Fix/schtasks xml encoding

packages/loop-js/src/cli/cron/schtasks.encoding.test.ts

@@ -0,0 +1,16 @@
+import { expect, test } from "bun:test"
+import { buildTaskXml, taskXmlBytes, wrapperCommand } from "./schtasks.ts"
+
+test("Task Scheduler XML declaration and bytes agree on UTF-16", () => {
+  const xml = buildTaskXml({
+    expr: "0 8 * * *",
+    dir: "C:\\proj",
+    command: wrapperCommand("C:\\proj\\.loop\\cron\\abc123.cmd"),
+    until: { settled: false },
+  })
+  const bytes = taskXmlBytes(xml)
+
+  expect(xml.startsWith('<?xml version="1.0" encoding="UTF-16"?>')).toBe(true)
+  expect([...bytes.subarray(0, 2)]).toEqual([0xff, 0xfe])
+  expect(bytes.toString("utf16le").startsWith('\uFEFF<?xml version="1.0" encoding="UTF-16"?>')).toBe(true)
+})

packages/loop-js/src/cli/cron/schtasks.ts

@@ -116,7 +116,7 @@ export function wrapperCommand(wrapper: string): TaskCommand {
 export function buildTaskXml(opts: { expr: string; dir: string; command: TaskCommand; until: Until }): string {
   const trigger = triggersXml(expr.schtasks.schedule(opts.expr)) // a refused expr throws before anything is installed
   const desc = `${DESC}${opts.expr} ${formatUntil(opts.until)}`
-  return `<?xml version="1.0" encoding="UTF-8"?>
+  return `<?xml version="1.0" encoding="UTF-16"?>
 <Task version="1.2" xmlns="http://schemas.microsoft.com/windows/2004/02/mit/task">
   <RegistrationInfo><Description>${xmlEscape(desc)}</Description></RegistrationInfo>
   ${trigger}
@@ -126,6 +126,11 @@ export function buildTaskXml(opts: { expr: string; dir: string; command: TaskCom
 </Task>`
 }
 
+/** The exact bytes `schtasks /Create /XML` receives: UTF-16LE plus an explicit byte-order mark. */
+export function taskXmlBytes(xml: string): Buffer {
+  return Buffer.from(`\uFEFF${xml}`, "utf16le")
+}
+
 /** Recover `{ expr, dir, until }` from a task XML we wrote; null if it is not one of ours. */
 export function parseTaskXml(xml: string): Pick<Entry, "expr" | "dir" | "until"> | null {
   const desc = firstMatch(xml, /<Description>([\s\S]*?)<\/Description>/)
@@ -176,7 +181,7 @@ export function systemSchtasks(): Schtasks {
       const dir = mkdtempSync(join(tmpdir(), "loop-cron-"))
       const file = join(dir, "task.xml")
       try {
-        writeFileSync(file, xml, "utf8")
+        writeFileSync(file, taskXmlBytes(xml))
         const r = run(["/Create", "/F", "/TN", taskPath, "/XML", file])
         if (r.status !== 0) throw bin.failure("schtasks create", r)
       } finally {