win, child_process: sanitize env variables

On Windows environment variables are case-insensitive. When spawning
child process certain apps can get confused if some of the variables are
duplicated.

This adds a step on Windows to normalizeSpawnArguments that removes such
duplicates, keeping only the first (in lexicographic order) entry in the
env key of options. This is partly already done for the PATH entry.

Fixes: #35129

doc/api/child_process.md

@@ -43,6 +43,10 @@ the first one case-insensitively matching `PATH` to perform command lookup.
 This may lead to issues on Windows when passing objects to `env` option that
 have multiple variants of `PATH` variable.
 
+On Windows Node.js will sanitize the `env` by removing case-insensitive
+duplicates. Only first (in lexicographic order) entry will be passed to the
+child process.
+
 The [`child_process.spawn()`][] method spawns the child process asynchronously,
 without blocking the Node.js event loop. The [`child_process.spawnSync()`][]
 function provides equivalent functionality in a synchronous manner that blocks

lib/child_process.js

@@ -29,6 +29,7 @@ const {
   ObjectDefineProperty,
   ObjectPrototypeHasOwnProperty,
   Promise,
+  Set,
 } = primordials;
 
 const {
@@ -524,8 +525,27 @@ function normalizeSpawnArguments(file, args, options) {
     env.NODE_V8_COVERAGE = process.env.NODE_V8_COVERAGE;
   }
 
+  let envKeys = [];
   // Prototype values are intentionally included.
   for (const key in env) {
+    envKeys.push(key);
+  }
+
+  if (process.platform === 'win32') {
+    // On Windows env keys are case insensitive. Filter out duplicates,
+    // keeping only the first one (in lexicographic order)
+    const sawKey = new Set();
+    envKeys = envKeys.sort().filter((key) => {
+      const uppercaseKey = key.toUpperCase();
+      if (sawKey.has(uppercaseKey)) {
+        return false;
+      }
+      sawKey.add(uppercaseKey);
+      return true;
+    });
+  }
+
+  for (const key of envKeys) {
     const value = env[key];
     if (value !== undefined) {
       envPairs.push(`${key}=${value}`);

test/parallel/test-child-process-env.js

@@ -36,7 +36,9 @@ const env = {
   'HELLO': 'WORLD',
   'UNDEFINED': undefined,
   'NULL': null,
-  'EMPTY': ''
+  'EMPTY': '',
+  'duplicate': 'lowercase',
+  'DUPLICATE': 'uppercase',
 };
 Object.setPrototypeOf(env, {
   'FOO': 'BAR'
@@ -65,4 +67,11 @@ child.stdout.on('end', mustCall(() => {
   assert.ok(!response.includes('UNDEFINED=undefined'));
   assert.ok(response.includes('NULL=null'));
   assert.ok(response.includes(`EMPTY=${os.EOL}`));
+  if (isWindows) {
+    assert.ok(response.includes('DUPLICATE=uppercase'));
+    assert.ok(!response.includes('duplicate=lowercase'));
+  } else {
+    assert.ok(response.includes('DUPLICATE=uppercase'));
+    assert.ok(response.includes('duplicate=lowercase'));
+  }
 }));