ENH: Optimize GitHub Actions ccache strategy for faster C++ builds

Key changes from upstream:

1. Remove CCACHE_NODIRECT=1: Enables direct mode so ccache skips the
   preprocessor for cache lookups, significantly faster on hits.

2. Add CCACHE_SLOPPINESS=pch_defines,time_macros: Avoids cache misses
   caused by __DATE__/__TIME__ macros that change every build. Standard
   CI-safe setting.

3. CCACHE_MAXSIZE 2.4G -> 5G: ITK has ~4000 translation units; a
   larger cache retains more objects across incremental builds.

4. SHA-based cache key with restore-keys fallback:
   - key: ccache-v4-<os>-<config>-<sha>  (unique per commit)
   - restore-keys: ccache-v4-<os>-<config>-  (most recent match)
   Each build creates a new cache entry; restore-keys always finds
   the most recent cache for the same OS and configuration.
   Replaces the old static key that could never be updated once written.

5. Save on !cancelled() instead of main-only: PR builds now persist
   their cache for subsequent pushes, but cancelled runs do not save
   potentially incomplete caches.

6. Use runner.os (Linux/macOS/Windows) instead of matrix.os
   (ubuntu-24.04-arm/macos-15) for cache key consistency across
   runner image updates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hjmjohnson

.github/workflows/arm.yml

@@ -94,9 +94,9 @@ jobs:
           echo "CCACHE_BASEDIR=${GITHUB_WORKSPACE}" >> "$GITHUB_ENV"
           echo "CCACHE_COMPILERCHECK=content" >> "$GITHUB_ENV"
           echo "CCACHE_NOHASHDIR=true" >> "$GITHUB_ENV"
-          echo "CCACHE_NODIRECT=1" >> "$GITHUB_ENV"
+          echo "CCACHE_SLOPPINESS=pch_defines,time_macros" >> "$GITHUB_ENV"
           echo "CCACHE_DIR=${{ runner.temp }}/ccache" >> "$GITHUB_ENV"
-          echo "CCACHE_MAXSIZE=2.4G" >> "$GITHUB_ENV"
+          echo "CCACHE_MAXSIZE=5G" >> "$GITHUB_ENV"
           if [ "$RUNNER_OS" == "Linux" ]; then
             sudo apt-get update -qq && sudo apt-get install -y ccache locales
             sudo locale-gen de_DE.UTF-8
@@ -109,7 +109,9 @@ jobs:
         uses: actions/cache/restore@v5
         with:
           path: ${{ runner.temp }}/ccache
-          key: ccache-v1-${{ matrix.os }}-${{ matrix.name }}
+          key: ccache-v4-${{ runner.os }}-${{ matrix.name }}-${{ github.sha }}
+          restore-keys: |
+            ccache-v4-${{ runner.os }}-${{ matrix.name }}-
 
       - name: Show ccache configuration, stats and maintenance
         shell: bash
@@ -170,11 +172,11 @@ jobs:
         env:
           CTEST_OUTPUT_ON_FAILURE: 1
       - name: Save compiler cache
-        if: github.ref == 'refs/heads/main'
+        if: ${{ !cancelled() }}
         uses: actions/cache/save@v5
         with:
           path: ${{ runner.temp }}/ccache
-          key: ccache-v1-${{ matrix.os }}-${{ matrix.name }}
+          key: ccache-v4-${{ runner.os }}-${{ matrix.name }}-${{ github.sha }}
 
       - name: ccache stats
         if: always()

.github/workflows/pixi.yml

@@ -53,9 +53,9 @@ jobs:
             echo "CCACHE_BASEDIR=${GITHUB_WORKSPACE}" >> "$GITHUB_ENV"
             echo "CCACHE_COMPILERCHECK=content" >> "$GITHUB_ENV"
             echo "CCACHE_NOHASHDIR=true" >> "$GITHUB_ENV"
-            echo "CCACHE_NODIRECT=1" >> "$GITHUB_ENV"
+            echo "CCACHE_SLOPPINESS=pch_defines,time_macros" >> "$GITHUB_ENV"
             echo "CCACHE_DIR=${{ runner.temp }}/ccache" >> "$GITHUB_ENV"
-            echo "CCACHE_MAXSIZE=2.4G" >> "$GITHUB_ENV"
+            echo "CCACHE_MAXSIZE=5G" >> "$GITHUB_ENV"
             if [ "$RUNNER_OS" == "Linux" ]; then
               sudo apt-get update -qq && sudo apt-get install -y ccache locales
               sudo locale-gen de_DE.UTF-8
@@ -70,7 +70,9 @@ jobs:
           uses: actions/cache/restore@v5
           with:
             path: ${{ runner.temp }}/ccache
-            key: ccache-v1-${{ matrix.os }}-pixi-cxx
+            key: ccache-v4-${{ runner.os }}-pixi-cxx-${{ github.sha }}
+            restore-keys: |
+              ccache-v4-${{ runner.os }}-pixi-cxx-
 
         - name: Show ccache configuration, stats and maintenance
           shell: bash
@@ -150,11 +152,11 @@ jobs:
             echo "****** df -h /"
             df -h /
         - name: Save compiler cache
-          if: github.ref == 'refs/heads/main'
+          if: ${{ !cancelled() }}
           uses: actions/cache/save@v5
           with:
             path: ${{ runner.temp }}/ccache
-            key: ccache-v1-${{ matrix.os }}-pixi-cxx
+            key: ccache-v4-${{ runner.os }}-pixi-cxx-${{ github.sha }}
 
         - name: ccache stats
           if: always()