enhance(actions): only create filtered-out workflow commit status for required contexts (#38371)

Follow #38237

#38237 posts "skipped" commit statuses for every workflow that is not
triggered due to a filter (e.g. `paths` or `branches`) mismatch.
However, for non-required workflows, creating "skipped" commit statuses
for them would generate a lot of noise.

To address this issue, this PR adds a check before creating commit
status:

- For the context that matches any required status check patterns, a
"skipped" commit status will be created. The `Required` label can inform
users that this status check is required, but has been skipped because
of a filter mismatch.
- For a non-required context, nothing will be created.

NOTE: Reducing noise is a best-effort approach and isn't entirely
accurate. When creating commit statuses, it is impossible to predict
which branch protection rule will take effect. Therefore, we have to
compare the commit status context against the required patterns from all
rules. If any rule matches, the context is considered "required".

services/pull/commit_status.go

@@ -156,11 +156,16 @@ func GetPullRequestCommitStatusState(ctx context.Context, pr *issues_model.PullR
 	return MergeRequiredContextsCommitStatus(commitStatuses, requiredContexts), nil
 }
 
-// EffectiveRequiredContexts returns the required status-check contexts for a PR head:
+// EffectiveRequiredContexts returns the required status-check contexts to enforce, drawn from:
 //  1. every required scoped workflow's status-check patterns effective for the repo (always)
-//  2. the branch protection's own configured contexts, only when its status check is enabled
-func EffectiveRequiredContexts(ctx context.Context, repo *repo_model.Repository, pb *git_model.ProtectedBranch) ([]string, error) {
-	if pb == nil {
+//  2. each given protected branch rule's own configured contexts, only when that rule's status check is enabled
+//
+// Passing no rule or a single nil rule yields nothing, not even scoped patterns.
+// A single rule yields that rule's effective contexts.
+// Passing several rules unions their effective contexts; this is used when the governing rule is not yet known.
+func EffectiveRequiredContexts(ctx context.Context, repo *repo_model.Repository, pbs ...*git_model.ProtectedBranch) ([]string, error) {
+	// No protection rule in effect: nothing is required.
+	if len(pbs) == 0 || (len(pbs) == 1 && pbs[0] == nil) {
 		return nil, nil
 	}
 
@@ -169,36 +174,28 @@ func EffectiveRequiredContexts(ctx context.Context, repo *repo_model.Repository,
 		return nil, fmt.Errorf("GetEffectiveScopedWorkflowSources: %w", err)
 	}
 
+	required := make(container.Set[string])
+
 	// Every required scoped workflow's admin-authored status-check patterns, matched must-present-and-pass:
 	// a required scoped check that posts no matching status blocks the merge.
-	seen := make(container.Set[string])
-	var scoped []string
 	for _, source := range sources {
 		for _, cfg := range source.WorkflowConfigs {
 			if !cfg.Required {
 				continue
 			}
-			for _, p := range cfg.Patterns {
-				if seen.Add(p) {
-					scoped = append(scoped, p)
-				}
-			}
+			required.AddMultiple(cfg.Patterns...)
 		}
 	}
 
-	slices.Sort(scoped) // sort for stable output
-
-	// With the branch protection's own status check disabled, only the required scoped checks (mandated by the owner or instance admin) gate the merge.
-	if !pb.EnableStatusCheck {
-		return scoped, nil
-	}
-
-	// Status check enabled: the rule's configured contexts, then the scoped patterns not already among them.
-	required := slices.Clone(pb.StatusCheckContexts)
-	for _, p := range scoped {
-		if !slices.Contains(pb.StatusCheckContexts, p) {
-			required = append(required, p)
+	// Union the configured contexts of every rule whose own status check is enabled (a disabled rule contributes none)
+	for _, pb := range pbs {
+		if pb == nil || !pb.EnableStatusCheck {
+			continue
 		}
+		required.AddMultiple(pb.StatusCheckContexts...)
 	}
-	return required, nil
+
+	values := required.Values()
+	slices.Sort(values) // stable output
+	return values, nil
 }