fix(cli): keep UTF-8 log tails valid

ly85206559 · ly85206559 · commit ce6876e72762 · 2026-07-07T00:48:18.000+08:00
diff --git a/src/cli/logs-cli.runtime.test.ts b/src/cli/logs-cli.runtime.test.ts
@@ -54,4 +54,26 @@ describe("execFileUtf8Tail", () => {
     await expect(resultPromise).resolves.toMatchObject({ code: 1, stderr: "spawn failed" });
     expect(kill).not.toHaveBeenCalled();
   });
+
+  it("does not return a replacement character when stdout tail starts mid UTF-8 sequence", async () => {
+    const stdout = new EventEmitter();
+    const stderr = new EventEmitter();
+    const child = Object.assign(new EventEmitter(), {
+      kill: vi.fn(() => true),
+      stderr,
+      stdout,
+    });
+    spawnMock.mockReturnValue(child as unknown as ChildProcess);
+
+    const resultPromise = execFileUtf8Tail("journalctl", ["--no-pager"], { maxBytes: 1 });
+    stdout.emit("data", Buffer.from("é", "utf8"));
+    child.emit("close", 0);
+
+    await expect(resultPromise).resolves.toEqual({
+      code: 0,
+      stderr: "",
+      stdout: "",
+      truncated: true,
+    });
+  });
 });
diff --git a/src/cli/logs-cli.runtime.ts b/src/cli/logs-cli.runtime.ts
@@ -7,6 +7,18 @@ export { readSystemdServiceRuntime } from "../daemon/systemd.js";
 
 type ExecFileTailResult = { stdout: string; stderr: string; code: number; truncated: boolean };
 
+function decodeUtf8Tail(chunks: Buffer[], truncated: boolean): string {
+  const buffer = Buffer.concat(chunks);
+  if (!truncated || buffer.length === 0) {
+    return buffer.toString("utf8");
+  }
+  let offset = 0;
+  while (offset < buffer.length && (buffer[offset] & 0xc0) === 0x80) {
+    offset += 1;
+  }
+  return buffer.subarray(offset).toString("utf8");
+}
+
 export async function execFileUtf8Tail(
   command: string,
   args: string[],
@@ -68,7 +80,7 @@ export async function execFileUtf8Tail(
         child.kill();
       }
       resolve({
-        stdout: Buffer.concat(stdoutChunks).toString("utf8"),
+        stdout: decodeUtf8Tail(stdoutChunks, truncated),
         stderr: error instanceof Error ? error.message : String(error),
         code: 1,
         truncated,
@@ -84,7 +96,7 @@ export async function execFileUtf8Tail(
       }
       settled = true;
       resolve({
-        stdout: Buffer.concat(stdoutChunks).toString("utf8"),
+        stdout: decodeUtf8Tail(stdoutChunks, truncated),
         stderr: Buffer.concat(stderrChunks).toString("utf8"),
         code: typeof code === "number" ? code : 1,
         truncated,
