WIP: Step "gate"

Signed-off-by: Chao Li <[email protected]>

064-gate/proposal.md

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+# Summary
+
+This RFC proposes a new step: `gate`.
+
+# Motivation
+
+Concourse pipeline lacks function of conditional workflow. Some use cases are:
+
+1. A pipeline runs code coverage scan. If coverage is lower than 85%, then fail the
+build.
+
+2. A pipeline monitors GitLab merged MergeRequests to automatically generate releases, 
+but if a MR is labeled with "no-release", then silently discard the MR.
+
+For the first use case, there could be two solutions: 1) Add gating function to the
+resource type of code coverage scan. This solution is not generic. With this direction,
+all similar scan related resource types would have to add gating function; 2) Use a 
+generic resource type of gating. This solution is better, but using a resource to do 
+gating is kinda overhead.
+
+For the second use case, gating resource seems to not fit. Because if a resource needs
+to abort a build, only way is to fail the build. If a user just wants to silently abort
+the build, then gating resource won't work. So that only solution is to make the GitLab
+resource to support to filter by MR labels, but which will make GitLab heavier and heavier.
+
+# Proposal
+
+A native step `gate` may make things easier and better.
+
+For the above use case 1, `gate` can be used like:
+
+```yaml
+- put: sonar-scan
+
+- load_var: scan-result
+  file: sonar-scan/result.json
+
+- gate: code-coverage-gate
+  condition: ((.:scan-result.coverage_rate)) < 0.85
+
+```
+
+For the above use case 2, `gate` can be used like:
+
+```yaml
+- get: mr
+
+- load_var: mr_info
+  file: mr/mr_metadata.json
+
+- gate: no-release-gate
+  condition: no-release in ((.:mr_info.labels))
+  fail: false
+```
+
+Step `gate` takes a name for description of the gate, and takes the following
+parameters:
+
+* `condition` a boolean expression. when `condition` is true, abort the build.
+* `fail` if `fail` is true, fail the build `condition` is true; otherwise abort the build
+with succeeded result.
+
+# Open Questions
+
+How to define `condition`? 
+
+Basically `condition` will be evaluated to a boolean value.
+An expression may contains:
+
+* integer comparison
+* string comparison
+* not logic
+* check if an item is in a list
+
+# Answered Questions
+
+> If there were any major concerns that have already (or eventually, through
+> the RFC process) reached consensus, it can still help to include them along
+> with their resolution, if it's otherwise unclear.
+>
+> This can be especially useful for RFCs that have taken a long time and there
+> were some subtle yet important details to get right.
+>
+> This may very well be empty if the proposal is simple enough.
+
+
+# New Implications
+
+> What is the impact of this change, outside of the change itself? How might it
+> change peoples' workflows today, good or bad?