Mustache template evaluation failure for non-existent array indices

[ywangd]

When mustache expression requests an item with a non-existent index from a Collection object, an IndexOutOfBoundsException will be thrown which in turn results in template execution failure.

The mustache java library does not support indexing into Collection object. This feature is provided by a custom CustomReflectionObjectHandler which wraps a Collection object inside a specialised map of class CustomReflectionObjectHandler#CollectionMap. The exception is thrown when checking whether the given index is available in the collection through the Map#containsKey method, whose implementation is simply trying to retrieve the item at given index. Therefore when the index does not exist, an IndexOutOfBoundsException will be thrown.

Mustache is in general very lenient of missing items. When an item does not exist, it will be simply rendered as empty string. Hence there are values to have a consistent behaviour for collection items. On the flipside, it is worth note that silently ignore all missing items could be a separate issue on its own.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[ywangd]

Currentlty, an expression of a.b results into an empty string if a is a list. But a.9 results an MustacheException due to IndexOutOfBoundsException. This feels inconsistent to me.

The discussion point is around whether this failure behaviour is what we want or even intentional. Is it a bug since the exception happens when performing a key existence check (containsKey)?

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)