util: use a global symbol for util.promisify.custom

Define `util.promisify.custom`
as `Symbol.for("nodejs.util.inspect.custom")`, rather than
as `Symbol("util.inspect.custom")`.

This allows custom `promisify` wrappers to easily/safely be defined
in non‑Node.js environments.

Fixes: https://github.com/nodejs/node/issues/31647

PR-URL: https://github.com/nodejs/node/pull/31672
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/util.md

@@ -995,11 +995,32 @@ throw an error.
 ### `util.promisify.custom`
 <!-- YAML
 added: v8.0.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/31672
+    description: This is now defined as a shared symbol.
 -->
 
 * {symbol} that can be used to declare custom promisified variants of functions,
 see [Custom promisified functions][].
 
+In addition to being accessible through `util.promisify.custom`, this
+symbol is [registered globally][global symbol registry] and can be
+accessed in any environment as `Symbol.for('nodejs.util.promisify.custom')`.
+
+For example, with a function that takes in
+`(foo, onSuccessCallback, onErrorCallback)`:
+
+```js
+const kCustomPromisifiedSymbol = Symbol.for('nodejs.util.promisify.custom');
+
+doSomething[kCustomPromisifiedSymbol] = (foo) => {
+  return new Promise((resolve, reject) => {
+    doSomething(foo, resolve, reject);
+  });
+};
+```
+
 ## Class: `util.TextDecoder`
 <!-- YAML
 added: v8.3.0

lib/internal/util.js

@@ -271,7 +271,7 @@ function getSystemErrorName(err) {
   return entry ? entry[0] : `Unknown system error ${err}`;
 }
 
-const kCustomPromisifiedSymbol = Symbol('util.promisify.custom');
+const kCustomPromisifiedSymbol = SymbolFor('nodejs.util.promisify.custom');
 const kCustomPromisifyArgsSymbol = Symbol('customPromisifyArgs');
 
 function promisify(original) {

test/parallel/test-util-promisify.js

@@ -33,6 +33,21 @@ const stat = promisify(fs.stat);
   assert.strictEqual(promisify(promisify(fn)), promisifedFn);
 }
 
+{
+  function fn() {}
+
+  function promisifiedFn() {}
+
+  // util.promisify.custom is a shared symbol which can be accessed
+  // as `Symbol.for("nodejs.util.promisify.custom")`.
+  const kCustomPromisifiedSymbol = Symbol.for('nodejs.util.promisify.custom');
+  fn[kCustomPromisifiedSymbol] = promisifiedFn;
+
+  assert.strictEqual(kCustomPromisifiedSymbol, promisify.custom);
+  assert.strictEqual(promisify(fn), promisifiedFn);
+  assert.strictEqual(promisify(promisify(fn)), promisifiedFn);
+}
+
 {
   function fn() {}
   fn[promisify.custom] = 42;