# UNDF: UNDF-2026-000000385
# elasticsearch-004: IndexGraveyard.containsIndex O(n²) List scan in DanglingIndicesState loop

## Classification
- **CWE**: CWE-407 (Inefficient Algorithmic Complexity)
- **Severity**: MEDIUM
- **Path**: Node startup and periodic dangling-index detection

## Location
`server/src/main/java/org/elasticsearch/cluster/metadata/IndexGraveyard.java:120`
`server/src/main/java/org/elasticsearch/gateway/DanglingIndicesState.java:67`

## Defect

```java
// IndexGraveyard.java
private final List<Tombstone> tombstones;   // up to 500 tombstones (cluster.indices.tombstones.size)

public boolean containsIndex(final Index index) {
    for (Tombstone tombstone : tombstones) {   // O(T) — linear scan
        if (tombstone.getIndex().equals(index)) {
            return true;
        }
    }
    return false;
}

// DanglingIndicesState.java — called at node startup and on cluster state change
for (IndexMetadata indexMetadata : indexMetadataList) {   // O(I) — for each index file on disk
    Index index = indexMetadata.getIndex();
    if (graveyard.containsIndex(index) == false) {        // O(T) — linear scan per index
        danglingIndices.put(index, stripAliases(indexMetadata));
    }
}
```

**Total complexity:** O(I × T) where I = index files on disk, T = tombstones (default max 500).

In a cluster that has experienced heavy index churn (many create/delete cycles), both I and T
approach their limits, producing 500 × 500 = 250,000 equality checks on every dangling-index
scan. The scan runs at node startup (blocking) and on every cluster state update.

## Fix

Pre-build a `HashSet<Index>` from the tombstones once, then do O(1) membership tests:

```java
// IndexGraveyard.java — add a helper or cache
public Set<Index> tombstoneIndexSet() {
    Set<Index> set = new HashSet<>(tombstones.size() * 2);
    for (Tombstone t : tombstones) {
        set.add(t.getIndex());
    }
    return set;
}

// DanglingIndicesState.java
Set<Index> graveyardSet = graveyard.tombstoneIndexSet();   // O(T) once
for (IndexMetadata indexMetadata : indexMetadataList) {    // O(I)
    Index index = indexMetadata.getIndex();
    if (graveyardSet.contains(index) == false) {           // O(1)
        danglingIndices.put(index, stripAliases(indexMetadata));
    }
}
```

**Result:** O(I + T) — linear rather than quadratic.

## Overhead Measurement

| I (indices) | T (tombstones) | Slow ops | Fast ops | Ratio |
|-------------|----------------|----------|----------|-------|
| 100         | 100            | 10,000   | 200      | 50x   |
| 500         | 500            | 250,000  | 1,000    | 250x  |
| 1000        | 500            | 500,000  | 1,500    | 333x  |
