module: remove async module customization hooks

Remove `module.register()` (DEP0205), the `--experimental-loader` and
`--loader` flags, and the machinery that supported them: the loader
hooks worker thread, the hooks proxy, the async module customization
hook chains for resolving and loading, the internal worker class used to
run the loader thread, and the asynchronous module job linking. Module
customization is now provided only by `module.registerHooks()`, which
runs synchronously on the thread that loads the modules.

As the loader thread no longer exists, the ES module loader always loads
and links modules synchronously, so the code that handled promises and
the re-invented `require()` for imported CommonJS customized by the
async module customization hooks is removed too. `import()` still defers
evaluation of the module to a later microtask, as before.

`worker_threads.isInternalThread` is kept for compatibility and is now
always `false`; removing it is left for a future semver-major change.

The `ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED` and
`ERR_LOADER_CHAIN_INCOMPLETE` error codes are no longer used and are
documented as removed.

Tests that exist only to exercise the async module customization hooks
are removed.

Assisted-by: Claude
Signed-off-by: Geoffrey Booth <webadmin@geoffreybooth.com>

Author: GeoffreyBooth

doc/api/cli.md

@@ -1515,33 +1515,6 @@ added:
 
 Enable experimental support for inspector network resources.
 
-### `--experimental-loader=module`
-
-<!-- YAML
-added: v8.8.0
-changes:
-  - version:
-    - v23.6.1
-    - v22.13.1
-    - v20.18.2
-    pr-url: https://github.com/nodejs-private/node-private/pull/629
-    description: Using this feature with the permission model enabled requires
-                 passing `--allow-worker`.
-  - version: v12.11.1
-    pr-url: https://github.com/nodejs/node/pull/29752
-    description: This flag was renamed from `--loader` to
-                 `--experimental-loader`.
--->
-
-> This flag is discouraged and may be removed in a future version of Node.js.
-> Please use
-> [`--import` with `register()`][preloading asynchronous module customization hooks] instead.
-
-Specify the `module` containing exported [asynchronous module customization hooks][].
-`module` may be any string accepted as an [`import` specifier][].
-
-This feature requires `--allow-worker` if used with the [Permission Model][].
-
 ### `--experimental-network-inspection`
 
 <!-- YAML
@@ -4312,7 +4285,6 @@ one is included in the list below.
 * `--experimental-import-meta-resolve`
 * `--experimental-import-text`
 * `--experimental-json-modules`
-* `--experimental-loader`
 * `--experimental-modules`
 * `--experimental-package-map`
 * `--experimental-print-required-tla`
@@ -4996,7 +4968,6 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [`dns.setDefaultResultOrder()`]: dns.md#dnssetdefaultresultorderorder
 [`dnsPromises.lookup()`]: dns.md#dnspromiseslookuphostname-options
 [`import.meta.url`]: esm.md#importmetaurl
-[`import` specifier]: esm.md#import-specifiers
 [`inspector.open()`]: inspector.md#inspectoropenport-host-wait
 [`net.getDefaultAutoSelectFamilyAttemptTimeout()`]: net.md#netgetdefaultautoselectfamilyattempttimeout
 [`node:ffi`]: ffi.md
@@ -5013,7 +4984,6 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [`v8.startupSnapshot.setDeserializeMainFunction()`]: v8.md#v8startupsnapshotsetdeserializemainfunctioncallback-data
 [`v8.startupSnapshot` API]: v8.md#startup-snapshot-api
 [`vfs.mount()`]: vfs.md#vfsmount
-[asynchronous module customization hooks]: module.md#asynchronous-customization-hooks
 [benchmark runner]: bench.md#command-line-runner
 [captured by the built-in snapshot of Node.js]: https://github.com/nodejs/node/blob/b19525a33cc84033af4addd0f80acd4dc33ce0cf/test/parallel/test-bootstrap-modules.js#L24
 [collecting code coverage from tests]: test.md#collecting-code-coverage
@@ -5031,7 +5001,6 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [jitless]: https://v8.dev/blog/jitless
 [libuv threadpool documentation]: https://docs.libuv.org/en/latest/threadpool.html
 [module compile cache]: module.md#module-compile-cache
-[preloading asynchronous module customization hooks]: module.md#registration-of-asynchronous-customization-hooks
 [randomizing tests execution order]: test.md#randomizing-tests-execution-order
 [remote code execution]: https://www.owasp.org/index.php/Code_Injection
 [running tests from the command line]: test.md#running-tests-from-the-command-line

