test: fix flaky test-child-process-emfile

Require the test setup to obtain an EMFILE error and not ENFILE as
ENFILE means there is a race condition with other processes that may
close files before `spawn()` is called by the test.

Fixes: #2666
PR-URL: #3430
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/sequential/test-child-process-emfile.js

@@ -1,30 +1,46 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var spawn = require('child_process').spawn;
-var fs = require('fs');
+const common = require('../common');
+const assert = require('assert');
+const child_process = require('child_process');
+const fs = require('fs');
 
 if (common.isWindows) {
   console.log('1..0 # Skipped: no RLIMIT_NOFILE on Windows');
   return;
 }
 
-var openFds = [];
+const ulimit = Number(child_process.execSync('ulimit -Hn'));
+if (ulimit > 64 || Number.isNaN(ulimit)) {
+  // Sorry about this nonsense. It can be replaced if
+  // https://github.com/nodejs/node-v0.x-archive/pull/2143#issuecomment-2847886
+  // ever happens.
+  const result = child_process.spawnSync(
+    '/bin/sh',
+    ['-c', `ulimit -n 64 && '${process.execPath}' '${__filename}'`]
+  );
+  assert.strictEqual(result.stdout.toString(), '');
+  assert.strictEqual(result.stderr.toString(), '');
+  assert.strictEqual(result.status, 0);
+  assert.strictEqual(result.error, undefined);
+  return;
+}
+
+const openFds = [];
 
 for (;;) {
   try {
     openFds.push(fs.openSync(__filename, 'r'));
   } catch (err) {
-    assert(err.code === 'EMFILE' || err.code === 'ENFILE');
+    assert(err.code === 'EMFILE');
     break;
   }
 }
 
 // Should emit an error, not throw.
-var proc = spawn(process.execPath, ['-e', '0']);
+const proc = child_process.spawn(process.execPath, ['-e', '0']);
 
 proc.on('error', common.mustCall(function(err) {
-  assert(err.code === 'EMFILE' || err.code === 'ENFILE');
+  assert.strictEqual(err.code, 'EMFILE');
 }));
 
 proc.on('exit', function() {