ci: Effective cache sharing between PRs

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,17 +1,20 @@
-**Description**
+# Description
+
 Include a summary of the change and which issue is fixed. Please also include
 relevant motivation and context. List any dependencies that are required for
 this change.
 
 Fixes # (issue)
 If this is a hotfix to a released version, please specify it.
 
-**How Has This Been Tested?**
+## How has this been tested?
+
 Please describe the tests that you ran to verify your changes. Please also note
 any relevant details for your test configuration (e.g. compiler, OS).  Include
 enough information so someone can reproduce your tests.
 
-**Checklist:**
+## Checklist
+
 - [ ] My code follows the style guidelines of this project
 - [ ] I have performed a self-review of my own code
 - [ ] I have commented my code, particularly in hard-to-understand areas

.github/workflows/create-cache.yaml

@@ -0,0 +1,44 @@
+name: "Create GHA cache"
+
+# GitHub puts the following restrictions on cache sharing. PRs can access
+#
+# - caches that were created by the PR / earlier runs of the PR
+# - caches that were created on the target branch
+#
+# To get effective cache sharing between PRs, we create caches on the `develop`
+# branch (which is where almost all PRs merge into).
+
+on:
+  push:
+    branches: [develop]
+
+# Cancel running jobs if there's a newer push
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  # GitHub Actions cache of the pre-commit environment
+  pre-commit:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Setup Python 3.11
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - uses: actions/cache@v4
+        id: cache
+        with:
+          path: ~/.cache/pre-commit
+          key: pre-commit_${{ env.pythonLocation }}_${{ hashFiles('.pre-commit-config.yaml') }}
+          lookup-only: true  # don't actually download the cache
+
+      - name: Populate pre-commit environment (if not cached)
+        if: steps.cache.outputs.cache-hit != 'true'
+        run: |
+          pip install pre-commit
+          pre-commit install --install-hooks

.github/workflows/lint.yaml

@@ -25,7 +25,9 @@ jobs:
         with:
           python-version: '3.11'
 
-      - uses: actions/cache@v4
+      # Only restore (don't save) caches on PRs. New caches created from PRs won't be
+      # accessible from other PRs, see workflows/create-cache.yaml.
+      - uses: actions/cache/restore@v4
         with:
           path: ~/.cache/pre-commit
           key: pre-commit_${{ env.pythonLocation }}_${{ hashFiles('.pre-commit-config.yaml') }}