EventSource "Resource" label selector doesn't support "exists" operator (and possibly others)

[azazel75]

Describe the bug
I've added the following EventSource to get events when the Workflow instances are completed:

apiVersion: argoproj.io/v1alpha1
kind: EventSource
metadata:
  name: notification
spec:
  template:
    serviceAccountName: viewer
    container:
      env:
        - name: DEBUG_LOG
          value: "true"
  resource:
    completed-workflows:
      namespace: devops
      group: argoproj.io
      version: v1alpha1
      resource: workflows
      eventTypes:
        - UPDATE
      filter:
        afterStart: true
        labels:
          - key: workflows.argoproj.io/completed
            operation: "=="
            value: "true"
          - key: workflows.argoproj.io/phase
            operation: exists

but the event source containers crashes during startup, this the relevant log:

2023-08-26T14:33:06.147Z	ERROR	argo-events.eventsource	eventsources/eventing.go:564	Failed to start listening eventsource	{"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "completed-workflows", "error": "failed after retries: failed to create the label selector for the event source completed-workflows, values: Invalid value: []string{\"\"}: values set must be empty for exists and does not exist"}

I believe the error is where the requirements are created: there it always initializes a string value

To Reproduce
Add the already mentioned eventsource with kubectl apply -f eventsource.yaml

Expected behavior
To not crash and filter updates that only ave that label

Environment (please complete the following information):

• Kubernetes: 1.26.4
• Argo: 3.4.8
• Argo Events: 1.7.6

---

Message from the maintainers:

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[gokulav137]

If we check the documentation for selectors here https://github.com/argoproj/argo-events/blob/master/api/event-source.md#selector. It says "Supported operations like ==, !=, <=, >= etc"

But if we use a "<=" operator we will get an error.
operator: Unsupported value: "<=": supported values: "in", "notin", "=", "==", "!=", "gt", "lt", "exists", "!"

Argo uses NewRequirements under the hood: https://pkg.go.dev/k8s.io/[email protected]/pkg/labels#NewRequirement
Which has following constraints that is throwing the error

// NewRequirement is the constructor for a Requirement.
// If any of these rules is violated, an error is returned:
// (1) The operator can only be In, NotIn, Equals, DoubleEquals, Gt, Lt, NotEquals, Exists, or DoesNotExist.
// (2) If the operator is In or NotIn, the values set must be non-empty.
// (3) If the operator is Equals, DoubleEquals, or NotEquals, the values set must contain one value.
// (4) If the operator is Exists or DoesNotExist, the value set must be empty.
// (5) If the operator is Gt or Lt, the values set must contain only one value, which will be interpreted as an integer.
// (6) The key is invalid due to its length, or sequence
//     of characters. See validateLabelKey for more details.
//
// The empty string is a valid value in the input values set.
// Returned error, if not nil, is guaranteed to be an aggregated field.ErrorList

operators : In, NotIn etc are defined here: https://pkg.go.dev/k8s.io/[email protected]/pkg/selection#Operator

Argo creates a requirement using the following line

req, err := labels.NewRequirement(sel.Key, op, []string{sel.Value})

So for both:

- key: workflows.argoproj.io/phase
  operation: exists
- key: workflows.argoproj.io/phase
  operation: exists
  value: ""

a list [""] is passed to NewRequirements where it is expecting an empty list [] for operators exist and !. This is throwing the error in the issue.
Invalid value: []string{""}: values set must be empty for exists and does not exist

Also only one value is used to create the requirement, this means in and notin are the same as == and !=

Possible fixes

For fixing exist and !:

• pass an empty list for exists and ! when value is "" to add support for these operators

For fixing <= and >= error:

• update the documentation to specify the correct operators gt and lt
  OR
• map "<= value" to "lt value+1" and ">= value" to "gt value-1" (as value for gt and lt is restricted to int). This would provide backward compatibility is what I am thinking if required.

For in and notin:

• maybe we can look into using a separator to split the input string for in and notin to specify different values. "," can be used as value is currently constrained to be alphanumeric.
  OR
• maybe reject in and notin as its kinda confusing, as it does the same thing as == and !=?
  OR
• Change the Selector type to something similar to LabelSelectorRequirement defined as in k8s here: https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/types.go#L1226
  It would make life easier than trying to convert between the two.
  But changing value's type from string to []string could lead to backward comparability issues

I would like to contribute to fix this issue.

[azazel75]

Then propose a pull request

[gokulav137]

Before changes

2023-09-11T09:13:00.623Z        INFO    argo-events.eventsource resource/start.go:108   configuring label selectors if filters are selected...  {"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "example"}
2023-09-11T09:13:00.623Z        ERROR   argo-events.eventsource eventsources/eventing.go:586    Failed to start listening eventsource   {"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "example", "error": "failed after retries: failed to create the label selector for the event source example, values: Invalid value: []string{\"\"}: values set must be empty for exists and does not exist"}

After changes

2023-09-11T09:21:21.184Z        INFO    argo-events.eventsource resource/start.go:108   configuring label selectors if filters are selected...  {"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "example"}
2023-09-11T09:21:21.184Z        INFO    argo-events.eventsource resource/start.go:121   setting up informer factory...  {"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "example"}
2023-09-11T09:21:21.184Z        INFO    argo-events.eventsource resource/start.go:234   running informer...     {"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "example"}
2023-09-11T09:21:21.184Z        INFO    argo-events.eventsource resource/start.go:174   listening to resource events... {"eventSourceName": "notification", "eventSourceType": "resource", "eventName": "example"}