doc/api/deprecations.md

@@ -4615,6 +4615,9 @@ no longer supported.
 
 <!-- YAML
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/67090
+    description: End-of-Life.
   - version: v26.0.0
     pr-url: https://github.com/nodejs/node/pull/62401
     description: Runtime deprecation.
@@ -4625,18 +4628,18 @@ changes:
     description: Documentation-only deprecation.
 -->
 
-Type: Runtime
+Type: End-of-Life
 
-[`module.register()`][] is deprecated. Use [`module.registerHooks()`][]
-instead.
+The `module.register()` API and the `--experimental-loader` and `--loader`
+flags have been removed. Use [`module.registerHooks()`][] instead.
 
-The `module.register()` API provides off-thread async hooks for customizing ES modules;
-the `module.registerHooks()` API provides similar hooks that are synchronous, in-thread, and
-work for all types of modules.
-Supporting async hooks has proven to be complex, involving worker threads orchestration, and there are issues
-that have proven unresolvable. See [caveats of asynchronous customization hooks][]. Please migrate to
-`module.registerHooks()` as soon as possible as `module.register()` will be
-removed in a future version of Node.js.
+The `module.register()` API provided off-thread async module customization
+hooks for customizing ES modules. The `module.registerHooks()` API provides
+similar hooks that are synchronous, in-thread and work for all types of modules.
+To continue running async module customization off-thread, users can spawn their
+own threads to run the async customizations, communicate from the in-thread
+synchronous hooks and block for the results on the main thread, similar to how
+`module.register()` worked internally.
 
 ### DEP0206: Calling `digest()` on an already-finalized `Hmac` instance
 
@@ -4935,7 +4938,6 @@ Type: Documentation-only
 [`message.trailersDistinct`]: http.md#messagetrailersdistinct
 [`message.trailers`]: http.md#messagetrailers
 [`module.createRequire()`]: module.md#modulecreaterequirefilename
-[`module.register()`]: module.md#moduleregisterspecifier-parenturl-options
 [`module.registerHooks()`]: module.md#moduleregisterhooksoptions
 [`os.networkInterfaces()`]: os.md#osnetworkinterfaces
 [`os.tmpdir()`]: os.md#ostmpdir
@@ -4991,7 +4993,6 @@ Type: Documentation-only
 [`zlib.bytesWritten`]: zlib.md#zlibbyteswritten
 [alloc]: buffer.md#static-method-bufferallocsize-fill-encoding
 [alloc_unsafe_size]: buffer.md#static-method-bufferallocunsafesize-alignment
-[caveats of asynchronous customization hooks]: module.md#caveats-of-asynchronous-customization-hooks
 [from_arraybuffer]: buffer.md#static-method-bufferfromarraybuffer-byteoffset-length
 [from_string_encoding]: buffer.md#static-method-bufferfromstring-encoding
 [legacy URL API]: url.md#legacy-url-api

doc/api/errors.md

@@ -733,13 +733,6 @@ by the `node:assert` module.
 An attempt was made to register something that is not a function as an
 `AsyncHooks` callback.
 
-<a id="ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED"></a>
-
-### `ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED`
-
-An operation related to module loading is customized by an asynchronous loader
-hook that never settled the promise before the loader thread exits.
-
 <a id="ERR_ASYNC_RESOURCE_DOMAIN_REMOVED"></a>
 
 ### `ERR_ASYNC_RESOURCE_DOMAIN_REMOVED`
@@ -2364,19 +2357,6 @@ for more information.
 
 IP is blocked by `net.BlockList`.
 
