infra: add Mergify serial merge queue for dev branch

.github/PULL_REQUEST_TEMPLATE.md

@@ -25,6 +25,11 @@
 ## Related issue
 Closes #
 
+## Dependencies
+<!-- List any PRs or issues that must merge before this one. -->
+<!-- Replace "none" with one or more references, e.g. Depends-On: #123 -->
+Depends-On: none
+
 ## Checklist
 - [ ] Every commit includes a DCO `Signed-off-by` trailer (`git commit -s`; see `docs/dco.md`)
 - [ ] My code follows the rule template in CONTRIBUTING.md

.mergify.yml

@@ -0,0 +1,141 @@
+# .mergify.yml
+# Merge-queue configuration for the OpenShield dev branch.
+#
+# This file is inactive until the Mergify GitHub App is installed and
+# authorized for this repository. GitHub Actions remains the CI system.
+# Mergify reads pull-request state and operates the queue; it does not
+# run CI, replace existing workflows, or bypass GitHub branch protection.
+#
+# Related: https://github.com/openshield-org/openshield/issues/334
+
+# Limit speculative checks globally so at most one PR is tested at a time.
+# This enforces serial behaviour across the queue.
+merge_queue:
+  max_parallel_checks: 1
+
+queue_rules:
+  - name: default
+    # Restrict queue entry to PRs targeting dev, including manual
+    # @mergifyio queue commands. Without this, a maintainer could
+    # manually queue a PR targeting any branch.
+    queue_conditions:
+      - base=dev
+
+    # One PR merged per operation. Combined with max_parallel_checks: 1
+    # above, this gives a fully serial merge queue.
+    batch_size: 1
+
+    # Dequeue a PR whose required checks have not posted within this
+    # window instead of stalling the entire queue indefinitely. The
+    # default ("auto") applies no timeout until enough queue history has
+    # been gathered, so a stuck check on the very first queued PR would
+    # block every subsequent PR forever.
+    checks_timeout: 2 hours
+
+    # These conditions must hold at the moment Mergify merges the PR.
+    # Mergify re-evaluates them after rebasing onto the current dev HEAD.
+    #
+    # CI Summary aggregates all jobs inside ci.yml, including the Astro
+    # website build and rendered-site verification added by PR #329
+    # (now on dev). CodeQL runs in a separate workflow (codeql.yml) and
+    # is NOT included in CI Summary, so it is gated explicitly below.
+    # Build site is gated explicitly for belt-and-braces coverage: it
+    # runs unconditionally on every dev-targeted PR (no paths filter)
+    # so it cannot stall the queue.
+    #
+    # External Semgrep app checks (Semgrep OSS, semgrep-cloud-platform/scan)
+    # are deliberately not gated: their coverage duplicates the SAST
+    # (Semgrep) job already gated through CI Summary, and third-party app
+    # checks can vanish if the app is uninstalled or its plan changes,
+    # which would stall the queue permanently.
+    merge_conditions:
+      - check-success=CI Summary
+      - check-success=DCO sign-off
+      - check-success=dependency-review
+      - check-success=CodeQL
+      - check-success=Analyze (python)
+      - check-success=Analyze (javascript)
+      - check-success=Build site
+      # terraform-plan only runs for PRs touching infra/terraform/**. An
