feat: Add winexec helper for running PowerShell from the agent

agent/internal/winexec/encode.go

@@ -0,0 +1,24 @@
+// Package winexec runs PowerShell on Windows hosts.
+//
+// It exists because three subsystems — updates, workload collection and
+// workload logs — all need the same invocation, and because getting a
+// multi-line script past Go quoting, cmd.exe quoting and PowerShell's own
+// parser is a problem worth solving once.
+package winexec
+
+import (
+	"encoding/base64"
+	"unicode/utf16"
+)
+
+// EncodeCommand renders a script for powershell.exe -EncodedCommand: UTF-16LE,
+// no byte-order mark, base64. This is deliberately free of build tags so it is
+// tested on a Linux development machine like every other pure function here.
+func EncodeCommand(script string) string {
+	units := utf16.Encode([]rune(script))
+	b := make([]byte, 0, len(units)*2)
+	for _, u := range units {
+		b = append(b, byte(u), byte(u>>8))
+	}
+	return base64.StdEncoding.EncodeToString(b)
+}

agent/internal/winexec/encode_test.go

@@ -0,0 +1,20 @@
+package winexec
+
+import "testing"
+
+func TestEncodeCommand(t *testing.T) {
+	// "hi" as UTF-16LE is 68 00 69 00, which base64-encodes to aABpAA==.
+	if got := EncodeCommand("hi"); got != "aABpAA==" {
+		t.Fatalf("EncodeCommand(hi) = %q, want aABpAA==", got)
+	}
+}
+
+func TestEncodeCommandMultiline(t *testing.T) {
+	// Only that it round-trips through the same encoding PowerShell expects:
+	// every ASCII byte followed by a zero byte, no BOM.
+	got := EncodeCommand("a\nb")
+	want := "YQAKAGIA"
+	if got != want {
+		t.Fatalf("EncodeCommand = %q, want %q", got, want)
+	}
+}

agent/internal/winexec/run_windows.go

@@ -0,0 +1,31 @@
+package winexec
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// Run executes a PowerShell script and returns its stdout.
+//
+// powershell.exe rather than pwsh: everything this agent runs through here
+// touches Windows Update COM or CIM, both of which are most reliable under
+// Windows PowerShell 5.1, and 5.1 is present on every supported Windows while
+// pwsh is an optional install.
+func Run(ctx context.Context, script string) (string, error) {
+	cmd := exec.CommandContext(ctx, "powershell.exe",
+		"-NoProfile", "-NonInteractive", "-EncodedCommand", EncodeCommand(script))
+
+	out, err := cmd.Output()
+	if err != nil {
+		if ee, ok := err.(*exec.ExitError); ok && len(ee.Stderr) > 0 {
+			return "", fmt.Errorf("powershell: %s", strings.TrimSpace(string(ee.Stderr)))
+		}
+		if ctx.Err() == context.DeadlineExceeded {
+			return "", fmt.Errorf("powershell: timed out")
+		}
+		return "", fmt.Errorf("powershell: %w", err)
+	}
+	return string(out), nil
+}