CI: give Windows a persistent Cargo target and enable sccache

.github/workflows/ci.yml

@@ -106,8 +106,19 @@ jobs:
         name: Set Cargo target directory
         shell: bash
         run: |
+          # The self-hosted Windows runner root, matching $RunnerDir in
+          # clockworklabs/infra runner-manager.sh.j2 and the warmed layout
+          # created by warm-cache-windows.ps1.
+          windows_runner_work="C:/actions-runner/_work"
           if [[ "${RUNNER_OS}" == "Linux" ]]; then
             cargo_target_dir="${HOME}/actions-runner/_work/target"
+          elif [[ "${RUNNER_OS}" == "Windows" && -d "${windows_runner_work}" ]]; then
+            # Our Windows image ships a warmed target outside the workspace,
+            # because checkout wipes GITHUB_WORKSPACE on every run -- that one
+            # `else` branch was the entire reason Windows built cold. The
+            # directory only exists on the self-hosted image, so GitHub-hosted
+            # windows-latest (the update-flow job) still falls through below.
+            cargo_target_dir="${windows_runner_work}/target"
           else
             # Forward slashes keep the Windows path compatible with Cargo,
             # PowerShell, GitHub Actions, and Git Bash filesystem commands.
@@ -117,15 +128,28 @@ jobs:
 
       - &configure-sccache
         name: Configure sccache
-        if: runner.os == 'Linux'
+        if: runner.os == 'Linux' || runner.os == 'Windows'
         shell: bash
         run: |
+          # `command -v` decides whether this runner was warmed at all, so an
+          # unwarmed image degrades to a cold-but-working build. It cannot
+          # supply RUSTC_WRAPPER on Windows though: it reports an MSYS path
+          # (/c/sccache/sccache.exe) that Cargo, a native Windows process,
+          # cannot execute. Hence the explicit per-OS paths below, which are
+          # written by the warm scripts in clockworklabs/infra.
           if command -v sccache >/dev/null 2>&1; then
+            if [[ "${RUNNER_OS}" == "Windows" ]]; then
+              sccache_bin="C:/sccache/sccache.exe"
+              sccache_dir="C:/actions-runner/_work/sccache"
+            else
+              sccache_bin="/usr/local/bin/sccache"
+              sccache_dir="${HOME}/.cache/sccache"
+            fi
             # sccache cannot cache incremental compilation artifacts, so
             # disable Cargo's incremental mode whenever the wrapper is active.
             echo "CARGO_INCREMENTAL=0" >>"${GITHUB_ENV}"
-            echo "RUSTC_WRAPPER=/usr/local/bin/sccache" >>"${GITHUB_ENV}"
-            echo "SCCACHE_DIR=${HOME}/.cache/sccache" >>"${GITHUB_ENV}"
+            echo "RUSTC_WRAPPER=${sccache_bin}" >>"${GITHUB_ENV}"
+            echo "SCCACHE_DIR=${sccache_dir}" >>"${GITHUB_ENV}"
             echo "SCCACHE_CACHE_SIZE=20G" >>"${GITHUB_ENV}"
           fi
 
@@ -243,13 +267,13 @@ jobs:
 
       - &show-sccache-stats
         name: Show sccache statistics
-        if: always() && runner.os == 'Linux'
+        if: always() && (runner.os == 'Linux' || runner.os == 'Windows')
         shell: bash
         run: |
           if command -v sccache >/dev/null 2>&1; then
             sccache --show-stats
           else
-            echo "::notice::sccache is not installed on this Linux runner"
+            echo "::notice::sccache is not installed on this ${RUNNER_OS} runner"
           fi
 
       - name: Upload Cargo timing reports
@@ -275,7 +299,7 @@ jobs:
     needs: [merge_queue_noop, lints]
     if: ${{ needs.merge_queue_noop.outputs.skip != 'true' }}
     name: Upload build artifacts (Windows)
-    runs-on: spacetimedb-windows-runner
+    runs-on: spacetimedb-windows-runner-staging
     timeout-minutes: 15
     env:
       RUST_BACKTRACE: full
@@ -361,7 +385,7 @@ jobs:
   smoketest_build_windows:
     needs: [upload-build-artifacts-windows]
     name: Build smoketests (Windows)
-    runs-on: spacetimedb-windows-runner
+    runs-on: spacetimedb-windows-runner-staging
     timeout-minutes: 15
     env:
       RUST_BACKTRACE: full
@@ -537,7 +561,7 @@ jobs:
       fail-fast: false
       matrix:
         partition: [1, 2, 3, 4]
-    runs-on: spacetimedb-windows-runner
+    runs-on: spacetimedb-windows-runner-staging
     timeout-minutes: 30
     env:
       RUST_BACKTRACE: full