S3 NotificationConfiguration - unable to validate destination configuration - amazon-web-services

Question

I'm trying to setup an S3 bucket which notifies a Lambda function when a new object is created.
The stack below works fine but I want to added the SourceArn to the Lambda permission, following best practices.
There's some literature on this which suggests the way to do it is via a string rather than Fn::GetAtt/Arn -
https://aws.amazon.com/premiumsupport/knowledge-center/unable-validate-circular-dependency-cloudformation/
But if I uncomment the relevant SourceArn line and redeploy, I get -
Unable to validate the following destination configurations (Service: Amazon S3; Status Code: 400; Error Code: InvalidArgument; Request ID: TBZDEVZ1HVD9EN0Q; S3 Extended Request ID: gL3CRz6UayvHup5i5oC4+/RMm0p1oRaRrPVtfZrykeAaJ1BVuhNSKkqxQ8TL5sy749d9PtbMOEQ=; Proxy: null)
Ho hum. Looking at the article again, I see that MyBucket needs to depend on MyBucketFunctionPermission - but if I uncomment that and redeploy I now get -
An error occurred (ValidationError) when calling the CreateChangeSet operation: Circular dependency between resources: [MyBucketFunctionPermission, MyBucket]
This is some fresh circle of hell. Am I missing something from the article or is there some other combination of SourceArn format + DependsOn that would get this to work ?
TIA.
AWSTemplateFormatVersion: '2010-09-09'
Outputs: {}
Parameters: {}
Resources:
MyBucket:
# DependsOn:
# - MyBucketFunctionPermission
Properties:
NotificationConfiguration:
LambdaConfigurations:
- Event: s3:ObjectCreated:*
Function:
Fn::GetAtt:
- MyBucketFunction
- Arn
Type: AWS::S3::Bucket
MyBucketFunction:
Properties:
Code:
ZipFile: "def handler(event, context):\n print (event)"
Handler: index.handler
Role:
Fn::GetAtt:
- MyBucketRole
- Arn
Runtime: "python3.8"
Type: AWS::Lambda::Function
MyBucketFunctionPermission:
Properties:
Action: lambda:InvokeFunction
FunctionName:
Ref: MyBucketFunction
Principal: s3.amazonaws.com
# SourceArn:
# Fn::Sub: arn:aws:s3:::${MyBucket}
Type: AWS::Lambda::Permission
MyBucketRole:
Properties:
AssumeRolePolicyDocument:
Statement:
- Action: sts:AssumeRole
Effect: Allow
Principal:
Service: lambda.amazonaws.com
Version: '2012-10-17'
Policies:
- PolicyDocument:
Statement:
- Action: logs:*
Effect: Allow
Resource: '*'
Version: '2012-10-17'
PolicyName:
Fn::Sub: my-bucket-role-policy-1234567890
Type: AWS::IAM::Role

Answer 1

There is nothing seemingly wrong with your template. It works fine, at least for me. But a race condition is possible between MyBucket and MyBucketFunctionPermission. Thus, protect against this, you have to use DependsOn. But for that to work you have to explicitly set your bucket name. For example:
AWSTemplateFormatVersion: '2010-09-09'
Outputs: {}
Parameters: {}
Resources:
MyBucket:
DependsOn:
- MyBucketFunctionPermission
Properties:
BucketName: !Sub "my-bucket-323323-${AWS::StackName}-${AWS::Region}"
NotificationConfiguration:
LambdaConfigurations:
- Event: s3:ObjectCreated:*
Function:
Fn::GetAtt:
- MyBucketFunction
- Arn
Type: AWS::S3::Bucket
MyBucketFunction:
Properties:
Code:
ZipFile: "def handler(event, context):\n print (event)"
Handler: index.handler
Role:
Fn::GetAtt:
- MyBucketRole
- Arn
Runtime: "python3.8"
Type: AWS::Lambda::Function
MyBucketFunctionPermission:
Properties:
Action: lambda:InvokeFunction
FunctionName:
Ref: MyBucketFunction
Principal: s3.amazonaws.com
SourceArn: !Sub "arn:aws:s3:::my-bucket-323323-${AWS::StackName}-${AWS::Region}"
Type: AWS::Lambda::Permission
MyBucketRole:
Properties:
AssumeRolePolicyDocument:
Statement:
- Action: sts:AssumeRole
Effect: Allow
Principal:
Service: lambda.amazonaws.com
Version: '2012-10-17'
Policies:
- PolicyDocument:
Statement:
- Action: logs:*
Effect: Allow
Resource: '*'
Version: '2012-10-17'
PolicyName:
Fn::Sub: my-bucket-role-policy-1234567890
Type: AWS::IAM::Role