prevent windows backend tests from running out of disk space (#9325)

.github/workflows/backend-test-windows.yml

@@ -98,6 +98,21 @@ jobs:
           vcpkg.exe install openssl:x64-windows-static
           vcpkg.exe integrate install
 
+      - name: Free disk space (post-vcpkg)
+        shell: pwsh
+        run: |
+          # vcpkg leaves multi-GB of buildtrees/downloads after installing openssl;
+          # we only need the installed/ dir for linking.
+          $vcpkgRoot = $env:VCPKG_INSTALLATION_ROOT
+          foreach ($sub in @("buildtrees", "downloads", "packages")) {
+            $path = Join-Path $vcpkgRoot $sub
+            if (Test-Path $path) {
+              Write-Host "Removing $path"
+              Remove-Item -Recurse -Force -ErrorAction SilentlyContinue $path
+            }
+          }
+          Get-PSDrive C | Select-Object Used,Free | Format-Table -AutoSize
+
       - name: Get runtime paths
         id: runtime-paths
         shell: pwsh
@@ -119,6 +134,10 @@ jobs:
           cargo build --release -p windmill_duckdb_ffi_internal
           New-Item -ItemType Directory -Path ..\target\debug -Force
           Copy-Item target\release\windmill_duckdb_ffi_internal.dll ..\target\debug\
+          # duckdb is bundled (~2GB of build artifacts); the DLL is the only
+          # thing we need from this excluded-crate target dir.
+          Remove-Item -Recurse -Force -ErrorAction SilentlyContinue target
+          Get-PSDrive C | Select-Object Used,Free | Format-Table -AutoSize
 
       - name: Print runtime versions and env
         shell: pwsh
@@ -136,6 +155,10 @@ jobs:
           echo "USERPROFILE=$env:USERPROFILE"
           echo "HOME=$env:HOME"
 
+      - name: Disk space before cargo test
+        shell: pwsh
+        run: Get-PSDrive C | Select-Object Used,Free | Format-Table -AutoSize
+
       - name: cargo test
         working-directory: backend
         timeout-minutes: 60
@@ -144,13 +167,16 @@ jobs:
           RUST_LOG: "off"
           RUST_LOG_STYLE: never
           CARGO_NET_GIT_FETCH_WITH_CLI: true
-          CARGO_BUILD_JOBS: 12
+          # 16-vcpu runners with disabled PDB still hit LNK1180 ("insufficient
+          # disk space") at link time with 12 parallel link jobs: each test
+          # binary link spikes several hundred MB of transient I/O. Capping at
+          # 8 trades ~25% wall time for headroom on the ~75GB runner disk.
+          CARGO_BUILD_JOBS: 8
           # backend/Cargo.toml sets split-debuginfo = "unpacked", which on
           # windows-msvc is coerced to "packed": every test-binary link spawns
-          # the mspdbsrv.exe PDB type server and writes a large .pdb. With 12
-          # parallel link jobs this races the type-server cap (LNK1318 "LIMIT
-          # (12)") and exhausts the runner disk (LNK1180). CI needs no debug
-          # info, so disable PDB generation for the dev/test profiles here.
+          # the mspdbsrv.exe PDB type server and writes a large .pdb. CI needs
+          # no debug info, so disable PDB generation for the dev/test profiles
+          # here (avoids both LNK1318 type-server limit and PDB disk usage).
           CARGO_PROFILE_DEV_SPLIT_DEBUGINFO: "off"
           CARGO_PROFILE_TEST_SPLIT_DEBUGINFO: "off"
           # Tests' poll-time stack frames (deep nested async fn chains in