Adding TEP for task results in final tasks

Proposal for enabling final tasks to get value from non-final
tasks.

teps/0004-task-results-in-final-tasks.md

@@ -0,0 +1,304 @@
+---
+title: task-results-in-final-tasks
+authors:
+  - "@pritidesai"
+  - "@bobcatfish"
+creation-date: 2020-07-16
+last-updated: 2020-07-16
+status: proposed
+---
+
+# TEP-NNNN: Task Results in Final Tasks
+
+<!-- toc -->
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+  - [User Stories](#user-stories)
+- [Design Details](#design-details)
+- [Advantages](#advantages)
+- [Test Plan](#test-plan)
+- [Drawbacks](#drawbacks)
+- [Alternatives](#alternatives)
+<!-- /toc -->
+
+## Summary
+
+A `Task` in a `Pipeline` can emit `Results` which can be consumed by any other `Tasks` within the same
+`Pipeline`. `Result` of a `Task` in a `Pipeline` is received through the input parameter using variable
+substitution such as `$(tasks.<pipeline-task-name>.results.<result-name>)`.
+
+Today, final `Tasks` can not receive `Results` of any other `Tasks` in a `Pipeline`. This feature is
+limited to non-finally `Tasks` of a `Pipeline`.
+
+This proposal is enabling final `Tasks` to receive `Results` of any `Tasks` within the same `Pipeline`.
+
+## Motivation
+
+A `Pipeline` author wants to clean up resources that is done by the final `Tasks` in the `Pipeline`.
+The resources are acquired by the setup `Task` which has the names of resources. The clean up `Tasks`
+must clean up only those resources.
+
+Also, cleanup must be done only if the setup was successful but regardless of the failure of the rest of
+the `Tasks` in the `Pipeline`.
+
+```none
+setup
+  |
+  | e.g. Project Name
+  v
+cleanup
+```
+
+### Goals
+
+One goal of this proposal is to allow final `Tasks` to receive `Results` from non-finally `Tasks`.
+
+Second goal of this proposal is to determine what happens to the final `Task` when referenced
+task results are not initialized because either dependent task failed or dependent task skipped
+due to its own condition or if its parent was skipped.
+
+### Non-Goals
+
+This proposal is not enabling final `Task` to receive `Results` from other final `Tasks`.
+
+This proposal is also not enabling final `Tasks` to produce `Task` `Results`.
+
+## Proposal
+
+Allow a finally `Task` to get a value from a non-finally `Task` via a `Task` `Result`.
+If the `Task` producing the value fails, the finally `Task` that requires that value will be marked as failed.
+That final `Task` will be `attempted` i.e. evaluated by the `Pipeline Controller` for execution,
+but the validation will fail before the `TaskRun` is created since the required `Result` is not available.
+However if the `Task` being attempted has a default value for that parameter, it can still run,
+using the default when result is not available.
+
+### User Stories
+
+A `Pipeline` author can send the name of the project to finally task after non-finally task creates/acquires
+resources for that project. The finally `Task` can then clean up those project resources regardless of the
+failure of the rest of the `Tasks` in the Pipeline.
+
+## Design Details
+
+`Tasks` can produce one or more execution `Results` under `/tekton/results/` by specifying `results`
+in the `Task` specification:
+
+```yaml
+spec:
+  results:
+    - name: leased-resource
+      description: The name of the leased resource
+  steps:
+```
+
+This task produces a result file at `/tekton/results/leased-resource`.
+
+Any other `Pipeline` `Tasks` can use these `Results` as parameter values through variable substitution:
+
+```yaml
+spec:
+  tasks:
+    - name: boskos-release
+      params:
+        - name: leased-resource
+          value: $(tasks.boskos-acquire.results.leased-resource)
+```
+
+No need to specify explicit runAfter here, `Tekton` infers the execution order by setting `boskos-acquire`
+as a dependency so that the task emitting the referenced results executes before the task that consumes them.
+
+Similar to non-final tasks, final task use `Results` through variable substitution:
+
+```yaml
+spec:
+ tasks:
+   - name: boskos-acquire
+     taskRef:
+       name: boskos-acquire
+   - name: do-stuff-with-resource
+     taskRef:
+       name: do-stuff-with-resource
+     params:
+       - name: resource-name
+         value:  $(tasks.boskos-acquire.results.leased-resource)
+ finally:
+   - name: boskos-release
+     taskRef:
+       name: boskos-release
+     params:
+       - name: leased-resource
+         value:  $(tasks.boskos-acquire.results.leased-resource)
+```
+
+**Note:** Today final tasks are executed all in parallel and can not depend on any other `Task` in the `Pipeline`.
+With adding support for `Task` `Results`, we are introducing dependencies to finally `Tasks` but that
+does not change scheduling of finally tasks. All final tasks will still be executed in parallel after
+all non-final tasks finishes.
+
+**Q. What happens to `Pipeline` when the dependent task `boskos-acquire` either failed or not executed and
+the task result `leased-resource` is not initialized?**
+
+**A.** The finally `Task` `boskos-release` is attempted but declared failure since the task result
+`leased-resource` is not initialized. Pipeline controller logs the reason of this failure as invalid result
+reference `InvalidTaskResultReference`. `Pipeline` continues executing rest of the final tasks and exits with failure.
+
+**Q. What happens when the dependent `Task` succeeds but the `Task` `Result` is empty?**
