[FLINK-17083] Restrict users of accumulators not to return null in Accumulator#getLocalValue

• Type: Improvement
• Status: Closed
• Priority: Not a Priority
• Resolution: Abandoned
• Affects Version/s: 1.11.0
• Component/s: API / Core
• Labels: auto-deprioritized-major auto-deprioritized-minor
• Assignee: Unassigned
• Reporter: Caizhi Weng
• Created: 10/Apr/20 08:37
• Updated: 10/Dec/21 07:54
• Resolved: 10/Dec/21 07:54

Description

This discussion raises a problem that, if a user returns a null value in Accumulator#getLocalValue, he will be notified that the failureCause should not be null when fetching the accumulator result, which seems to be really wired.

The problem is that we're not explicitly restricting the users not to return null values in Accumulator#getLocalValue.

aljoscha is it legal to return null values? If not, we should explicitly throw a related exception to the users instead of giving the users a somewhat unrelated exception.