Feature Request: `AWS::StepFunctions::StateMachine` - Recognize if `state` is unreachable

[r-heimann]

Is this feature request related to a new rule or cfn-lint capabilities?

New capability

Describe the feature you'd like to request

Currently cfn-lint is unable to recognize if a state is unreachable. The following CloudFormation Template will cause a CloudFormation error:

AWSTemplateFormatVersion: 2010-09-09
Description: Test

Resources:
  StepFunctionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Statement:
          - Effect: Allow
            Principal:
              Service: states.amazonaws.com
            Action: sts:AssumeRole

  StateMachine:
    Type: AWS::StepFunctions::StateMachine
    Properties:
      RoleArn: !GetAtt StepFunctionRole.Arn
      Definition:
        StartAt: Pass One
        States:
          Pass One:
            Type: Pass
            Next: Pass Two
          Pass Two:
            Type: Pass
            Next: Success
          Unreachable pass: # <-------------------------- This state is unreachable
            Type: Pass
            Next: Success
          Success:
            Type: Succeed

CloudFormation error:

Resource handler returned message:
"Invalid State Machine Definition: 'MISSING_TRANSITION_TARGET: State "Unreachable pass" is not reachable.
at /States/Unreachable pass' (Service: Sfn, Status Code: 400, Request ID: e...f)
(SDK Attempt Count: 1)"
(RequestToken: 6...f8, HandlerErrorCode: InvalidRequest)

It would be great if cfn-lint could give an error on these.

Describe the solution you'd like

A new/updated rule to notify the user of this error.

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request

Would this feature include a breaking change?

• ⚠️ This feature might incur a breaking change

[kddejong]

Thanks for reporting. Detecting unreachable states requires graph traversal of the state machine definition which is more involved than a schema change. Labeling as enhancement.

[r-heimann]

I am planning, with the help of a LLM, to work on this feature sometimes this year. It is incredibly useful to have when working with AWS::StepFunctions::StateMachine.