Allow submodule cache to persist across runs

Although it only takes a few minutes to do the shallow clones, and we
don't trigger runs that frequently, let's keep the submodule cache
around, and invalidate it based on the hash of the 'git submodules'
command, instead on each run. This will save us space on the repo's
cache storage (10GB) and speed up the process a bit.

Signed-off-by: Nick Kossifidis <[email protected]>

.github/workflows/build.yaml

@@ -29,28 +29,39 @@ jobs:
   submodule_cache:
     name: Initialize submodule cache
     runs-on: ubuntu-latest
+    outputs:
+      key: ${{ steps.keygen.outputs.smcache_key }}
     steps:
       - uses: actions/checkout@v4
 
       - name: Remove unneeded frameworks to recover disk space
         run: sudo ./.github/cleanup-rootfs.sh
 
+      - name: Generate submodule cache key
+        id: keygen
+        run: echo "smcache_key=smcache-$(printf $(git submodule | sha1sum))" >> $GITHUB_OUTPUT
+
+      - name: Setup submodule cache
+        id: smcache
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.submodule_paths }}
+          key: ${{ steps.keygen.outputs.smcache_key }}
+
       - name: Checkout required submodules
+        if: steps.smcache.outputs.cache-hit != 'true'
         run: git submodule update --init -j $(nproc) --depth 1 $(echo ${submodule_paths} | sed '$d' | tr '\n' ' ')
 
       - name: Storage size optimization
+        if: steps.smcache.outputs.cache-hit != 'true'
         run: |
           git submodule foreach 'git maintenance run'
 
-      - name: Setup submodule cache
-        uses: actions/cache@v4
-        with:
-          path: ${{ env.submodule_paths }}
-          key: submodule-cache-${{ github.run_id }}-${{ github.run_attempt }}
-
   build:
     runs-on: ${{ matrix.os }}
     needs: [submodule_cache]
+    env:
+      smcache_key: ${{ needs.submodule_cache.outputs.key }}
     strategy:
       matrix:
         os:     [ubuntu-22.04, ubuntu-24.04]
@@ -75,7 +86,7 @@ jobs:
         uses: actions/cache/restore@v4
         with:
           path: ${{ env.submodule_paths }}
-          key: submodule-cache-${{ github.run_id }}-${{ github.run_attempt }}
+          key: ${{ env.smcache_key }}
 
       - name: build toolchain
         run: |
@@ -134,6 +145,8 @@ jobs:
   test-sim:
     runs-on: ${{ matrix.os }}
     needs: [submodule_cache]
+    env:
+      smcache_key: ${{ needs.submodule_cache.outputs.key }}
     strategy:
       matrix:
         os:     [ubuntu-24.04]
@@ -153,7 +166,7 @@ jobs:
         uses: actions/cache/restore@v4
         with:
           path: ${{ env.submodule_paths }}
-          key: submodule-cache-${{ github.run_id }}-${{ github.run_attempt }}
+          key: ${{ env.smcache_key }}
 
       - name: build toolchain
         run: |
@@ -170,6 +183,8 @@ jobs:
     if: ${{ false }} # Disable until multilib errors are triaged
     runs-on: ${{ matrix.os }}
     needs: [submodule_cache]
+    env:
+      smcache_key: ${{ needs.submodule_cache.outputs.key }}
     strategy:
       matrix:
         os:     [ubuntu-24.04]
@@ -188,7 +203,7 @@ jobs:
         uses: actions/cache/restore@v4
         with:
           path: ${{ env.submodule_paths }}
-          key: submodule-cache-${{ github.run_id }}-${{ github.run_attempt }}
+          key: ${{ env.smcache_key }}
 
       - name: build toolchain
         run: |