esm: unflag Module.register and allow nested loader import()

Major functional changes:

- Allow `import()` to work within loaders that require other loaders,
- Unflag the use of `Module.register`.

A new interface `Customizations` has been created in order to unify
`ModuleLoader` (previously `DefaultModuleLoader`), `Hooks` and
`CustomizedModuleLoader` all of which now implement it:

```ts
interface LoadResult {
  format: ModuleFormat;
  source: ModuleSource;
}

interface ResolveResult {
  format: string;
  url: URL['href'];
}

interface Customizations {
  allowImportMetaResolve: boolean;
  load(url: string, context: object): Promise<LoadResult>
  resolve(
    originalSpecifier:
    string, parentURL: string,
    importAssertions: Record<string, string>
  ): Promise<ResolveResult>
  resolveSync(
    originalSpecifier:
    string, parentURL: string,
    importAssertions: Record<string, string>
  ) ResolveResult;
  register(specifier: string, parentUrl: string): any;
  forceLoadHooks(): void;
  importMetaInitialize(meta, context, loader): void;
}
```

The `ModuleLoader` class now has `setCustomizations` which takes an
object of this shape and delegates its responsibilities to this object
if present.

Note that two properties `allowImportMetaResolve` and `resolveSync`
exist now as a mechanism for `import.meta.resolve` – since `Hooks`
does not implement `resolveSync` other loaders cannot use
`import.meta.resolve`; `allowImportMetaResolve` is a way of checking
for that case instead of invoking `resolveSync` and erroring.

Fixes nodejs#48515
Closes nodejs#48439

Author: izaakschroeder

doc/api/errors.md

@@ -1233,23 +1233,6 @@ provided.
 Encoding provided to `TextDecoder()` API was not one of the
 [WHATWG Supported Encodings][].
 
-<a id="ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE"></a>
-
-### `ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE`
-
-<!-- YAML
-added: REPLACEME
--->
-
-Programmatically registering custom ESM loaders
-currently requires at least one custom loader to have been
-registered via the `--experimental-loader` flag. A no-op
-loader registered via CLI is sufficient
-(for example: `--experimental-loader data:text/javascript,`;
-do not omit the necessary trailing comma).
-A future version of Node.js will support the programmatic
-registration of loaders without needing to also use the flag.
-
 <a id="ERR_EVAL_ESM_CANNOT_PRINT"></a>
 
 ### `ERR_EVAL_ESM_CANNOT_PRINT`

lib/internal/errors.js

@@ -1036,11 +1036,6 @@ E('ERR_ENCODING_INVALID_ENCODED_DATA', function(encoding, ret) {
 }, TypeError);
 E('ERR_ENCODING_NOT_SUPPORTED', 'The "%s" encoding is not supported',
   RangeError);
-E('ERR_ESM_LOADER_REGISTRATION_UNAVAILABLE', 'Programmatically registering custom ESM loaders ' +
-  'currently requires at least one custom loader to have been registered via the --experimental-loader ' +
-  'flag. A no-op loader registered via CLI is sufficient (for example: `--experimental-loader ' +
-  '"data:text/javascript,"` with the necessary trailing comma). A future version of Node.js ' +
-  'will remove this requirement.', Error);
 E('ERR_EVAL_ESM_CANNOT_PRINT', '--print cannot be used with ESM input', Error);
 E('ERR_EVENT_RECURSION', 'The event "%s" is already being dispatched', Error);
 E('ERR_FALSY_VALUE_REJECTION', function(reason) {

lib/internal/modules/esm/hooks.js

@@ -31,6 +31,7 @@ const {
   ERR_INVALID_RETURN_PROPERTY_VALUE,
   ERR_INVALID_RETURN_VALUE,
   ERR_LOADER_CHAIN_INCOMPLETE,
+  ERR_METHOD_NOT_IMPLEMENTED,
   ERR_UNKNOWN_BUILTIN_MODULE,
   ERR_WORKER_UNSERIALIZABLE_ERROR,
 } = require('internal/errors').codes;
@@ -65,6 +66,8 @@ const {
 let debug = require('internal/util/debuglog').debuglog('esm', (fn) => {
   debug = fn;
 });
+let importMetaInitializer;
+
 
