Empty String variables are deserialized as null with Oracle DB

Given

• Oracle 19c DB
• DMN with an empty string A as output
• UserTask with CandidateGroup or CandidateUser referring to the value of A
• wait state between business rule task and user task

When

• DMN is evaluated, Oracle DB stores empty string value as NULL

Then

• Following exception is thrown when the instance reaches UserTask:

  14:23:19.144 [camundaTaskExecutor-1] ERROR org.camunda.bpm.engine.context - ENGINE-16004 Exception while closing command context: Expression did not resolve to a string or collection of strings
  org.camunda.bpm.engine.ProcessEngineException: Expression did not resolve to a string or collection of strings
  	at org.camunda.bpm.engine.impl.task.TaskDecorator.initializeTaskCandidateUsers(TaskDecorator.java:196)
  	at org.camunda.bpm.engine.impl.task.TaskDecorator.initializeTaskAssignments(TaskDecorator.java:150)
  	at org.camunda.bpm.engine.impl.task.TaskDecorator.decorate(TaskDecorator.java:63)
  	at org.camunda.bpm.engine.impl.bpmn.behavior.UserTaskActivityBehavior.performExecution(UserTaskActivityBehavior.java:59)
  	at org.camunda.bpm.engine.impl.bpmn.behavior.TaskActivityBehavior.execute(TaskActivityBehavior.java:69)
  	...
  

Expected

• Empty string is not transferred into NULL values.

Rootcause

• String variables with an empty String as a value are persisted as NULL in the field ACT_RU_VARIABLE.TEXT_. On deserialization, this value is deserialized to null instead of an empty String

Solution idea

• Extend the String value serializer so that for empty Strings it writes a value into TEXT2_ that indicates that this is an empty String and not null (e.g. by writing the value "empty")
• Evaluate the text2 value when deserializing the variable
• Extend the value matching conditions of the queries so that
  • Empty String values can be matched on Oracle (for equals and notEquals)
    • only evaluate this extra query condition if the text value is null => only in this case we need to make the distinction, so let's avoid the query complexity otherwise
    • only evaluate this extra query condition on Oracle (same reason)
  • The value is not matched if the query condition filters for a variable of another type that also writes to text2 (e.g. file variables)
• Create sufficient test coverage to ensure that the variable filtering works in various APIs
• Create tests that ensure that filtering by other values that write to text2 does not return String values