ci: use GitHub merge queue (#40130)

Requiring branches to be up to date reruns the full CI matrix on every
waiting PR after each merge, which saturates our 20 concurrent jobs. A
merge queue still tests against the latest `main`, but once per batch.

This runs the PR workflows on `merge_group` and reports `giteabot/lgtm`
on queue commits, since GiteaBot only sets it on PR heads. Mergers queue
PRs with "Merge when ready" instead of `reviewed/wait-merge`, so
giteabot no longer updates branches. Nothing changes until the
`merge-queue` ruleset is enabled and "Require branches to be up to date"
is turned off. Backport to `release/v28` before the ruleset covers it.

---------

Co-authored-by: silverwind <me@silverwind.io>

.github/workflows/giteabot-backport.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
-      - uses: go-gitea/giteabot@36ea99f821fefa3e8fb26aeac6211735664cd9c0 # v1.3.1
+      - uses: go-gitea/giteabot@b830f7aa810e43afc1b2f8e8968a30754e6ff5d3 # v1.4.1
         with:
           github_token: ${{ secrets.GITEABOT_TOKEN }}
           gitea_fork: giteabot/gitea

.github/workflows/giteabot.yml

@@ -1,16 +1,11 @@
 name: giteabot
 
 on:
-  # When main advances, rerun merge queue maintenance so the oldest
-  # reviewed/wait-merge PR can be updated against the new base promptly.
-  push:
-    branches:
-      - main
   # pull_request_target gives this workflow access to GITEABOT_TOKEN on PRs from
   # forks, which the bot needs to write labels, statuses and comments. Safe here
   # because the job only runs a pinned action and never checks out PR HEAD.
-  # These PR lifecycle events drive label maintenance, queue maintenance, and
-  # explicit bot actions triggered by relevant label changes.
+  # These PR lifecycle events drive label maintenance, lgtm, and explicit bot
+  # actions triggered by relevant label changes.
   pull_request_target: # zizmor: ignore[dangerous-triggers]
     types:
       - opened
@@ -26,8 +21,7 @@ on:
       - giteabot-review
     types:
       - requested
-  # Periodic maintenance is still useful as a backstop for queue cleanup and
-  # other housekeeping, even though main pushes now trigger it promptly.
+  # Daily housekeeping like locks, stale feedback issues, last calls and milestones
   schedule:
     - cron: "15 3 * * *"
   # Allow maintainers to rerun selected checks manually when debugging bot
@@ -37,7 +31,7 @@ on:
       checks:
         description: Comma-separated list of non-backport checks to run
         required: false
-        default: backport_branches,labels,merge_queue,lock,feedback,last_call,milestones,lgtm,translation_comment,pr_actions
+        default: backport_branches,labels,lock,feedback,last_call,milestones,lgtm,translation_comment,pr_actions
 
 permissions: {}
 
@@ -56,7 +50,7 @@ jobs:
       pull-requests: write
       statuses: write
     steps:
-      - uses: go-gitea/giteabot@36ea99f821fefa3e8fb26aeac6211735664cd9c0 # v1.3.1
+      - uses: go-gitea/giteabot@b830f7aa810e43afc1b2f8e8968a30754e6ff5d3 # v1.4.1
         with:
           github_token: ${{ secrets.GITEABOT_TOKEN }}
-          checks: ${{ github.event.inputs.checks || 'backport_branches,labels,merge_queue,lock,feedback,last_call,milestones,lgtm,translation_comment,pr_actions' }}
+          checks: ${{ github.event.inputs.checks || 'backport_branches,labels,lock,feedback,last_call,milestones,lgtm,translation_comment,pr_actions' }}

.github/workflows/merge-group.yml

@@ -0,0 +1,16 @@
+name: merge-group
+
+# Reports the required giteabot/lgtm check on merge queue commits. giteabot only sets it
+# on PR heads, which already passed it before entering the queue.
+
+on:
+  merge_group:
+
+permissions: {}
+
+jobs:
+  lgtm:
+    name: giteabot/lgtm
+    runs-on: ubuntu-latest
+    steps:
+      - run: "true"

.github/workflows/pull-compliance.yml

@@ -2,6 +2,7 @@ name: compliance
 
 on:
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}

.github/workflows/pull-db-tests.yml

@@ -2,6 +2,7 @@ name: db-tests
 
 on:
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}

.github/workflows/pull-e2e-tests.yml

@@ -2,6 +2,7 @@ name: e2e-tests
 
 on:
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}