ci: Only save caches on main and release branches

This restores caches on all branches and pull requests, but only stores
them on the main branch and release branches. This prevents those caches
from being evicted early when we hit the 10 GB limit quickly.

.github/actions/cache-on-main/action.yaml

@@ -0,0 +1,31 @@
+name: Cache on main
+
+description: Stores caches on main only, but restores them on all branches.
+
+inputs:
+  path:
+    description: Path(s) to cache
+    required: true
+  prefix:
+    description: Cache key prefix to be used in both primary key and restore-keys.
+    required: true
+  suffix:
+    description: Cache key suffix to be used only in primary key.
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
+      if: ${{ startsWith(github.ref, 'refs/heads/') }}
+      with:
+        path: ${{ inputs.path }}
+        key: ${{ runner.os }}-${{ inputs.prefix }}-${{ inputs.suffix }}
+        restore-keys: |
+          ${{ runner.os }}-${{ inputs.prefix }}-
+    - uses: actions/cache/restore@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
+      if: ${{ !startsWith(github.ref, 'refs/heads/') }}
+      with:
+        path: ${{ inputs.path }}
+        key: ${{ runner.os }}-${{ inputs.prefix }}-${{ inputs.suffix }}
+        restore-keys: |
+          ${{ runner.os }}-${{ inputs.prefix }}-

.github/workflows/build.yaml

@@ -115,12 +115,11 @@ jobs:
     steps:
       - uses: actions/checkout@a5ac7e51b41094c92402da3b24376905380afc29 # v4.1.6
       - name: Stack working files cache
-        uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
+        uses: ./.github/actions/cache-on-main
         with:
           path: ${{ matrix.cache }}
-          key: stack-${{ runner.os }}-${{ hashFiles('stack.yaml.lock') }}
-          restore-keys: |
-            stack-${{ runner.os }}-
+          prefix: stack
+          suffix: ${{ hashFiles('stack.yaml.lock') }}
       - name: Install dependencies
         if: matrix.deps
         run: ${{ matrix.deps }}
@@ -171,15 +170,14 @@ jobs:
         run: |
           mkdir ~/.cabal
       - name: Cache
-        uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
+        uses: ./.github/actions/cache-on-main
         with:
           path: |
             ~/.cabal/packages
             ~/.cabal/store
             dist-newstyle
-          key: cabal-${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('postgrest.cabal', 'cabal.project', 'cabal.project.freeze') }}
-          restore-keys: |
-            cabal-${{ runner.os }}-${{ matrix.ghc }}-
+          prefix: cabal-${{ matrix.ghc }}
+          suffix: ${{ hashFiles('postgrest.cabal', 'cabal.project', 'cabal.project.freeze') }}
       - name: Install dependencies
         run: |
           cabal update