fix(e2e): keep text file tails UTF-8 safe (#109669)

* fix(e2e): keep text file tails UTF-8 safe

* fix: preserve malformed UTF-8 tail bytes

Co-authored-by: qingminlong <0668001063@xydigit.com>

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

scripts/e2e/lib/text-file-utils.mjs

@@ -1,11 +1,21 @@
 // Text file tail helpers for E2E assertions.
 import fs from "node:fs";
 
+function decodeUtf8Tail(buffer, truncated) {
+  let start = 0;
+  if (truncated) {
+    while (start < buffer.length && (buffer[start] & 0b1100_0000) === 0b1000_0000) {
+      start += 1;
+    }
+  }
+  return buffer.subarray(start).toString("utf8");
+}
+
 export function tailText(text, maxBytes) {
   if (Buffer.byteLength(text, "utf8") <= maxBytes) {
     return text;
   }
-  return Buffer.from(text, "utf8").subarray(-maxBytes).toString("utf8");
+  return decodeUtf8Tail(Buffer.from(text, "utf8").subarray(-maxBytes), true);
 }
 
 export function readTextFileTail(file, maxBytes) {
@@ -26,7 +36,7 @@ export function readTextFileTail(file, maxBytes) {
     fd = fs.openSync(file, "r");
     const buffer = Buffer.alloc(length);
     const bytesRead = fs.readSync(fd, buffer, 0, length, start);
-    return buffer.subarray(0, bytesRead).toString("utf8");
+    return decodeUtf8Tail(buffer.subarray(0, bytesRead), start > 0);
   } catch {
     return "";
   } finally {

test/scripts/e2e-text-file-utils.test.ts

@@ -23,6 +23,8 @@ describe("e2e text file utilities", () => {
   it("keeps short diagnostic text intact and trims long text by byte count", () => {
     expect(tailText("short", 8)).toBe("short");
     expect(tailText("prefix-tail", 4)).toBe("tail");
+    expect(tailText("prefix \u{1f600}tail", 7)).toBe("tail");
+    expect(tailText("prefix \u{1f600}tail", 8)).toBe("\u{1f600}tail");
   });
 
   it("reads only the requested file tail and treats missing or non-file paths as empty", () => {
@@ -33,6 +35,11 @@ describe("e2e text file utilities", () => {
     writeFileSync(file, "line-one\nline-two\nline-three", "utf8");
 
     expect(readTextFileTail(file, 10)).toBe("line-three");
+    writeFileSync(file, "prefix \u{1f600}tail", "utf8");
+    expect(readTextFileTail(file, 7)).toBe("tail");
+    expect(readTextFileTail(file, 8)).toBe("\u{1f600}tail");
+    writeFileSync(file, Buffer.concat([Buffer.from("prefix tail"), Buffer.from([0xf0])]));
+    expect(readTextFileTail(file, 5)).toBe("tail\ufffd");
     expect(readTextFileTail(path.join(root, "missing.log"), 10)).toBe("");
     expect(readTextFileTail(directory, 10)).toBe("");
   });