Add sccache to CI builds

.github/workflows/ci.yml

@@ -7,6 +7,11 @@ on:
   pull_request:
     branches: [ master ]
   workflow_dispatch:
+    inputs:
+      sccache:
+        description: Use the sccache compilation cache
+        type: boolean
+        default: true
 
 permissions:
   contents: read
@@ -21,13 +26,16 @@ jobs:
       AEGISUB_BUILD_BRANCH: ${{ github.head_ref || github.ref_name }}
       # Same condition as windows-sign-executable
       AEGISUB_SIGN_WINDOWS: ${{ matrix.config.os == 'windows-latest' && (startsWith(github.ref, 'refs/tags/v') || github.ref == 'refs/heads/ci/windows-signing') }}
+      SCCACHE_GHA_ENABLED: "true"
+      SCCACHE_C_CUSTOM_CACHE_BUSTER: ${{ matrix.config['sccache-key'] }}
 
     strategy:
       fail-fast: false
       matrix:
         config:
           - name: Windows MSVC Release
             os: windows-latest
+            sccache-key: windows-x86_64-msvc
             msvc: true
             devenv: |
               Import-Module "$env:VS\Common7\Tools\Microsoft.VisualStudio.DevShell.dll"
@@ -49,17 +57,22 @@ jobs:
               -Dlibass:fontconfig=disabled
           - name: Ubuntu Debug
             os: ubuntu-latest
+            sccache-key: linux-x86_64-gcc
             buildtype: debugoptimized
             args: -Db_pch=false
           - name: Ubuntu Release
             os: ubuntu-latest
+            sccache-key: linux-x86_64-gcc
             buildtype: release
             args: -Db_pch=false
           - name: macOS x86_64 Release
             os: macos-15
-            cross_args: --cross-file tools/macos-release.ini --cross-file tools/macos-x86_64.ini
+            sccache-key: macos-x86_64-clang
+            cross_args: --cross-file tools/macos-release.ini --cross-file tools/macos-x86_64.ini --cross-file tools/sccache.ini
             buildtype: release
+            # sccache serves clang PCHs that fail mtime validation on a fresh checkout
             args: &macos_source_args >-
+              -Db_pch=false
               -Ddefault_library=static
               -Dbuild_osx_bundle=true
               -Dfftw3=disabled
@@ -88,15 +101,18 @@ jobs:
               --force-fallback-for=boost,wxWidgets,libass,zlib,freetype2,fribidi,harfbuzz,ffms2,libavformat,libavcodec,libswscale,libavutil,libswresample,dav1d,hunspell,uchardet,icu-uc,icu-i18n,libcurl
           - name: macOS arm64 Debug
             os: macos-15
+            sccache-key: macos-arm64-clang
             buildtype: debugoptimized
             args: >-
+              -Db_pch=false
               -Dmacos_deployment_target=15.0
               -Dbuild_osx_bundle=false
               -Dfftw3=enabled
               -Dlibpulse=disabled
               -Dopenal=enabled
           - name: macOS arm64 Release
             os: macos-15
+            sccache-key: macos-arm64-clang
             native_args: --native-file tools/macos-release.ini
             buildtype: release
             args: *macos_source_args
@@ -110,6 +126,11 @@ jobs:
         with:
           python-version: '3.x'
 
+      # Release builds compile from scratch; manual runs can opt out too
+      - name: Set up sccache
+        if: github.ref_type != 'tag' && (github.event_name != 'workflow_dispatch' || inputs.sccache)
+        uses: mozilla-actions/sccache-action@v0.0.11
+
       - name: Setup Meson
         run: |
           python -m pip install --upgrade pip setuptools

tools/macos-x86_64.ini

@@ -1,10 +1,14 @@
 # Build Intel binaries with native Apple Silicon tools; run tests via Rosetta.
 # Combine with macos-release.ini for target dependency search paths.
+[constants]
+# Overridden by tools/sccache.ini when layered after this file
+compiler_cache = []
+
 [binaries]
-c = ['/usr/bin/clang', '-arch', 'x86_64']
-cpp = ['/usr/bin/clang++', '-arch', 'x86_64']
-objc = ['/usr/bin/clang', '-arch', 'x86_64']
-objcpp = ['/usr/bin/clang++', '-arch', 'x86_64']
+c = compiler_cache + ['/usr/bin/clang', '-arch', 'x86_64']
+cpp = compiler_cache + ['/usr/bin/clang++', '-arch', 'x86_64']
+objc = compiler_cache + ['/usr/bin/clang', '-arch', 'x86_64']
+objcpp = compiler_cache + ['/usr/bin/clang++', '-arch', 'x86_64']
 ar = '/usr/bin/ar'
 strip = '/usr/bin/strip'
 pkg-config = 'pkg-config'

tools/sccache.ini

@@ -0,0 +1,5 @@
+# Layer after a cross file that defines compiler_cache (e.g. macos-x86_64.ini)
+# to compile through sccache. Meson only adds sccache automatically for
+# compilers it finds itself, not ones listed in machine files.
+[constants]
+compiler_cache = ['sccache']