+      # unconditional check-success would stall every non-terraform PR
+      # forever. This condition passes when the PR touches no terraform
+      # files; otherwise the check must be green.
+      - or:
+          - -files~=^infra/terraform/
+          - check-success=Terraform fmt / validate / plan
+      - "#approved-reviews-by>=1"
+      - "#changes-requested-reviews-by=0"
+      - "#review-threads-unresolved=0"
+      - -draft
+      - -label=blocked
+
+pull_request_rules:
+  # Dismiss stale approvals when a new commit is pushed to a dev-targeted PR.
+  # CHANGES_REQUESTED reviews are intentionally NOT dismissed (changes_requested
+  # is explicitly set to false). A reviewer's objection must survive the
+  # contributor's push and be cleared only by the reviewer re-reviewing, or
+  # by a maintainer manually dismissing it via GitHub after verifying the fix
+  # (see the inactive-reviewer process in docs/merge-queue.md).
+  # Auto-dismissing CHANGES_REQUESTED reviews would allow any push followed
+  # by a third-party approval to silently erase a blocking concern without
+  # the original reviewer ever checking the fix.
+  #
+  # Reviewers: use inline conversation threads for every blocking concern.
+  # Threads survive this dismissal and continue to block via
+  # #review-threads-unresolved=0, giving an independent gate beyond the
+  # review verdict itself.
+  - name: dismiss stale approvals on new push
+    conditions:
+      - base=dev
+    actions:
+      dismiss_reviews:
+        approved: true
+        changes_requested: false
+        message: "A new commit was pushed. Please re-review and re-approve before this PR can merge."
+
+  # Queue an eligible PR automatically once all conditions are met.
+  # Entry conditions mirror merge_conditions so only fully green PRs
+  # enter the queue. A PR admitted while a check is still pending
+  # occupies the single queue slot and stalls every PR behind it.
+  - name: add to merge queue when eligible
+    conditions:
+      - base=dev
+      - check-success=CI Summary
+      - check-success=DCO sign-off
+      - check-success=dependency-review
+      - check-success=CodeQL
+      - check-success=Analyze (python)
+      - check-success=Analyze (javascript)
+      - check-success=Build site
+      - or:
+          - -files~=^infra/terraform/
+          - check-success=Terraform fmt / validate / plan
+      - "#approved-reviews-by>=1"
+      - "#changes-requested-reviews-by=0"
+      - "#review-threads-unresolved=0"
+      - -draft
+      - -label=blocked
+    actions:
+      queue:
+        name: default
+
+  # Apply awaiting-author when a reviewer has requested changes.
+  - name: label awaiting-author when changes are requested
+    conditions:
+      - base=dev
+      - "#changes-requested-reviews-by>0"
+    actions:
+      label:
+        add:
+          - awaiting-author
+
+  # Remove awaiting-author once all change requests are resolved.
+  - name: remove awaiting-author when no changes are requested
+    conditions:
+      - base=dev
+      - label=awaiting-author
+      - "#changes-requested-reviews-by=0"
+    actions:
+      label:
+        remove:
+          - awaiting-author

