Sceptre: Fix sceptre_user_data with !stack_output

[ngfgrant]

In v2.0 When referencing sceptre_user_data with a stack_output resolver an error occurs where the resolvable property is read Sceptre enters into a recursion and eventually returns a RecursionError: maximum recursion depth exceeded.

[m1keil]

An easy way to reproduce:

template.yaml
just a simple NOOP template that creates no resources

---
AWSTemplateFormatVersion: '2010-09-09'
Description: 'Master stack: PathToMasterStackFile'

Conditions:
  HasNot: !Equals [ 'true', 'false' ]

Resources:
  NullResource:
    Type: 'Custom::NullResource'
    Condition: HasNot

Outputs:
  TestOutput:
    Value: !Ref 'AWS::StackName'

config.yaml
just general testing config

project_code: testing
region: ap-southeast-2

stack1.yaml

template_path: template.yaml

stack2.yaml
stack2 depends on stack1 directly or indirectly

template_path: template.yaml
sceptre_user_data:
  # both test & test2 will cause RecursionError. uncomment one or the other to test
  #test: !stack_output stack1.yaml::TestOutput
  #test2: !stack_output_external testing-stack1::TestOutput

to execute:

$ sceptre create stack2.yaml --yes

The root issue (as correctly pointed by @pha6d in #550) seem to be the call to fetch connection manager in line 65 of stack_output.py:

connection_manager = self.stack.template.connection_manager

I've tried to go through the recursion, but I'm still unable to grasp the concepts behind Resolver and ResolvableProperty classes. I suspect that what happens is the stack_output resolver keeps attempting to resolve itself.

Issue can be fixed by getting connection manager from the stack itself:

connection_manager = self.stack.connection_manager

However, given that connection_manager is created under template property, it makes me wonder if it will work in all the cases. (tests pass)

[ryanpagel]

Any idea when this might get released? We make heave use of stack_output_external so this issue is preventing us from upgrading to sceptre 2.x.

[chris-h-barrett]

I would echo @ryanpagel 's comment - this is blocking our upgrade to v2 sceptre, and given the timelines on end-of-support for v1 I'd ideally like to start our upgrade sooner than the 2.1.1 milestone is currently scheduled (end of April). If this was to be released sooner I would be very happy!