Add a checkCancelled public method to ContextIndexSearcher

[scampi]

Description

The ContextIndexSearcher has some API to deal with cancelled tasks, see for example hasCancellations() below. Acting on cancelled tasks can be useful in ValueFetchers (which can be implemented by plugins via custom runtime fields), where resources can be released in such a case.
Although the method hasCancellations() is public and accessible to plugins, there is however no access point to its counterpart checkCancelled() (see below): that method would be needed to check if the search's task got cancelled (and act upon it).

• Would it be possible to add a public access method to checkCancelled(), in order to allow plugins to handle cancellations in custom runtime fields ?
• Related question: are IndexSearchers used in Elasticsearch all ContextIndexSearcher instances ?

elasticsearch/server/src/main/java/org/elasticsearch/search/internal/ContextIndexSearcher.java

Line 193 in 0d2db06

public boolean hasCancellations() {

elasticsearch/server/src/main/java/org/elasticsearch/index/mapper/MappedFieldType.java

Line 118 in 0d2db06

public abstract ValueFetcher valueFetcher(SearchExecutionContext context, @Nullable String format);

elasticsearch/server/src/main/java/org/elasticsearch/search/internal/ContextIndexSearcher.java

Line 594 in 0d2db06

public void checkCancelled() {

[scampi]

Maybe an easier access would be to have an access point to SearchContext#isCancelled from SearchExecutionContext ?

elasticsearch/server/src/main/java/org/elasticsearch/search/internal/SearchContext.java

Line 107 in 0d2db06

public abstract boolean isCancelled();

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[javanna]

Hey @scampi the fact that we expose ContextIndexSearcher#hasCancellations is a bit of a coincidence. As far as I can tell we use it only in tests. I would suggest relying instead on SearchContext#getCancellationChecks. We don't really have a super convenient isCancelled method that you can call, but you can reliably loop over the runnable that getCancellationChecks returns and handle potential exceptions that may be thrown by them to signal cancellation or timeout. Would that work for you?

[scampi]

sorry for the late reply @javanna

I would suggest relying instead on SearchContext#getCancellationChecks

This would be nice, however SearchContext is not accessible from a SearchExecutionContext.

• Within the MappedFieldType#valueFetcher we can only access the SearchExecutionContext.
• A bit similar for MappedFieldType#fielddataBuilder where the FieldDataContext doesn't give access to the SearchContext in order to react upon cancellation.

[javanna]

Thanks for clarifying @scampi , that makes sense to me.