.github/workflows: shard the Windows builder

It's one of the slower ones, so split it up into chunks.

Updates tailscale/corp#28679

Change-Id: I16a5ba667678bf238c84417a51dda61baefbecf7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -210,6 +210,17 @@ jobs:
   windows:
     runs-on: windows-2022
     needs: gomod-cache
+    name: Windows (${{ matrix.name || matrix.shard}})
+    strategy:
+      fail-fast: false # don't abort the entire matrix if one element fails
+      matrix:
+        include:
+          - key: "win-bench"
+            name: "benchmarks"
+          - key: "win-shard-1-2"
+            shard: "1/2"
+          - key: "win-shard-2-2"
+            shard: "2/2"
     steps:
     - name: checkout
       uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -237,14 +248,16 @@ jobs:
           ~\AppData\Local\go-build
         # The -2- here should be incremented when the scheme of data to be
         # cached changes (e.g. path above changes).
-        key: ${{ github.job }}-${{ runner.os }}-go-2-${{ hashFiles('**/go.sum') }}-${{ github.run_id }}
+        key: ${{ github.job }}-${{ matrix.key }}-go-2-${{ hashFiles('**/go.sum') }}-${{ github.run_id }}
         restore-keys: |
-          ${{ github.job }}-${{ runner.os }}-go-2-${{ hashFiles('**/go.sum') }}
-          ${{ github.job }}-${{ runner.os }}-go-2-
+          ${{ github.job }}-${{ matrix.key }}-go-2-${{ hashFiles('**/go.sum') }}
+          ${{ github.job }}-${{ matrix.key }}-go-2-
     - name: test
+      if: matrix.key != 'win-bench' # skip on bench builder
       working-directory: src
-      run: go run ./cmd/testwrapper ./...
+      run: go run ./cmd/testwrapper sharded:${{ matrix.shard }}
     - name: bench all
+      if: matrix.key == 'win-bench'
       working-directory: src
       # Don't use -bench=. -benchtime=1x.
       # Somewhere in the layers (powershell?)

cmd/testwrapper/testwrapper.go

@@ -213,6 +213,16 @@ func main() {
 		return
 	}
 
+	// As a special case, if the packages looks like "sharded:1/2" then shell out to
+	// ./tool/listpkgs to cut up the package list pieces for each sharded builder.
+	if nOfM, ok := strings.CutPrefix(packages[0], "sharded:"); ok && len(packages) == 1 {
+		out, err := exec.Command("go", "run", "tailscale.com/tool/listpkgs", "-shard", nOfM, "./...").Output()
+		if err != nil {
+			log.Fatalf("failed to list packages for sharded test: %v", err)
+		}
+		packages = strings.Split(strings.TrimSpace(string(out)), "\n")
+	}
+
 	ctx := context.Background()
 	type nextRun struct {
 		tests   []*packageTests