A missing variable/nested property can not be handled

[saig0]

Describe the bug
In an expression, a missing variable or a non-existing nested property can not be handled. It is handled as a failure (i.e. ValError) in the FEEL engine. Some boolean expressions, like an equal comparison (e.g. = x = "here"), can deal with it and returns false.

But it is not possible to check if a value/property is missing using the null value. For example,

not_existing = null
// false

not_existing != null
// true

This behavior is not intuitive.

To Reproduce

• workflow with an exclusive gateway with a condition x != null
• create a workflow instance without variables
• verify that the sequence flow with the condition is taken

Expected behavior
A missing variable/nested property can be compared to null and returns true.

Alternatively, provide a method to check if a variable/property is not present (e.g. isPresent(x)).

Environment:

• OS: [e.g. Linux]
• Zeebe Version: 0.23.0-SNAPSHOT
• Configuration: [e.g. exporters etc.]

[saig0]

Current workaround:

• check if a variable has a specific type

if(x instance of string) then x else "default"

[korthout]

I'd like to re-evaluate the priority of this issue.

[asaf]

@korthout this is a big issue, we avoided upgrading because of this.

some examples we ended up with but felt like it's way to compex to be used:

=(get value (products, "lists") instance of list and count(get value (product, "lists")) > 0
=not(get value(person.org, "manager") instance of context

btw, in general, it's very annoying that Zeebe fails when accessing a nonexisting variable, this makes it very hard to ask if "foo.bar.baz" exists in gateways and such.

Thanks.

[saig0]

@asaf, thank you for your input. We'll work on this soon.

My idea is to use the null check for non-existing variables and variables with null values. This should work also for nested properties if the property or the variables doesn't exist.

x = null 
// true if x is null or doesn't exist

x.y = null  
// true if x is null or doesn't exist
// true if y is null or doesn't exist

If the value or nested property is accessed without null check then an incident can be raised.

Do you have any proposals for your other examples?

[asaf]

@saig0 I think that fixing null equality is top priority
and safe get (e.g., "x.y.z") even if x or y does not exist will solve most of the issues

If you can't do it, then at least get value (obj, "x.y.z") should do the trick

[asaf]

@saig0 per your last comment, please pay attention that in case y does not exist on x an expr such:

x.y != null

will cause an error that x context does not contain y.

this behavior blocks checking x.y == null

[saig0]

Open PR: camunda/feel-scala#176

[npepinpe]

Will this be fixed by #5355?

[saig0]

I'll open a new PR. I need to add some integration code :)