Fix multi-byte UTF-8 decoding in Executable stream handling

common/changes/@rushstack/node-core-library/fix-multibyte-decoding_2026-01-07-19-31.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Fix incorrect decoding of multi-byte UTF-8 characters in Executable.waitForExitAsync and getProcessInfoByIdAsync by explicitly setting stream encoding before attaching handlers",
+      "type": "patch",
+      "packageName": "@rushstack/node-core-library"
+    }
+  ],
+  "packageName": "@rushstack/node-core-library",
+  "email": "copilot@github.com"
+}

libraries/node-core-library/src/Executable.ts

@@ -597,6 +597,13 @@ export class Executable {
     const { exitCode, signal } = await new Promise<ISignalAndExitCode>(
       (resolve: (result: ISignalAndExitCode) => void, reject: (error: Error) => void) => {
         if (encoding) {
+          // Set the encoding on the streams to ensure proper handling of multi-byte characters.
+          // When encoding is set, Node.js uses StringDecoder internally which properly handles
+          // character boundaries that may be split across chunks.
+          if (encoding !== 'buffer') {
+            childProcess.stdout!.setEncoding(encoding);
+            childProcess.stderr!.setEncoding(encoding);
+          }
           childProcess.stdout!.on('data', (chunk: Buffer | string) => {
             collectedStdout.push(normalizeChunk(chunk));
           });
@@ -667,6 +674,8 @@ export class Executable {
     if (process.stdout === null) {
       throw new InternalError('Child process did not provide stdout');
     }
+    // Set the encoding to ensure proper handling of multi-byte characters
+    process.stdout.setEncoding('utf8');
     const [processInfoByIdMap] = await Promise.all([
       parseProcessListOutputAsync(process.stdout),
       // Don't collect output in the result since we process it directly

libraries/node-core-library/src/test/Executable.test.ts

@@ -350,6 +350,30 @@ describe('Executable process tests', () => {
       Executable.waitForExitAsync(childProcess, { encoding: 'utf8', throwOnSignal: true })
     ).rejects.toThrowError(/Process terminated by SIGTERM/);
   });
+
+  test('Executable.waitForExitAsync() handles multi-byte UTF-8 characters correctly', async () => {
+    // Test that multi-byte characters are properly decoded even when split across chunks
+    const executablePath: string = path.join(executableFolder, 'multibyte', 'output-multibyte.js');
+    const childProcess: child_process.ChildProcess = Executable.spawn(process.argv0, [executablePath], {
+      environment,
+      currentWorkingDirectory: executableFolder,
+      stdio: ['ignore', 'pipe', 'pipe']
+    });
+
+    const result: IWaitForExitResult<string> = await Executable.waitForExitAsync(childProcess, {
+      encoding: 'utf8'
+    });
+
+    expect(result.exitCode).toEqual(0);
+    expect(result.signal).toBeNull();
+    expect(typeof result.stdout).toEqual('string');
+
+    // The output should contain properly decoded multi-byte characters
+    // Chinese characters (世界) and emoji (🎉)
+    expect(result.stdout).toContain('Hello, 世界! 🎉');
+    // Ensure no replacement characters (�) which would indicate improper decoding
+    expect(result.stdout).not.toContain('�');
+  });
 });
 
 describe('Executable process list', () => {

libraries/node-core-library/src/test/test-data/executable/multibyte/output-multibyte.js

@@ -0,0 +1,20 @@
+// This script writes multi-byte UTF-8 characters byte by byte to test proper decoding
+const { setTimeout } = require('node:timers/promises');
+
+const unicodeString = "Hello, 世界! 🎉"; // "Hello, World" in Chinese with emoji
+const encoded = Buffer.from(unicodeString, 'utf8');
+
+async function writeChars() {
+  // Write each byte individually to force chunks that split multi-byte characters
+  for (let i = 0; i < encoded.length; i++) {
+    process.stdout.write(encoded.subarray(i, i + 1));
+    // Small delay to ensure each byte is in a separate chunk
+    await setTimeout(1);
+  }
+  process.stdout.write('\n');
+}
+
+writeChars().catch((error) => {
+  console.error('Error:', error);
+  process.exit(1);
+});