Composite Actions Support ADR (#1144)

Composite Actions ADR

Author: thboop

docs/adrs/1144-composite-actions.md

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+**Date**: 2021-06-10
+
+**Status**: Accepted
+
+## Context
+
+We released [composite run steps](https://github.com/actions/runner/pull/554) last year which started our journey of reusing steps across different workflow files. To continue that journey, we want to expand composite run steps into composite actions.
+
+We want to support the `uses` steps from workflows in composite actions, including:
+  - Container actions
+  - Javascript actions
+  - Other Composite actions (up to a limit of course!)
+  - The pre and post steps these actions can generate
+
+## Guiding Principles
+
+- Composite Actions should function as a single step or action, no matter how many steps it is composed of or how many levels of recursion it has
+  - In the future we may add a configurable option to make this no longer the case
+- A workflow author should not need to understand the inner workings of a composite action in order to use it
+- Composite actions should leverage inputs to get values they need, they will not have full access to the `context` objects. The secrets context will **not** be available to composite actions, users will need to pass these values in as an input.
+- Other Actions should **just work** inside a composite action, without any code changes
+
+## Decisions
+
+### Composite Recursion Limit
+
+- We will start with supporting a recursion limit of `10` composite actions deep
+- We are free to bump this limit in the future, the code will be written to just require updating a variable. If the graph evaluates beyond the recursion limit, the job will fail in the pre-job phase (The `Set up job` step).
+- A composite actions interface is its inputs and outputs, nothing else is carried over when invoking recursively.
+
+### Pre/Post Steps in nested Actions
+
+- We do not plan on adding the ability to configure a customizable pre or post step for composite actions at this time. However, we will execute the pre and post steps of any actions referenced in a composite action.
+- Composite actions will generate a single pre-step and post-step for the entire composite action, even if there are multiple pre-steps and post-steps in the referenced actions.
+  - These steps will execute following the same ordering rules we have today, first to run has their pre step run first and their post step run last.
+  - For example, if you had a composite action with two pre steps and two posts steps:
+
+  ```
+  - uses: action1
+  - uses: composite1
+  - uses: action2
+  ```
+
+  The order of execution would be:
+
+  ```
+  - prestep-action1
+  - prestep-composite1
+    - prestep-composite1-first-action-referenced
+    - prestep-composite1-second-action-referenced
+  - prestep-action2
+  - the job steps
+  - poststep-action2
+  - poststep-composite1
+    - poststep-composite1-the-second-action-referenced
+    - poststep-composite1-first-action-referenced
+  - poststep-action1
+  ```
+
+#### Set-state
+
+- While the composite action has an individual combined pre/post action, the `set-state` command will not be shared.
+- If the `set-state` command is used during a composite step, only the action that originally called `set-state` will have access to the env variable during the post run step.
+  - This prevents multiple actions that set the same state from interfering with the execution of another action's post step.
+
+### Resolve Action Endpoint changes
+
+- The resolve actions endpoint will now validate policy to ensure that the given workflow run has access to download that action.
+  - Older GHES/GHAE customers with newer runners will be locked out of composite uses steps until they upgrade their instance.
+
+### Local actions
+- Local actions will expand the tree, perform policy checks, and download actions Just in Time when the step is running.
+- Like current local actions, we will not support presteps. If an action is running local, by the time we know that, the time to run presteps have already passed.
+
+### If, continue-on-error, timeout-minutes - Not being considered at this time
+
+- `if`, `continue-on-error`, `timeout-minutes` could be supported in composite run/uses steps. These values were not originally supported in our composite run steps implementation.
+  - Browsing the community forums and runner repo, there hasn't been a lot of noise asking for these features, so we will hold off on them.
+- These values passed as input into the composite action will **not** be carried over as input into the individual steps the composite action runs.
+
+### Defaults - Not being considered at this time
+
+- In actions, we have the idea of [defaults](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#defaultsrun) , which allow you to specify a shell and working directory in one location, rather then on each step.
+- However, `shell` is currently required in composite run steps
+  - In regular run steps, it is optional, and defaults to a different value based on the OS.
+- We want to prioritize the right experience for the consumer, and make the action author continue to explicitly set these values. We can consider improving this experience in the future.
+
+## Consequences
+
+- Workflows are now more reusable across multiple workflow files
+- Composite actions implement most of the existing workflow run steps, with room to expand these in the future
+- Feature flags will control this rollout