 
 /**
@@ -82,7 +85,6 @@ let debug = require('internal/util/debuglog').debuglog('esm', (fn) => {
 
 // [2] `validate...()`s throw the wrong error
 
-
 class Hooks {
   #chains = {
     /**
@@ -121,20 +123,20 @@ class Hooks {
   // Cache URLs we've already validated to avoid repeated validation
   #validatedUrls = new SafeSet();
 
+  allowImportMetaResolve = false;
+
   /**
    * Import and register custom/user-defined module loader hook(s).
    * @param {string} urlOrSpecifier
    * @param {string} parentURL
    */
   async register(urlOrSpecifier, parentURL) {
     const moduleLoader = require('internal/process/esm_loader').esmLoader;
-
     const keyedExports = await moduleLoader.import(
       urlOrSpecifier,
       parentURL,
       kEmptyObject,
     );
-
     this.addCustomLoader(urlOrSpecifier, keyedExports);
   }
 
@@ -152,13 +154,15 @@ class Hooks {
     } = pluckHooks(exports);
 
     if (globalPreload) {
-      ArrayPrototypePush(this.#chains.globalPreload, { fn: globalPreload, url });
+      ArrayPrototypePush(this.#chains.globalPreload, { __proto__: null, fn: globalPreload, url });
     }
     if (resolve) {
-      ArrayPrototypePush(this.#chains.resolve, { fn: resolve, url });
+      const next = this.#chains.resolve[this.#chains.resolve.length - 1];
+      ArrayPrototypePush(this.#chains.resolve, { __proto__: null, fn: resolve, url, next });
     }
     if (load) {
-      ArrayPrototypePush(this.#chains.load, { fn: load, url });
+      const next = this.#chains.load[this.#chains.load.length - 1];
+      ArrayPrototypePush(this.#chains.load, { __proto__: null, fn: load, url, next });
     }
   }
 
@@ -235,7 +239,6 @@ class Hooks {
       chainFinished: null,
       context,
       hookErrIdentifier: '',
-      hookIndex: chain.length - 1,
       hookName: 'resolve',
       shortCircuited: false,
     };
@@ -258,7 +261,7 @@ class Hooks {
       }
     };
 
-    const nextResolve = nextHookFactory(chain, meta, { validateArgs, validateOutput });
+    const nextResolve = nextHookFactory(chain[chain.length - 1], meta, { validateArgs, validateOutput });
 
     const resolution = await nextResolve(originalSpecifier, context);
     const { hookErrIdentifier } = meta; // Retrieve the value after all settled
@@ -335,6 +338,10 @@ class Hooks {
     };
   }
 
+  resolveSync(_originalSpecifier, _parentURL, _importAssertions) {
+    throw new ERR_METHOD_NOT_IMPLEMENTED('resolveSync()');
+  }
+
   /**
    * Provide source that is understood by one of Node's translators.
    *
@@ -351,7 +358,6 @@ class Hooks {
       chainFinished: null,
       context,
       hookErrIdentifier: '',
-      hookIndex: chain.length - 1,
       hookName: 'load',
       shortCircuited: false,
     };
@@ -393,7 +399,7 @@ class Hooks {
       }
     };
 
-    const nextLoad = nextHookFactory(chain, meta, { validateArgs, validateOutput });
+    const nextLoad = nextHookFactory(chain[chain.length - 1], meta, { validateArgs, validateOutput });
 
     const loaded = await nextLoad(url, context);
     const { hookErrIdentifier } = meta; // Retrieve the value after all settled
@@ -468,6 +474,16 @@ class Hooks {
       source,
     };
   }
+
+  forceLoadHooks() {
+    // No-op
+  }
+
+  importMetaInitialize(meta, context, loader) {
+    importMetaInitializer ??= require('internal/modules/esm/initialize_import_meta').initializeImportMeta;
+    meta = importMetaInitializer(meta, context, loader);
+    return meta;
+  }
 }
 ObjectSetPrototypeOf(Hooks.prototype, null);
 
