⚙️ config(ci): Factor out custom github/event action.

.github/actions/github/event/action.yml

@@ -0,0 +1,54 @@
+name: github/event
+description: >
+  Fetch generic info about GitHub event.
+
+outputs:
+
+  merge-base:
+    description: >
+      The merge base for this event.
+    value: ${{ steps.merge-base.outputs.sha }}
+
+runs:
+
+  using: composite
+
+  steps:
+
+    - name: Compute BASE_SHA and HEAD_SHA for push event 📜
+      if: github.event_name == 'push'
+      shell: bash
+      run: |
+        BASE_SHA="${{ github.event.before }}"
+        echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
+        HEAD_SHA="${{ github.event.after }}"
+        echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
+
+    - name: Compute BASE_SHA and HEAD_SHA for pull_request event 📜
+      if: github.event_name == 'pull_request'
+      shell: bash
+      run: |
+        BASE_SHA="${{ github.event.pull_request.base.sha }}"
+        echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
+        HEAD_SHA="${{ github.event.pull_request.head.sha }}"
+        echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
+
+    - name: Compute BASE_SHA and HEAD_SHA for merge_group event 📜
+      if: github.event_name == 'merge_group'
+      shell: bash
+      run: |
+        BASE_SHA="${{ github.event.merge_group.base_sha }}"
+        echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
+        HEAD_SHA="${{ github.event.merge_group.head_sha }}"
+        echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
+
+    - name: Compute MERGE_BASE 🌱
+      id: merge-base
+      env:
+        BASE_SHA: ${{ env.BASE_SHA }}
+        HEAD_SHA: ${{ env.HEAD_SHA }}
+      shell: bash
+      run: |
+        FIRST_NEW_COMMIT="$(git log "${BASE_SHA}..${HEAD_SHA}" --pretty=format:%H | tail -1)"
+        MERGE_BASE="$(git rev-parse "${FIRST_NEW_COMMIT}~")"
+        echo "sha=${MERGE_BASE}" >> "${GITHUB_OUTPUT}"

.github/workflows/ci:commit.yml

@@ -29,45 +29,14 @@ jobs:
           # See https://stackoverflow.com/a/76573878
           fetch-depth: 0
 
-      # TODO Make this a reusable action
-      - name: Compute BASE_SHA and HEAD_SHA for push event 📜
-        if: github.event_name == 'push'
-        run: |
-          BASE_SHA="${{ github.event.before }}"
-          echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
-          HEAD_SHA="${{ github.event.after }}"
-          echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
-
-      - name: Compute BASE_SHA and HEAD_SHA for pull_request event 📜
-        if: github.event_name == 'pull_request'
-        run: |
-          BASE_SHA="${{ github.event.pull_request.base.sha }}"
-          echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
-          HEAD_SHA="${{ github.event.pull_request.head.sha }}"
-          echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
-
-      - name: Compute BASE_SHA and HEAD_SHA for merge_group event 📜
-        if: github.event_name == 'merge_group'
-        run: |
-          BASE_SHA="${{ github.event.merge_group.base_sha }}"
-          echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
-          HEAD_SHA="${{ github.event.merge_group.head_sha }}"
-          echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
-
-      - name: Compute MERGE_BASE 🌱
-        id: history
-        env:
-          BASE_SHA: ${{ env.BASE_SHA }}
-          HEAD_SHA: ${{ env.HEAD_SHA }}
-        run: |
-          FIRST_NEW_COMMIT="$(git log "${BASE_SHA}..${HEAD_SHA}" --pretty=format:%H | tail -1)"
-          MERGE_BASE="$(git rev-parse "${FIRST_NEW_COMMIT}~")"
-          echo "merge-base=${MERGE_BASE}" >> "${GITHUB_OUTPUT}"
+      - name: Compute BASE_SHA, HEAD_SHA, and MERGE_BASE 📜
+        id: event
+        uses: .github/actions/github/event
 
       - name: Stage MERGE_BASE...HEAD_SHA ⏮️
         env:
           HEAD_SHA: ${{ env.HEAD_SHA }}
-          MERGE_BASE: ${{ steps.history.outputs.merge-base }}
+          MERGE_BASE: ${{ steps.event.outputs.merge-base }}
         run: |
           git reset --hard "${HEAD_SHA}"
           git reset --soft "${MERGE_BASE}"

.github/workflows/ci:precommit.yml

@@ -29,45 +29,14 @@ jobs:
           # See https://stackoverflow.com/a/76573878
           fetch-depth: 0
 
-      # TODO Make this a reusable action
-      - name: Compute BASE_SHA and HEAD_SHA for push event 📜
-        if: github.event_name == 'push'
-        run: |
-          BASE_SHA="${{ github.event.before }}"
-          echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
-          HEAD_SHA="${{ github.event.after }}"
-          echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
-
-      - name: Compute BASE_SHA and HEAD_SHA for pull_request event 📜
-        if: github.event_name == 'pull_request'
-        run: |
-          BASE_SHA="${{ github.event.pull_request.base.sha }}"
-          echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
-          HEAD_SHA="${{ github.event.pull_request.head.sha }}"
-          echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
-
-      - name: Compute BASE_SHA and HEAD_SHA for merge_group event 📜
-        if: github.event_name == 'merge_group'
-        run: |
-          BASE_SHA="${{ github.event.merge_group.base_sha }}"
-          echo "BASE_SHA=${BASE_SHA}" >> "${GITHUB_ENV}"
-          HEAD_SHA="${{ github.event.merge_group.head_sha }}"
-          echo "HEAD_SHA=${HEAD_SHA}" >> "${GITHUB_ENV}"
-
-      - name: Compute MERGE_BASE 🌱
-        id: history
-        env:
-          BASE_SHA: ${{ env.BASE_SHA }}
-          HEAD_SHA: ${{ env.HEAD_SHA }}
-        run: |
-          FIRST_NEW_COMMIT="$(git log "${BASE_SHA}..${HEAD_SHA}" --pretty=format:%H | tail -1)"
-          MERGE_BASE="$(git rev-list -n1 "${FIRST_NEW_COMMIT}~")"
-          echo "merge-base=${MERGE_BASE}" >> "${GITHUB_OUTPUT}"
+      - name: Compute BASE_SHA, HEAD_SHA, and MERGE_BASE 📜
+        id: event
+        uses: .github/actions/github/event
 
       - name: Stage MERGE_BASE...HEAD_SHA ⏮️
         env:
           HEAD_SHA: ${{ env.HEAD_SHA }}
-          MERGE_BASE: ${{ steps.history.outputs.merge-base }}
+          MERGE_BASE: ${{ steps.event.outputs.merge-base }}
         run: |
           git reset --hard "${HEAD_SHA}"
           git reset --soft "${MERGE_BASE}"