Replace mergify with GHAs (#3346)

Replacing `mergify` that gets hard to maintain with GitHub actions with
features:
1. automerge for PRs
* `automerge` label is attached if and only if the PR author is
`softwaremill-ci` and exactly one file from the specified list is
changed,
* auto-merge action is performed if and only if there is an `automerge`
label attached and `ci` and `mima` jobs complete successfully.
2. `/rebase` as PR comment to rebase it.

.github/labeler.yml

@@ -0,0 +1,14 @@
+version: 1
+labels:
+  - label: "automerge"
+    authors: ["softwaremill-ci"]
+    files:
+      - "build.sbt"
+      - "project/Versions.scala"
+      - "project/plugins.sbt"
+  - label: "dependency"
+    authors: ["softwaremill-ci"]
+    files:
+      - "build.sbt"
+      - "project/Versions.scala"
+      - "project/plugins.sbt"

.github/workflows/ci.yml

@@ -1,7 +1,6 @@
 name: CI
 on:
   pull_request:
-    branches: ['**']
   push:
     branches: ['**']
     tags: [v*]
@@ -10,8 +9,12 @@ env:
   SBT_JAVA_OPTS: -J-Xms4g -J-Xmx4g
 jobs:
   ci:
-    # run on external PRs, but not on internal PRs since those will be run by push to branch
-    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+    # run on 1) push, 2) external PRs, 3) softwaremill-ci PRs
+    # do not run on internal, non-steward PRs since those will be run by push to branch
+    if: |
+      github.event_name == 'push' || 
+      github.event.pull_request.head.repo.full_name != github.repository ||
+      github.event.pull_request.user.login == 'softwaremill-ci'
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
@@ -102,9 +105,14 @@ jobs:
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+  # identify binary incompatibilities (check build.sbt for details)
   mima:
-    # run on external PRs, but not on internal PRs since those will be run by push to branch
-    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+    # run on 1) push, 2) external PRs, 3) softwaremill-ci PRs
+    # do not run on internal, non-steward PRs since those will be run by push to branch
+    if: |
+      github.event_name == 'push' || 
+      github.event.pull_request.head.repo.full_name != github.repository ||
+      github.event.pull_request.user.login == 'softwaremill-ci'
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
@@ -175,3 +183,41 @@ jobs:
           version: "v${{ env.VERSION }}"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+
+  # `automerge` label is attached iff there is exactly one file changed by steward and this file belongs to a
+  # whitelist specified by `labeler.yml`
+  label:
+    name: Attach automerge label
+    # only for PRs by softwaremill-ci
+    if: github.event.pull_request.user.login == 'softwaremill-ci'
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 2
+      # count number of files changed
+      - name: Count number of files changed
+        id: count-changed-files
+        run: |
+               N=$(git diff --name-only -r HEAD^1 HEAD | wc -w)
+               echo "changed_files_num=$N" >> $GITHUB_OUTPUT
+      - name: Launch labeler
+        # skip if more than one file changed
+        if: steps.count-changed-files.outputs.changed_files_num == 1
+        uses: srvaroa/labeler@master
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+
+  auto-merge:
+    name: Auto merge
+    # only for PRs by softwaremill-ci
+    if: github.event.pull_request.user.login == 'softwaremill-ci'
+    needs: [ ci, mima, label ]
+    runs-on: ubuntu-22.04
+    steps:
+      - id: automerge
+        name: automerge
+        uses: "pascalgn/[email protected]"
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"

.github/workflows/rebase-cmd-dispatch.yml

@@ -0,0 +1,19 @@
+# On any comment, it will look for '/rebase' in the comment body and in case of hit, it dispatches rebase cmd
+# with event type 'rebase-command' which triggers 'rebase-command` WF that performs the rebase operation.
+name: Slash Command Dispatch
+on:
+  issue_comment:
+    types: [created]
+jobs:
+  slashCommandDispatch:
+    # to fast-skip for typical, non-rebase comments
+    if: contains(github.event.comment.body, '/rebase')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Slash Command Dispatch
+        uses: peter-evans/slash-command-dispatch@v3
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          commands: rebase
+          permission: write
+          issue-type: pull-request

.github/workflows/rebase-cmd.yml

@@ -0,0 +1,20 @@
+name: rebase-command
+on:
+  repository_dispatch:
+    types: [rebase-command]
+jobs:
+  rebase:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: peter-evans/rebase@v2
+        id: rebase
+        with:
+          head: ${{ github.event.client_payload.pull_request.head.label }}
+      - name: Add reaction
+        if: steps.rebase.outputs.rebased-count == 1
+        uses: peter-evans/create-or-update-comment@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          repository: ${{ github.event.client_payload.github.payload.repository.full_name }}
+          comment-id: ${{ github.event.client_payload.github.payload.comment.id }}
+          reaction-type: hooray