Factor out incremental build prep into an action

.github/workflows/kernel-build.yml

@@ -49,6 +49,11 @@ jobs:
         REPO_ROOT: ${{ github.workspace }}
         REPO_PATH: ""
         KBUILD_OUTPUT: kbuild-output/
+        BASE_BRANCH: >-
+          ${{ github.event_name == 'push' && github.ref_name
+              || github.base_ref
+              || 'bpf-next_base'
+          }}
     steps:
       - uses: actions/checkout@v4
         # We fetch an actual bit of history here to facilitate incremental
@@ -67,23 +72,12 @@ jobs:
           rm -rf .kernel/.git
           cp -rf .kernel/. .
           rm -rf .kernel
-      - name: Get commit meta-data
-        id: get-commit-metadata
-        run: |
-          bash .github/scripts/get-commit-metadata.sh
-      - name: Pull recent KBUILD_OUTPUT contents
-        uses: actions/cache@v4
+      - uses: ./prepare-incremental-build
         with:
-          path: ${{ env.KBUILD_OUTPUT }}
-          key: kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-${{ steps.get-commit-metadata.outputs.branch }}-${{ steps.get-commit-metadata.outputs.timestamp }}-${{ steps.get-commit-metadata.outputs.commit }}
-          restore-keys: |
-            kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-${{ steps.get-commit-metadata.outputs.branch }}-${{ steps.get-commit-metadata.outputs.timestamp }}-
-            kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-${{ steps.get-commit-metadata.outputs.branch }}-
-            kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-
-      - name: Prepare incremental build
-        shell: bash
-        run: |
-          bash .github/scripts/prepare-incremental-builds.sh ${{ steps.get-commit-metadata.outputs.commit }}
+          repo-root: ${{ env.REPO_ROOT }}
+          base-branch: ${{ env.BASE_BRANCH }}
+          arch: ${{ inputs.arch }}
+          toolchain_full: ${{ inputs.toolchain_full }}
       - uses:  ./patch-kernel
         with:
           repo-root: '${{ github.workspace }}'

prepare-incremental-build/action.yml

@@ -0,0 +1,52 @@
+name: 'Prepare incremental kernel build'
+description: 'Pull cached kernel build output from previous runs and prepare the repo for incremental build'
+inputs:
+  repo-root:
+    description: "Root of the kernel repository"
+    required: true
+    default: ${{ github.workspace }}
+  base-branch:
+    description: "Base branch for cache lookup"
+    required: true
+    default: bpf-next_base
+  arch:
+    required: true
+    type: string
+    description: "Part of cache lookup key"
+  toolchain_full:
+    required: true
+    type: string
+    description: "Part of cache lookup key"
+  kbuild-output:
+    required: true
+    type: string
+    description: "Path to KBUILD_OUTPUT"
+    default: kbuild-output
+
+runs:
+  using: "composite"
+  steps:
+
+    - name: Get commit meta-data for cache lookup
+      id: get-commit-metadata
+      shell: bash
+      working-directory: ${{ inputs.repo-root }}
+      run: ${GITHUB_ACTION_PATH}/get-commit-metadata.sh ${{ inputs.base-branch }}
+
+    - name: Pull recent KBUILD_OUTPUT contents
+      uses: actions/cache@v4
+      with:
+        path: ${{ inputs.kbuild-output }}
+        key: kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-${{ steps.get-commit-metadata.outputs.branch }}-${{ steps.get-commit-metadata.outputs.timestamp }}-${{ steps.get-commit-metadata.outputs.commit }}
+        restore-keys: |
+          kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-${{ steps.get-commit-metadata.outputs.branch }}-${{ steps.get-commit-metadata.outputs.timestamp }}-
+          kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-${{ steps.get-commit-metadata.outputs.branch }}-
+          kbuild-output-${{ inputs.arch }}-${{ inputs.toolchain_full }}-
+
+    - name: Prepare incremental build
+      shell: bash
+      env:
+        KBUILD_OUTPUT: ${{ inputs.kbuild-output }}
+      working-directory: ${{ inputs.repo-root }}
+      run: ${GITHUB_ACTION_PATH}/prepare-incremental-builds.sh ${{ steps.get-commit-metadata.outputs.commit }}
+

prepare-incremental-build/get-commit-metadata.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# branch="${GITHUB_BASE_REF}"
+
+# if [ "${GITHUB_EVENT_NAME}" = 'push' ]; then
+#   branch="${GITHUB_REF_NAME}"
+# fi
+
+set -eux
+
+branch=${1:-bpf-next_base}
+
+echo "branch=${branch}" >> "${GITHUB_OUTPUT}"
+
+upstream="${branch//_base/}"
+
+git fetch --quiet --prune --no-tags --depth=1 --no-recurse-submodules \
+    origin "+refs/heads/${upstream}:refs/remotes/origin/${upstream}"
+commit=$(git rev-parse "origin/${upstream}")
+
+timestamp_utc="$(TZ=utc git show --format='%cd' --no-patch --date=iso-strict-local "${commit}")"
+
+echo "timestamp=${timestamp_utc}" >> "${GITHUB_OUTPUT}"
+echo "commit=${commit}" >> "${GITHUB_OUTPUT}"
+echo "Most recent upstream commit is ${commit}"
+