fix(process-host): accept string stdin in synchronous runs (#28225)

fix(process-host): accept string stdin in synchronous runs

Request raw subprocess output with null encoding so Node can encode string stdin as UTF-8. Preserve explicit text decoding and byte capture, with real-runner coverage for both output modes.

Co-authored-by: Joe Esposito <joeespo@gmail.com>

src/packages/process-host/src/run-process.test.ts

@@ -83,6 +83,29 @@ describe('resolveSpawn', () => {
 })
 
 describe('runProcessSync', () => {
+  it.each([false, true])('accepts string stdin with byte capture %s', (captureStdoutAsBytes) => {
+    const input = 'nonsecret-probe-λ'
+    const result = runProcessSync({
+      program: process.execPath,
+      args: [
+        '-e',
+        'process.stdin.pipe(process.stdout, { end: false }); process.stdin.on("end", () => { process.stdout.write(Buffer.from([0,255])); process.stderr.write("diagnostic λ") })'
+      ],
+      input,
+      captureStdoutAsBytes
+    })
+    expect(result).toMatchObject({
+      code: 0,
+      stdout: captureStdoutAsBytes ? '' : `${input}\u0000�`,
+      stderr: 'diagnostic λ',
+      timedOut: false,
+      outputTruncated: false
+    })
+    expect(result.stdoutBytes).toEqual(
+      captureStdoutAsBytes ? Buffer.concat([Buffer.from(input), Buffer.from([0, 255])]) : undefined
+    )
+  })
+
   it('reports a timeout as timedOut', () => {
     const result = runProcessSync({
       program: process.execPath,

src/packages/process-host/src/run-process.ts

@@ -357,7 +357,8 @@ function runResolvedProcessSync(spec: ProcessSpec, resolved: ResolvedSpawn): Pro
     input: spec.input,
     timeout: spec.timeoutMs === null ? undefined : (spec.timeoutMs ?? DEFAULT_PROCESS_TIMEOUT_MS),
     maxBuffer: spec.maxOutputBytes ?? DEFAULT_MAX_OUTPUT_BYTES,
-    encoding: 'buffer'
+    // Keep raw output without applying the nonexistent 'buffer' encoding to string stdin.
+    encoding: null
   })
   const timedOut = result.error
     ? 'code' in result.error && result.error.code === 'ETIMEDOUT'