Schema validation for Fn::Join flags empty delimiter

[glb]

The schema validation for Fn::Join seems incorrect. It is perfectly valid for the delimiter string for Fn::Join to be the empty string.

If a delimiter is the empty string, the set of values are concatenated with no delimiter.

(link)

Yet the tool complains when validating:

Serverless: Configuration warning at 'functions.example.environment.EXAMPLE['Fn::Join'][0]': should NOT be shorter than 1 characters

This is because the schema says that the minimum length for the delimiter parameter is 1:

serverless/lib/plugins/aws/provider/awsProvider.js

Line 262 in 7e4871b

items: [{ type: 'string', minLength: 1 }, { type: 'array' }],

serverless.yml

service: example
  
frameworkVersion: ">=2.0.0 <3.0.0"

provider:
  name: aws

functions:
  example:
    handler: example.example
    environment:
      EXAMPLE:
        "Fn::Join":
          - ""
          - - "ex"
            - "ample"

sls package --verbose output

Serverless: Configuration warning at 'functions.example.environment.EXAMPLE['Fn::Join'][0]': should NOT be shorter than 1 characters
Serverless:  
Serverless: Learn more about configuration validation here: http://slss.io/configuration-validation
Serverless:  
Serverless: Packaging service...
Serverless: Excluding development dependencies...
 
  Serverless Error ---------------------------------------
 
  No file matches include / exclude patterns
 
  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Issues:        forum.serverless.com
 
  Your Environment Information ---------------------------
     Operating System:          darwin
     Node Version:              14.7.0
     Framework Version:         2.4.0
     Plugin Version:            4.0.4
     SDK Version:               2.3.2
     Components Version:        3.2.1

Installed version

Framework Core: 2.4.0
Plugin: 4.0.4
SDK: 2.3.2
Components: 3.2.1

Suggested fix: remove the minLength: 1 for the first parameter to Fn::Join.

[medikoo]

@glb indeed, thanks for reporting that! PR with a fix is welcome

(cc @fredericbarthelet)