updateIndex on a mixed index fails an unrelated precondition when no key is in an applicable status

[batrived]

• Version: master (ac0eb23)
• Storage Backend: any
• Mixed Index Backend: any
• Expected Behavior: mgmt.updateIndex on a mixed index whose keys are all already in the target state either no-ops or reports a clear error, as it does for composite indexes.
• Current Behavior: it fails an internal Preconditions.checkArgument with a message about RelationTypeVertex and composite indexes, which gives no indication of the real problem.

Details

For a mixed index, updateIndex builds keySubset from the fields whose current status is in the action's applicable set:

janusgraph/janusgraph-core/src/main/java/org/janusgraph/graphdb/database/management/ManagementSystem.java

Lines 1070 to 1080 in ac0eb23

	setStatus(schemaVertex, SchemaStatus.DISCARDED, keySubset);
	updatedTypes.add(schemaVertex);
	if (!keySubset.isEmpty()) updatedTypes.addAll(dependentTypes);
	future = new EmptyScanJobFuture();
	break;
	case DISCARD_INDEX:
	if (index instanceof JanusGraphIndex && ((JanusGraphIndex) index).isMixedIndex()) {
	try {
	JanusGraphIndexWrapper indexWrapper = (JanusGraphIndexWrapper) index;
	MixedIndexType mixedIndex = (MixedIndexType) indexWrapper.getBaseIndex();
	IndexSerializer.clearStore(mixedIndex, transaction.getTxHandle());

keySubset = new HashSet<>();
MixedIndexType mixedIndexType = (MixedIndexType) indexType;
Set<SchemaStatus> applicableStatus = updateAction.getApplicableStatus();
for (ParameterIndexField field : mixedIndexType.getFieldKeys()) {
    if (applicableStatus.contains(field.getStatus()))
        keySubset.add((PropertyKeyVertex) field.getFieldKey());
}

If no field matches, keySubset is empty. setStatus then routes to the vertex-level path:

janusgraph/janusgraph-core/src/main/java/org/janusgraph/graphdb/database/management/ManagementSystem.java

Lines 1283 to 1291 in ac0eb23

	private void setStatus(JanusGraphSchemaVertex vertex, SchemaStatus status, Set<PropertyKeyVertex> keys) {
	if (keys.isEmpty()) setStatusVertex(vertex, status);
	else setStatusEdges(vertex, status, keys);
	vertex.resetCache();
	updateSchemaVertex(vertex);
	}
	private void setStatusVertex(JanusGraphSchemaVertex vertex, SchemaStatus status) {
	Preconditions.checkArgument(vertex instanceof RelationTypeVertex || vertex.asIndexType().isCompositeIndex());

private void setStatus(JanusGraphSchemaVertex vertex, SchemaStatus status, Set<PropertyKeyVertex> keys) {
    if (keys.isEmpty()) setStatusVertex(vertex, status);
    else setStatusEdges(vertex, status, keys);
    ...
}

private void setStatusVertex(JanusGraphSchemaVertex vertex, SchemaStatus status) {
    Preconditions.checkArgument(vertex instanceof RelationTypeVertex || vertex.asIndexType().isCompositeIndex());

which is guaranteed to fail for a mixed index.

Note the asymmetry: the composite-index branch calls updateAction.isApplicableStatus(schemaVertex.getStatus()) first, which either returns early or throws the helpful "Update action [%s] cannot be invoked for index with status [%s]". The mixed-index branch performs no such validation, so it falls through into an unrelated precondition.

Realistic triggers, all of them idempotent schema scripts being re-run:

• REGISTER_INDEX after all keys are already ENABLED — applicable status is {INSTALLED}, so nothing matches.
• DISABLE_INDEX when no key is ENABLED/DISABLED/REGISTERED.
• ENABLE_INDEX on a freshly created index whose keys are all INSTALLED.

Separately, and possibly worth its own issue: because the mixed-index branch never calls isApplicableStatus, DROP_INDEX on a live mixed index is not gated on DISCARDED either. schemaVertex.remove() runs regardless, dropping the schema vertex while leaving every document in the index backend orphaned with no JanusGraph-side handle to remove them.

Steps to Reproduce

mgmt = graph.openManagement();
mgmt.buildIndex("byName", Vertex.class).addKey(name).buildMixedIndex("search");
mgmt.commit();
ManagementSystem.awaitGraphIndexStatus(graph, "byName").call();

mgmt = graph.openManagement();
mgmt.updateIndex(mgmt.getGraphIndex("byName"), SchemaAction.REGISTER_INDEX);   // keys already ENABLED
// IllegalArgumentException from setStatusVertex

Suggested Fix

Give the mixed-index branch the same guard the composite branch has: when keySubset is empty, either return null as the composite path does for an inapplicable status, or throw the descriptive isApplicableStatus error naming the action and the current field statuses.