Internal transform failure for StageName defined by reference in a SAM template

[dinvlad]

Hi, I'm trying to create a simple stack for SAM that references a Swagger file using this configuration:

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Parameters:
  APIStage:
    Default: dev
    Description: Name of the API stage to use for deployment
    Type: String
    AllowedPattern: '[A-Za-z0-9-]*'

Resources:
  Api:
    Type: AWS::Serverless::Api
    Properties:
      StageName: !Ref APIStage
      DefinitionUri: models.yaml

However,

aws cloudformation package --template-file sam.yaml --output-template-file deploy.yaml --s3-bucket backend-dev-xxxx
aws cloudformation deploy --template-file deploy.yaml --stack-name backend-dev --capabilities CAPABILITY_IAM --parameter-overrides APIStage=dev

leads to

Failed to create the changeset: Waiter ChangeSetCreateComplete failed: Waiter encountered a terminal failure state Status: FAILED. Reason: Transform AWS::Serverless-2016-10-31 failed with: Internal transform failure.

When I remove the Parameters section and set StageName: dev explicitly, the stack is created successfully. So it seems like Transform fails because of the APIStage reference, even though the template is validated successfully before CloudFormation calls Transform.

[dinvlad]

Update: I got a bit confused but now it seems clear that this issue is specific to how Transform processes referenced StageName for AWS::Serverless::Api rather than being a generic issue with processing referenced variables. For example, the following template was adopted from the HelloWorld example and appears to work correctly:

AWSTemplateFormatVersion : '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Parameters:
  APIStage:
    Default: dev
    Description: Name of the API stage to use for deployment
    Type: String
    AllowedPattern: '[A-Za-z0-9-]*'

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: index.handler
      Runtime: nodejs4.3
      Environment:
        Variables:
          APIStage: !Ref APIStage

[sanathkr]

See #22. We don't support !Ref on the StageName parameter yet.

[sanathkr]

I am closing this issue in favor of tracking the problems thru #22 and #20