Fix dev supervisor readiness after UTF-8 log output

scripts/dev/supervisor/children.ts

@@ -93,7 +93,7 @@ export class Child {
       if (this.spec.readiness.kind === "log") {
         const pattern = this.spec.readiness.pattern;
         const offset = this.logOffset;
-        const seen = bestEffortValue(() => readFileSync(this.logFile(), "utf8").slice(offset).includes(pattern));
+        const seen = bestEffortValue(() => readFileSync(this.logFile()).includes(pattern, offset));
         if (seen) return { ok: true };
       } else {
         const url = this.spec.readiness.url;

test/dev-supervisor-child.test.ts

@@ -1,6 +1,6 @@
 import { test } from "node:test";
 import assert from "node:assert/strict";
-import { mkdtempSync, rmSync } from "node:fs";
+import { mkdtempSync, rmSync, statSync, writeFileSync } from "node:fs";
 import { createServer } from "node:net";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -34,6 +34,28 @@ function spec(lock: string, port: number, extraArgs: string[] = []): ChildSpec {
   };
 }
 
+test("child finds log readiness after existing multibyte UTF-8 content", async () => {
+  const lock = mkdtempSync(join(tmpdir(), "qm-child-"));
+  const port = await freeTcpPort();
+  const child = new Child(
+    spec(lock, port),
+    lock,
+    () => {},
+    () => {},
+  );
+  const prefix = "以前の起動\n";
+  writeFileSync(child.logFile(), prefix);
+  assert.equal(statSync(child.logFile()).size, Buffer.byteLength(prefix));
+
+  try {
+    const res = await child.start();
+    assert.equal(res.ok, true, res.detail);
+  } finally {
+    await child.stop();
+    rmSync(lock, { recursive: true, force: true });
+  }
+});
+
 test("child starts, reports ready via log pattern, and stops with the port released", async () => {
   const lock = mkdtempSync(join(tmpdir(), "qm-child-"));
   const port = await freeTcpPort();