts both in the now-cleared caches. Since close() already ran, nothing will ever clean up these resources.

Scenario 3 - ConcurrentModificationException on perArtifactLocks: close() calls perArtifactLocks.clear() while synchronized on perArtifactLocks (the map itself). Meanwhile, loadClassData() calls perArtifactLocks.computeIfAbsent() without any coordination. While ConcurrentHashMap handles this safely at the map level, the semantic inconsistency means the lock object retrieved by computeIfAbsent() could be cleared immediately by close().

The ReentrantReadWriteLock fix addresses all three scenarios:

• The read lock in loadClassData() guarantees that the entire operation (from closed-flag check through JarFile usage) runs atomically with respect to close().
• The write lock in close() waits for all in-flight loadClassData() operations to complete before destroying resources.
• Multiple concurrent loadClassData() calls can still run in parallel (read locks are shared), so there is no loss of concurrency for the normal case.
• The additional closed check inside ensureJarCached() is a defense-in-depth measure for edge cases where close() could interleave with per-artifact lock acquisition.

This pattern is consistent with MavenNexusClassSource which uses synchronized(this) for the same purpose, but ReentrantReadWriteLock is superior here because it allows concurrent reads (class loads) while only blocking for the exclusive close() operation.

Arbiter Decision

• Arbiter Model: haiku
• Decision: Selected Fix Bump org.apache.kafka:kafka-clients from 3.6.1 to 3.9.2 #3
• Reasoning: Fix 3 is the best solution because it provides complete protection against all three race condition scenarios through a proven, production-ready pattern already used in MavenNexusClassSource.

Why Fix 3 wins:

1. Completeness: Fixes 1 and 2 are incomplete. Fix 1 only adds a closed check inside the per-artifact lock but fails to protect JarFile usage in extractClassFromCachedJar(). Fix 2 only protects the volatile flag check, not the actual JarFile operations. Fix 3 holds the read lock during the ENTIRE loadClassData() operation (including both cache retrieval and JarFile reading), preventing close() from closing resources while they're in use.

2. Correctness: The core race condition occurs when Thread A obtains a JarFile reference from cache (line 199-200) without holding a protective lock, then Thread B executes close() and closes that JarFile, then Thread A tries to read from the closed JarFile. Fix 3 solves this by holding the read lock across the entire operation. Fixes 1 and 2 leave critical gaps where JarFile usage is unprotected.

3. Proven Pattern: MavenNexusClassSource (already in the codebase) uses ReentrantReadWriteLock with the exact same design—holding the read lock for the entire loadClassData() and using the write lock in close(). This pattern is proven to work and handles the identical synchronization requirements.

4. Concurrency: Fix 3 maintains good concurrency because multiple threads can hold the read lock simultaneously (shared access), only blocking when close() acquires the exclusive write lock. Fixes 1 and 2 use coarser locking strategies that don't adequately address the problem.

5. Risk Assessment: Fix 3 changes the synchronization strategy but does so based on a proven pattern from a related class, minimizing implementation risk. The complexity is justified by the complete protection it provides.

The three failure scenarios are all prevented:

• Scenario 1 (JarFile closed mid-read): Read lock prevents close() from executing
• Scenario 2 (Resource leak after close): Entire operation is atomic; no new resources created after close()
• Scenario 3 (ConcurrentModificationException): No longer relies on perArtifactLocks synchronization for safety
• Consensus Score: 0%

Multi-Model Consensus

• Models Proposed Solutions: 3
• Best Solution Selected By: haiku

Alternative Proposals (Not Selected)

1. sonnet (Confidence: 92%)

   • Approach: Introduce proper synchronization coordination between close() and loadClassData()/ensureJarCached(). Add a closed check inside the synchronized block of ensureJarCached() after acquiring the per-artifact lock, and ensure close() properly coordinates with ongoing operations by checking/waiting for in-flight operations.
   • Reason Not Selected: Not selected by haiku arbiter

2. haiku (Confidence: 82%)

   • Approach: Synchronize the closed flag check with proper locking to prevent race conditions. Use the same perArtifactLocks synchronization mechanism that close() uses, ensuring that loadClassData() cannot proceed if close() is running or has completed. This prevents the scenario where a thread checks closed, finds it false, then another thread closes all resources before the first thread uses them.
   • Reason Not Selected: Not selected by haiku arbiter

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