persist fuzz corpus between CI runs

Author: Anyitechs

.github/workflows/build.yml

@@ -267,6 +267,27 @@ jobs:
       - name: Install Rust 1.80 toolchain
         run: |
           curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile=minimal --default-toolchain 1.80
+      # This is read-only for PRs. It seeds the fuzzer for a more effective run.
+      # NOTE: The `key` is unique and will always miss, forcing a fallback to
+      # the `restore-keys` to find the latest global cache from the `main` branch.
+      - name: Restore persistent fuzz corpus (PR)
+        if: ${{ github.ref != 'refs/heads/main' }}
+        uses: actions/cache/restore@v4
+        with:
+          path: fuzz/hfuzz_workspace
+          key: fuzz-corpus-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            fuzz-corpus-refs/heads/main-
+      # Only on the `main` branch, restores the latest corpus and also save a
+      # new, updated one.
+      - name: Restore/Save persistent honggfuzz corpus (Main)
+        if: ${{ github.ref == 'refs/heads/main' }}
+        uses: actions/cache@v4
+        with:
+          path: fuzz/hfuzz_workspace
+          key: fuzz-corpus-refs/heads/main-${{ github.sha }}
+          restore-keys: |
+            fuzz-corpus-refs/heads/main-
       - name: Sanity check fuzz targets on Rust 1.80
         run: |
           cd fuzz