IndexSerializer.removeElement rejects String element ids, breaking mixed-index recovery for custom vertex id types

[batrived]

• Version: master (ac0eb23)
• Storage Backend: any
• Mixed Index Backend: any
• Expected Behavior: mixed-index transaction recovery should work on graphs configured with graph.allow-custom-vid-types.
• Current Behavior: IndexSerializer.removeElement rejects String element ids, so recovering a deleted element throws IllegalArgumentException and aborts the restore.

Details

janusgraph/janusgraph-core/src/main/java/org/janusgraph/graphdb/database/IndexSerializer.java

Lines 334 to 338 in ac0eb23

	public void removeElement(Object elementId, MixedIndexType index, Map<String,Map<String,List<IndexEntry>>> documentsPerStore) {
	Preconditions.checkArgument((index.getElement()==ElementCategory.VERTEX && elementId instanceof Long) ||
	(index.getElement().isRelation() && elementId instanceof RelationIdentifier),"Invalid element id [%s] provided for index: %s",elementId,index);
	getDocuments(documentsPerStore,index).put(element2String(elementId),new ArrayList<>());
	}

public void removeElement(Object elementId, MixedIndexType index, Map<String,Map<String,List<IndexEntry>>> documentsPerStore) {
    Preconditions.checkArgument((index.getElement()==ElementCategory.VERTEX && elementId instanceof Long) ||
        (index.getElement().isRelation() && elementId instanceof RelationIdentifier),"Invalid element id [%s] provided for index: %s",elementId,index);
    getDocuments(documentsPerStore,index).put(element2String(elementId),new ArrayList<>());
}

The precondition allows only Long for vertices. But the very next line calls element2String, which explicitly supports String ids:

janusgraph/janusgraph-core/src/main/java/org/janusgraph/graphdb/database/util/IndexRecordUtil.java

Lines 100 to 107 in ac0eb23

	Preconditions.checkArgument(index!=null,"Index with name [%s] is unknown or not configured properly",indexName);
	Preconditions.checkArgument(index.isMixedIndex());
	return (MixedIndexType)index;
	}
	public static String element2String(JanusGraphElement element) {
	return element2String(element.id());
	}

public static String element2String(Object elementId) {
    Preconditions.checkArgument(elementId instanceof Long || elementId instanceof RelationIdentifier || elementId instanceof String);
    ...
}

So the two methods disagree about what a valid element id is, and the stricter one runs first.

The only caller is the transaction-log recovery path:

janusgraph/janusgraph-core/src/main/java/org/janusgraph/graphdb/log/StandardTransactionLogProcessor.java

Lines 246 to 255 in ac0eb23

	SetMultimap<String,IndexRestore> indexRestores = HashMultimap.create();
	BackendOperation.execute(() -> {
	final StandardJanusGraphTx tx = (StandardJanusGraphTx) graph.newTransaction();
	try {
	entry.getContentAsModifications(serializer).stream()
	.map(m -> ModificationDeserializer.parseRelation(m, tx))
	.forEach(rel -> {
	//Collect affected vertex indexes
	for (final MixedIndexType index : getMixedIndexes(rel.getType())) {
	if (index.getElement()== ElementCategory.VERTEX

JanusGraphElement element = restore.retrieve(tx);
if (element!=null) {
    graph.getIndexSerializer().reindexElement(element,index,restoredDocs);
} else { //Element is deleted
    graph.getIndexSerializer().removeElement(restore.elementId,index,restoredDocs);
}

So on a graph using custom String vertex ids, recovering a transaction whose element was deleted throws instead of removing the stale index document — meaning the recovery mechanism that exists to repair index divergence cannot complete.

Steps to Reproduce

1. Configure a graph with graph.allow-custom-vid-types=true and tx.log-tx=true, with a mixed index on vertices, using String vertex ids.
2. Start transaction recovery (JanusGraphFactory.startTransactionRecovery).
3. Cause a transaction that deletes an indexed vertex to fail on the index backend.
4. Observe: recovery throws IllegalArgumentException: Invalid element id [...] provided for index and the stale document is never removed.

Suggested Fix

Align the precondition with element2String:

Preconditions.checkArgument(
    (index.getElement()==ElementCategory.VERTEX && (elementId instanceof Long || elementId instanceof String)) ||
    (index.getElement().isRelation() && elementId instanceof RelationIdentifier),
    "Invalid element id [%s] provided for index: %s", elementId, index);