@@ -717,46 +733,39 @@ function pluckHooks({
  * A utility function to iterate through a hook chain, track advancement in the
  * chain, and generate and supply the `next<HookName>` argument to the custom
  * hook.
- * @param {KeyedHook[]} chain The whole hook chain.
+ * @param {Hook} current The (currently) first hook in the chain (this shifts
+ * on every call).
  * @param {object} meta Properties that change as the current hook advances
  * along the chain.
  * @param {boolean} meta.chainFinished Whether the end of the chain has been
  * reached AND invoked.
  * @param {string} meta.hookErrIdentifier A user-facing identifier to help
  *  pinpoint where an error occurred. Ex "file:///foo.mjs 'resolve'".
- * @param {number} meta.hookIndex A non-negative integer tracking the current
- * position in the hook chain.
  * @param {string} meta.hookName The kind of hook the chain is (ex 'resolve')
  * @param {boolean} meta.shortCircuited Whether a hook signaled a short-circuit.
  * @param {(hookErrIdentifier, hookArgs) => void} validate A wrapper function
  *  containing all validation of a custom loader hook's intermediary output. Any
  *  validation within MUST throw.
  * @returns {function next<HookName>(...hookArgs)} The next hook in the chain.
  */
-function nextHookFactory(chain, meta, { validateArgs, validateOutput }) {
+function nextHookFactory(current, meta, { validateArgs, validateOutput }) {
   // First, prepare the current
   const { hookName } = meta;
   const {
     fn: hook,
     url: hookFilePath,
-  } = chain[meta.hookIndex];
+    next,
+  } = current;
 
   // ex 'nextResolve'
   const nextHookName = `next${
     StringPrototypeToUpperCase(hookName[0]) +
     StringPrototypeSlice(hookName, 1)
   }`;
 
-  // When hookIndex is 0, it's reached the default, which does not call next()
-  // so feed it a noop that blows up if called, so the problem is obvious.
-  const generatedHookIndex = meta.hookIndex;
   let nextNextHook;
-  if (meta.hookIndex > 0) {
-    // Now, prepare the next: decrement the pointer so the next call to the
-    // factory generates the next link in the chain.
-    meta.hookIndex--;
-
-    nextNextHook = nextHookFactory(chain, meta, { validateArgs, validateOutput });
+  if (next) {
+    nextNextHook = nextHookFactory(next, meta, { validateArgs, validateOutput });
   } else {
     // eslint-disable-next-line func-name-matching
     nextNextHook = function chainAdvancedTooFar() {
@@ -773,17 +782,16 @@ function nextHookFactory(chain, meta, { validateArgs, validateOutput }) {
 
       validateArgs(`${meta.hookErrIdentifier} hook's ${nextHookName}()`, arg0, context);
 
-      const outputErrIdentifier = `${chain[generatedHookIndex].url} '${hookName}' hook's ${nextHookName}()`;
+      const outputErrIdentifier = `${hookFilePath} '${hookName}' hook's ${nextHookName}()`;
 
       // Set when next<HookName> is actually called, not just generated.
-      if (generatedHookIndex === 0) { meta.chainFinished = true; }
+      if (!next) { meta.chainFinished = true; }
 
       if (context) { // `context` has already been validated, so no fancy check needed.
         ObjectAssign(meta.context, context);
       }
 
       const output = await hook(arg0, meta.context, nextNextHook);
-
       validateOutput(outputErrIdentifier, output);
 
       if (output?.shortCircuit === true) { meta.shortCircuited = true; }

lib/internal/modules/esm/initialize_import_meta.js

@@ -14,7 +14,7 @@ function createImportMetaResolve(defaultParentUrl, loader) {
     let url;
 
     try {
-      ({ url } = loader.resolve(specifier, parentUrl));
+      ({ url } = loader.resolveSync(specifier, parentUrl));
     } catch (error) {
       if (error?.code === 'ERR_UNSUPPORTED_DIR_IMPORT') {
         ({ url } = error);
@@ -38,7 +38,7 @@ function initializeImportMeta(meta, context, loader) {
   const { url } = context;
 
   // Alphabetical
-  if (experimentalImportMetaResolve && loader.loaderType !== 'internal') {
+  if (experimentalImportMetaResolve && loader.allowImportMetaResolve) {
     meta.resolve = createImportMetaResolve(url, loader);
   }