-<a id="ERR_LOADER_CHAIN_INCOMPLETE"></a>
-
-### `ERR_LOADER_CHAIN_INCOMPLETE`
-
-<!-- YAML
-added:
-  - v18.6.0
-  - v16.17.0
--->
-
-An ESM loader hook returned without calling `next()` and without explicitly
-signaling a short circuit.
-
 <a id="ERR_LOAD_SQLITE_EXTENSION"></a>
 
 ### `ERR_LOAD_SQLITE_EXTENSION`
@@ -3925,6 +3905,17 @@ attempting a [`require()`][] operation or when loading the program entry point.
 > Stability: 0 - Deprecated. These error codes are either inconsistent, or have
 > been removed.
 
+<a id="ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED"></a>
+
+### `ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED`
+
+<!-- YAML
+removed: REPLACEME
+-->
+
+An operation related to module loading is customized by an asynchronous loader
+hook that never settled the promise before the loader thread exits.
+
 <a id="ERR_CANNOT_TRANSFER_OBJECT"></a>
 
 ### `ERR_CANNOT_TRANSFER_OBJECT`
@@ -4163,6 +4154,20 @@ changes:
 
 An invalid transfer object was passed to `postMessage()`.
 
+<a id="ERR_LOADER_CHAIN_INCOMPLETE"></a>
+
+### `ERR_LOADER_CHAIN_INCOMPLETE`
+
+<!-- YAML
+added:
+  - v18.6.0
+  - v16.17.0
+removed: REPLACEME
+-->
+
+An ESM loader hook returned without calling `next()` and without explicitly
+signaling a short circuit.
+
 <a id="ERR_MANIFEST_ASSERT_INTEGRITY"></a>
 
 ### `ERR_MANIFEST_ASSERT_INTEGRITY`

doc/api/esm.md

@@ -199,8 +199,7 @@ ES modules are resolved and cached as URLs. This means that special characters
 must be [percent-encoded][], such as `#` with `%23` and `?` with `%3F`.
 
 `file:`, `node:`, and `data:` URL schemes are supported. A specifier like
-`'https://example.com/app.js'` is not supported natively in Node.js unless using
-a [custom HTTPS loader][].
+`'https://example.com/app.js'` is not supported natively in Node.js.
 
 #### `file:` URLs
 
@@ -493,8 +492,6 @@ resolutions are subject to the permitted exports resolutions within the package.
 
 * This can result in synchronous file-system operations, which
   can impact performance similarly to `require.resolve`.
-* This feature is not available within custom loaders (it would
-  create a deadlock).
 
 **Non-standard API**:
 
@@ -1316,8 +1313,8 @@ _isImports_, _conditions_)
 ### Customizing ESM specifier resolution algorithm
 
 [Module customization hooks][] provide a mechanism for customizing the ESM
-specifier resolution algorithm. An example that provides CommonJS-style
-resolution for ESM specifiers is [commonjs-extension-resolution-loader][].
+specifier resolution algorithm, for example to provide CommonJS-style
+resolution for ESM specifiers.
 
 <!-- Note: The merve link should be kept in-sync with the deps version -->
 
@@ -1360,8 +1357,6 @@ resolution for ESM specifiers is [commonjs-extension-resolution-loader][].
 [`process.dlopen`]: process.md#processdlopenmodule-filename-flags
 [`require(esm)`]: modules.md#loading-ecmascript-modules-using-require
 [`url.fileURLToPath()`]: url.md#urlfileurltopathurl-options
-[commonjs-extension-resolution-loader]: https://github.com/nodejs/loaders-test/tree/main/commonjs-extension-resolution-loader
-[custom https loader]: module.md#import-from-https
 [import.meta.resolve]: #importmetaresolvespecifier
 [merve]: https://github.com/anonrig/merve/tree/v1.0.0
 [percent-encoded]: url.md#percent-encoding-in-urls