+
+**A.** It could be possible that the task author intentionally chooses to leave the `Task` `Result` empty
+(creating an empty file at `/tekton/results/`). `Pipeline` leaves that param value empty and executes
+the final task. Empty `Task` `Result` is considered a valid value and results in successful variable
+substitution. The error handling of such empty `Task` `Result` is left to the final `Task` author.
+
+**Q. Can a final `Task` produce `Task` `Results`?**
+
+**A.** Nope, this proposal does not enable final tasks to produce `Task` `Results`.
+
+
+**Q. Explain the overall workflow.**
+
+**A.**
+
+```
+                   ------------------------------------------
+                  |             Param Validation              |
+                  | (required params provided and their types)|
+                   ------------------------------------------
+                                    |
+                                    V
+                   -------------------------------------------
+                  | Retrieve Execution Queue (All Final Tasks)|
+                   -------------------------------------------
+                                    |
+                                    V
+                -------------------------------------------------
+               | Validate and Apply Results from Dependent Tasks |
+                -------------------------------------------------
+                                    | (if the validation fails, declare that task failure but
+                                    | continue executing rest of the final tasks)
+                                    V
+                    ------------------------------------
+                   | Execute the Task and Create TaskRun |
+                    ------------------------------------
+```
+
+## Advantages
+
+* No API changes.
+
+* Finally contract does not break and all final tasks are attempted.
+
+* Consistent across non-final and final tasks.
+
+* Finally tasks do not need to explicitly check if the Task they depend on ran or not
+
+* This proposal is extensible with a param validation proposal using Schema -
+[#1393](https://github.com/tektoncd/pipeline/issues/1393#issuecomment-558833526) by reading default from
+`Task` specification.
+
+> `PipelineRun` reads parameter defaults from `Pipeline` specification but in case of
+parameters associated with `Task` `Results`, the default values will be used from the `Task` since
+`Pipeline` has no direct mapping between `Pipeline` parameter and `Task` parameter.
+
+```yaml
+# Pipeline
+spec:
+  params:
+    - name: owner-name
+      default: "the-best-owner"
+  tasks:
+   - name: boskos-acquire
+     taskRef:
+       name: boskos-acquire
+     params:
+       - name: owner-name
+         value: $(params.owner-name)
+   - name: do-stuff-with-resource
+     taskRef:
+       name: do-stuff-with-resource
+     params:
+       - name: resource-name
+         value:  $(tasks.boskos-acquire.results.leased-resource)
+ finally:
+   - name: boskos-release
+     taskRef:
+       name: boskos-release
+     params:
+       - name: leased-resource
+         value:  $(tasks.boskos-acquire.results.leased-resource)
+---
+
+# Task
+name: boskos-release
+spec:
+  params:
+    - name: resource-name
+      default: "best-default-ever"
+```
+
+
+## Test Plan
+
+* e2e and unit tests
+
+## Drawbacks
+
+* One could argue that this proposal breaks the finally contract, no longer means "finally tasks always run",
+it now means "finally tasks always run unless they depend on something and that fails". But `PipelineRun`
+does attempt such finally task which we explicitly fail with the validation failure so it is considered
+`ran` by the `PipelineRun` Controller.
+
+## Alternatives
+
+1. Allow dependency to specify explicit default under `results`
+
+```yaml
+
+results:
+  - name: leased-resource
+    description: The name of the leased resource
+    default: "best-default-ever"
+```
+
+**Pros/Cons:**
+
+* The default value will likely be invalid; a Task consuming this value as a param would have to understand
+what an invalid value looks like and prepare for that case.
+
+* Same default applies to all consumers of that result.
+
+* Inconsistent when the same task is invoked as non-final and final task. This default only applies to
+final `TaskRuns` and non-final `Tasks` are not allowed to have this kind of default.
+
+2. Allow `Tasks` to express validation on params; use this to identify when a default has been supplied
+
+In this option, we allow `Tasks` to specify default values for results. However we also let Tasks specify
+validation on their params, and finally Tasks can use this to verify they got value arguments and so
+should run. Use a syntax similar to the schema syntax proposed in #1393 to express validation.
+
+3. Allow default for Task Result in PipelineTask
+
+PipelineTask release-env can specify default to a task result proj-name of referenced task.
+In absence of any default, task result is considered as required and the final task results in a
+validation failure if proj-name is not initialized.
+
+```yaml
+   kind: Pipeline
+   spec:
+    finally:
+     - name: release-env
+        taskRef:
+          Name: boskos-release
+        params:
+          - name: proj-name
+            value: $(tasks.get-env.results.project)
+            default: best-default-ever
+```
+
+4. Allow finally tasks to depend on other Tasks; do not run if they fail
+
+
+
+