UNDF: UNDF-2026-000000508

pulsar-0004 — PersistentTopic: shadowTopics List.contains() O(S×R) in checkShadowReplication()

Metadata

Project: Apache Pulsar
Component: pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentTopic.java
CWE: CWE-407 (Inefficient Algorithmic Complexity)
Severity: MEDIUM
Complexity: O(S × R) → O(R + S) where S = configured shadow topics, R = active shadow replicators

Location

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentTopic.java
Lines 2127–2155

Defective code

private CompletableFuture<Void> checkShadowReplication() {
    if (CollectionUtils.isEmpty(shadowTopics)) {
        return CompletableFuture.completedFuture(null);
    }
    List<String> configuredShadowTopics = shadowTopics;  // volatile List<String> (line 223)
    // ...
    // Check for replicators to be stopped
    shadowReplicators.forEach((shadowTopic, replicator) -> {
        ((PersistentReplicator) replicator).updateMessageTTL(newMessageTTLInSeconds);
        if (!configuredShadowTopics.contains(shadowTopic)) {  // O(S) List.contains per replicator
            futures.add(removeShadowReplicator(shadowTopic));
        }
    });
    return FutureUtil.waitForAll(futures);
}

shadowTopics is declared as private volatile List<String> (line 223). The configuredShadowTopics.contains(shadowTopic) call inside the shadowReplicators.forEach() iterates the list linearly for each active replicator.

Fix

// BEFORE:
List<String> configuredShadowTopics = shadowTopics;

// AFTER:
Set<String> configuredShadowTopics = new HashSet<>(shadowTopics);

One-line fix. The configuredShadowTopics local is used only for contains checks — a HashSet is semantically correct and provides O(1) lookup.

Complexity analysis

Scenario	Before	After
S shadow topics, R shadow replicators	O(R × S)	O(R + S)
S=50, R=50	2,500 ops per topic check	~100 ops
Large shadow-replication deployment	Multiplied across all topics × check frequency	Negligible

Notes

checkShadowReplication() is called from checkReplication(), which is invoked periodically per topic. In deployments with many shadow topics per topic (used for cross-cluster shadow replication of high-throughput topics), the cost compounds at each check interval. The fix is identical in spirit to pulsar-0003 (same method, same root cause, different field).

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