process: fix call process.reallyExit, vs., binding

Some user-land modules, e.g., nyc, mocha, currently rely on patching
process.reallyExit.

PR-URL: #25655
Fixes: #25650
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/internal/process/per_thread.js

@@ -149,7 +149,10 @@ function wrapProcessMethods(binding) {
       process._exiting = true;
       process.emit('exit', process.exitCode || 0);
     }
-    binding.reallyExit(process.exitCode || 0);
+    // FIXME(joyeecheung): This is an undocumented API that gets monkey-patched
+    // in the user land. Either document it, or deprecate it in favor of a
+    // better public alternative.
+    process.reallyExit(process.exitCode || 0);
   }
 
   function kill(pid, sig) {

test/parallel/test-process-really-exit.js

@@ -0,0 +1,17 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+// ensure that the reallyExit hook is executed.
+// see: https://github.com/nodejs/node/issues/25650
+if (process.argv[2] === 'subprocess') {
+  process.reallyExit = function() {
+    console.info('really exited');
+  };
+  process.exit();
+} else {
+  const { spawnSync } = require('child_process');
+  const out = spawnSync(process.execPath, [__filename, 'subprocess']);
+  const observed = out.output[1].toString('utf8').trim();
+  assert.strictEqual(observed, 'really exited');
+}