Add subprocess test utility for environment isolation

Author: jdalton

test/utils/subprocess.mts

@@ -0,0 +1,42 @@
+/** @fileoverview Test utilities for running code in subprocesses with custom environments. */
+
+import { spawn } from 'node:child_process'
+
+/**
+ * Run code in a subprocess with custom environment variables.
+ */
+export function runInSubprocess(
+  env: Record<string, string>,
+  testCode: string,
+): Promise<{ stdout: string; stderr: string; exitCode: number | null }> {
+  return new Promise((resolve, reject) => {
+    const child = spawn(
+      process.execPath,
+      ['--input-type=module', '--eval', testCode],
+      {
+        env: {
+          ...process.env,
+          ...env,
+        },
+        stdio: ['pipe', 'pipe', 'pipe'],
+      },
+    )
+
+    let stdout = ''
+    let stderr = ''
+
+    child.stdout.on('data', data => {
+      stdout += data.toString()
+    })
+
+    child.stderr.on('data', data => {
+      stderr += data.toString()
+    })
+
+    child.on('close', exitCode => {
+      resolve({ exitCode, stderr, stdout })
+    })
+
+    child.on('error', reject)
+  })
+}