Fixup caching (#185)

Author: bleggett

.github/workflows/cache-refresh.yml

@@ -28,10 +28,13 @@ jobs:
         env:
           GH_TOKEN: ${{ github.token }}
         run: |
-          # List all ccache-* cache keys, strip the run_id suffix to deduplicate by flavor/arch.
+          # List main-scoped ccache-* cache keys, strip the run_id suffix to deduplicate
+          # by flavor/arch. We specifically skip non-main branches, becuse we only want to save/restore
+          # from main - in GH, PR branches can use `main` caches, but main cannot see PR branch caches,
+          # and saving duplicated PR branch caches counts against our 10GB github limit
           matrix=$(gh api "/repos/${{ github.repository }}/actions/caches" --paginate \
             --jq '[.actions_caches[]
-                  | select(.key | startswith("ccache-"))
+                  | select(.ref == "refs/heads/main" and (.key | startswith("ccache-")))
                   | {prefix: (.key | gsub("-[0-9]+$"; ""))}]
                   | unique_by(.prefix)
                   | {entry: .}')

.github/workflows/matrix.yml

@@ -123,6 +123,11 @@ jobs:
           compression-level: 0
           retention-days: 1
       - name: save ccache
+        # Only save from main. GH's per-repo 10GB cache pool is shared across all refs,
+        # so PR-scoped saves evict main-scoped entries via LRU - and main is the only ref
+        # whose entries the next build can actually restore. PRs still benefit because
+        # they will fallthrough to/inherit parent branch caches
+        if: github.ref == 'refs/heads/main'
         uses: actions/cache/save@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.2
         with:
           path: ~/.cache/kernel-ccache