docs/merge-queue.md

@@ -0,0 +1,148 @@
+# Merge Queue
+
+OpenShield uses Mergify to run a serial merge queue on the `dev` branch.
+GitHub Actions remains the CI system. Mergify reads pull-request state,
+waits for all conditions to be met, rebases each PR onto the current `dev`
+HEAD, and merges it only after CI passes on the updated state.
+
+This configuration is inactive until the Mergify GitHub App is installed
+and authorized for this repository. The `.mergify.yml` file has no effect
+before installation.
+
+## Eligibility
+
+The queue applies only to pull requests that target the `dev` branch.
+PRs targeting any other branch are not affected by this configuration.
+
+A pull request enters the queue automatically when all of the following
+are true:
+
+- The PR targets the `dev` branch
+- `CI Summary` check is successful (aggregates all jobs in `ci.yml`,
+  including the Astro website build and rendered-site verification)
+- `DCO sign-off` check is successful
+- `dependency-review` check is successful
+- `CodeQL` check is successful
+- `Analyze (python)` check is successful
+- `Analyze (javascript)` check is successful
+- `Build site` check is successful
+- `Terraform fmt / validate / plan` check is successful, or the PR does
+  not touch any files under `infra/terraform/` (the check only runs for
+  terraform-touching PRs, so a plain `check-success` condition would stall
+  every other PR)
+- At least one approving review exists and is current for the latest push
+- No active `CHANGES_REQUESTED` review exists
+- All review conversations are resolved
+- The pull request is not a draft
+- The `blocked` label is not applied
+
+Mergify processes one pull request at a time. It rebases the queued PR onto
+the latest `dev` and runs CI again before merging, so the branch is always
+tested against what is actually on `dev` at merge time.
+
+`CI Summary` covers all jobs inside `ci.yml`. `CodeQL` and `Build site` run
+in separate workflows (`codeql.yml` and `website.yml`) and are not part of
+`CI Summary`, which is why they are listed explicitly above. The external
+Semgrep app checks are deliberately not gated: their coverage duplicates the
+`SAST (Semgrep)` job already in `CI Summary`, and gating third-party app
+checks would stall the queue if the app is ever uninstalled or its plan
+changes.
+
+## Keeping a PR out of the queue
+
+To prevent a PR from entering the queue while it is still in progress, either:
+
+- Open it as a **draft**. Mergify will not queue it until you mark it ready
+  for review.
+- Apply the **`blocked` label**. This removes the PR from queue eligibility
+  and also prevents it from merging even if it is already in the queue.
+  Remove the label when the PR is ready to proceed.
+
+## Declaring dependencies
+
+If your PR depends on another PR or issue merging first, declare it in the
+pull request description:
+
+```
+Depends-On: #123
+```
+
+Use one `Depends-On:` line per dependency. The PR can enter the queue
+immediately, but Mergify holds it there until every declared dependency has
+merged. Once all dependencies are satisfied, Mergify re-evaluates the PR
+against the latest `dev` state and proceeds.
+
+Leave the placeholder as `none` when there are no dependencies:
+
+```
+Depends-On: none
+```
+
+Do not delete the section. It makes dependency state visible to reviewers.
+
+If a dependency PR is closed without merging, Mergify will hold your PR
+indefinitely. To unblock it, edit the PR description and remove or replace
+the `Depends-On:` line for that closed PR, then update the branch to
+trigger re-evaluation.
+
+## awaiting-author label and inactive-author process
+
+Mergify applies `awaiting-author` automatically when a reviewer submits a
+`CHANGES_REQUESTED` review. No manual step is needed.
+
+If the author remains inactive after the label is applied:
+
+1. After five working days, a maintainer notes the inactivity in the PR
+   and may take over the branch, open a replacement PR, or remove a
+   dependency that review confirms is unnecessary.
+2. The decision is recorded in the PR before changing ownership or
+   dependency state.
+
+## Approval freshness
+
+When a contributor pushes a new commit to a dev-targeted PR, all existing
+approvals are dismissed automatically. `CHANGES_REQUESTED` reviews are NOT
+dismissed: a reviewer's objection survives the push and continues to block
+`#changes-requested-reviews-by=0` until the reviewer themselves re-reviews
+and clears it, or a maintainer manually dismisses it via GitHub (see
+inactive-reviewer process below).
+
+This means every approval in `merge_conditions` always reflects the code
+that will actually land on `dev`, and no blocking concern can be erased by
+a push followed by a third-party approval.
+
+If your PR gets rebased while waiting in the queue, expect your approval to
+be dismissed and the PR to return to "needs review" state before it can
+re-enter the queue.
+
+**For reviewers:** use inline conversation threads for every blocking
+concern, not just the review summary body. Inline threads survive approval
+dismissal and block independently via `#review-threads-unresolved=0`, so
+your concern is protected even if another reviewer later approves.
+
+## Inactive-reviewer process
+
+If a reviewer left `CHANGES_REQUESTED` and has not responded after the
+contributor pushed a fix:
+
+1. After three working days with no response, the contributor tags the
+   reviewer and a maintainer in the PR comments.
+2. If there is still no response after two more working days, a maintainer
+   reads the original review, verifies that the fix addresses the concern,
+   and manually dismisses the stale review via GitHub with a comment
+   recording what was checked and why the fix is accepted.
+3. The decision is recorded in the PR before any dismissal so the reasoning
+   is auditable.
+
+## GitHub branch protection
+
+Mergify does not replace or weaken GitHub branch protection. It operates
+on top of it. Any protection rules set directly in GitHub repository
+settings remain the source of truth and are enforced independently of
+Mergify.
+
+## Pausing the queue
+
+If the queue merges a PR that bypasses a protection or produces unexpected
+behavior, a maintainer pauses queue operation immediately and records the
+incident in issue #334 before resuming.