How to provide dynamic values for approvals and checks in yaml pipelines? - azure-devops

Question

I'm working on an integration between Azure Pipelines and ServiceNow's change management module. To achieve that the ServiceNow Change Management extension has been installed and configured according to this documentation page. In Azure DevOps we are using multistage yaml pipelines, which should create standard preapproved changes in ServiceNow.
The connection itself between the two applications works fine, I managed to put together a pipeline that creates change requests, waits until their status changes and then closes them. However, I'd like to pass some values set in the pipeline runs to the created change requests and I couldn't find a way to do it.
First I added a service connection to our Azure DevOps project, and created the ServiceNow check for it. I experimented a little with adding different expressions to it, like setting the short description to ${{ parameters.shortDescription }}, or defining a variable in the pipeline as ShortDescription: ${{ parameters.shortDescription }} and using that variable in the check as $(ShortDescription) or $[ variables.ShortDescription ]. Unfortunately none of these expressions got resolved. I also realized it is possible to use the predefined variables, but the values I'd like to set are not possible to describe by predefined variables. For example, selecting an assignment group would be pretty straightforward from a parameter defined as a list, but impossible to select from predefined variables.
So as a next idea, I tried to link a variable group to the check and update the variables through logging commands. Even though the variables from the group got resolved, they only showed the values I set them through the UI as a static default value. The dynamic values set via the logging commands were not visible. I played around some time and verified that I can update the definition of the variable groups through Azure CLI or REST API, so I can add new variables or update existing ones. Thus I tried to add a new variable to the linked group during the pipeline run named as ShortDescription_$(Build.BuildId). Even though it got added properly, I could not use it within the check, because it required double variable resolution, like $(ShortDescription_$(Build.BuildId)) and this expression was not resolved, not even partly. It remained $(ShortDescription_$(Build.BuildId)).
Then I started thinking about using only one variable from the group with a static name (e.g. ShortDescription) for all pipeline runs. However, I feel it would create a race condition and could cause some inconsistencies.
So as a last resort, I tried to put together an extension with an Agent and a ServerGate task, which are capable of storing the values I want to pass to change request and reading the stored values in an agentless environment. The problem here is, that the second task is not visible as a check for service connections. It's there as a release pipeline gate and looks good there, but I can't utilize it that way. Based on a question I found, this does not seem to be the problem with my task. To verify it, I copied the content of the same ServiceNow check I used before, and added it to my extension as a contribution with a different task id. And it did not show up as the question stated.
Which means now I can either
create a change request through my custom server task (as the ServerGate task can be used properly in yaml if it is changed to a Server task), but that way I can't wait for the state change of the ServiceNow ticket, or
create the change request in a separate stage where I want to use it, update it first in the same stage where I created it via the first-party check and wait for the state change in the stage where I would normally create it.
The second can work, but it has its own problems, like having misleading values stored in the changed request for the stage id field, or not having multiple change requests created for multiple run attempts of the deployment stage. Also I feel like it's not how the extension's task and check should be used.
Unfortunately, I'm out of ideas how this dynamic value passing can be achieved, if it's possible to do so in the first place. Could you please help me by sharing ideas, or pointing out errors in my attempts?