module: implement "exports" proposal for CommonJS

Refs: hybrist/proposal-pkg-exports#36
Refs: nodejs#28568

Author: hybrist

doc/api/errors.md

@@ -1585,6 +1585,13 @@ compiled with ICU support.
 
 A given value is out of the accepted range.
 
+<a id="ERR_PATH_NOT_EXPORTED"></a>
+### ERR_PATH_NOT_EXPORTED
+
+> Stability: 1 - Experimental
+
+An attempt was made to load a protected path from a package using `exports`.
+
 <a id="ERR_REQUIRE_ESM"></a>
 ### ERR_REQUIRE_ESM
 

doc/api/modules.md

@@ -202,6 +202,35 @@ NODE_MODULES_PATHS(START)
 5. return DIRS
 ```
 
+If `--experimental-exports` is enabled,
+node allows packages loaded via `LOAD_NODE_MODULES` to explicitly declare
+which filepaths to expose and how they should be interpreted.
+This expands on the control packages already had using the `main` field.
+With this feature enabled, the `LOAD_NODE_MODULES` changes as follows:
+
+```txt
+LOAD_NODE_MODULES(X, START)
+1. let DIRS = NODE_MODULES_PATHS(START)
+2. for each DIR in DIRS:
+   a. let FILE_PATH = RESOLVE_BARE_SPECIFIER(DIR, X)
+   a. LOAD_AS_FILE(FILE_PATH)
+   b. LOAD_AS_DIRECTORY(FILE_PATH)
+
+RESOLVE_BARE_SPECIFIER(DIR, X)
+1. Try to interpret X as a combination of name and subpath where the name
+   may have a @scope/ prefix and the subpath begins with a slash (`/`).
+2. If X matches this pattern and DIR/name/package.json is a file:
+   a. Parse DIR/name/package.json, and look for "exports" field.
+   b. If "exports" is null or undefined, GOTO 3.
+   c. Find the longest key in "exports" that the subpath starts with.
+   d. If no such key can be found, throw "not exported".
+   e. If the key matches the subpath entirely, return DIR/name/${exports[key]}.
+   f. If either the key or exports[key] do not end with a slash (`/`),
+      throw "not exported".
+   g. Return DIR/name/${exports[key]}${subpath.slice(key.length)}.
+3. return DIR/X
+```
+
 ## Caching
 
 <!--type=misc-->

lib/internal/errors.js

@@ -1098,6 +1098,8 @@ E('ERR_OUT_OF_RANGE',
     msg += ` It must be ${range}. Received ${received}`;
     return msg;
   }, RangeError);
+E('ERR_PATH_NOT_EXPORTED',
+  'Package exports for \'%s\' do not define a \'%s\' subpath', Error);
 E('ERR_REQUIRE_ESM', 'Must use import to load ES Module: %s', Error);
 E('ERR_SCRIPT_EXECUTION_INTERRUPTED',
   'Script execution was interrupted by `SIGINT`', Error);

lib/internal/modules/cjs/loader.js

@@ -53,10 +53,12 @@ const { compileFunction } = internalBinding('contextify');
 const {
   ERR_INVALID_ARG_VALUE,
   ERR_INVALID_OPT_VALUE,
+  ERR_PATH_NOT_EXPORTED,
   ERR_REQUIRE_ESM
 } = require('internal/errors').codes;
 const { validateString } = require('internal/validators');
 const pendingDeprecation = getOptionValue('--pending-deprecation');
+const experimentalExports = getOptionValue('--experimental-exports');
 
 module.exports = { wrapSafe, Module };
 
@@ -182,12 +184,10 @@ Module._debug = deprecate(debug, 'Module._debug is deprecated.', 'DEP0077');
 
 // Check if the directory is a package.json dir.
 const packageMainCache = Object.create(null);
+// Explicit exports from package.json files
+const packageExportsCache = new Map();
 
-function readPackage(requestPath) {
-  const entry = packageMainCache[requestPath];
-  if (entry)
-    return entry;
-
+function readPackageRaw(requestPath) {
   const jsonPath = path.resolve(requestPath, 'package.json');
   const json = internalModuleReadJSON(path.toNamespacedPath(jsonPath));
 
@@ -201,14 +201,44 @@ function readPackage(requestPath) {
   }
 
   try {
-    return packageMainCache[requestPath] = JSON.parse(json).main;
+    const parsed = JSON.parse(json);
+    packageMainCache[requestPath] = parsed.main;
+    if (experimentalExports) {
+      packageExportsCache.set(requestPath, parsed.exports);
+    }
+    return parsed;
   } catch (e) {
     e.path = jsonPath;
     e.message = 'Error parsing ' + jsonPath + ': ' + e.message;
     throw e;
   }
 }
 
+function readPackage(requestPath) {
+  const entry = packageMainCache[requestPath];
+  if (entry)
+    return entry;
+
+  const pkg = readPackageRaw(requestPath);
+  if (pkg === false) return false;
+
+  return pkg.main;
+}
+
+function readExports(requestPath) {
+  if (packageExportsCache.has(requestPath)) {
+    return packageExportsCache.get(requestPath);
+  }
+
+  const pkg = readPackageRaw(requestPath);
+  if (!pkg) {
+    packageExportsCache.set(requestPath, null);
+    return null;
+  }
+
+  return pkg.exports;
+}
+
 function tryPackage(requestPath, exts, isMain, originalPath) {
   const pkg = readPackage(requestPath);
 
@@ -297,8 +327,58 @@ function findLongestRegisteredExtension(filename) {
   return '.js';
 }
 
+// This only applies to requests of a specific form:
+// 1. name/.*
+// 2. @scope/name/.*
+const EXPORTS_PATTERN = /^((?:@[^./@\\][^/@\\]*\/)?[^@./\\][^/\\]*)(\/.*)$/;
+function resolveExports(nmPath, request, absoluteRequest) {
+  // The implementation's behavior is meant to mirror resolution in ESM.
+  if (experimentalExports && !absoluteRequest) {
+    const [, name, expansion] =
+      request.match(EXPORTS_PATTERN) || [];
+    if (!name) {
+      return path.resolve(nmPath, request);
+    }
+
+    const basePath = path.resolve(nmPath, name);
+    const pkgExports = readExports(basePath);
+
+    if (pkgExports != null) {
+      const mappingKey = `.${expansion}`;
+      const mapping = pkgExports[mappingKey];
+      if (typeof mapping === 'string') {
+        return path.resolve(basePath, mapping);
+      }
+
+      let dirMatch = '';
+      for (const [candidateKey, candidateValue] of Object.entries(pkgExports)) {
+        if (candidateKey[candidateKey.length - 1] !== '/') continue;
+        if (candidateValue[candidateValue.length - 1] !== '/') continue;
+        if (candidateKey.length > dirMatch.length &&
+            mappingKey.startsWith(candidateKey)) {
+          dirMatch = candidateKey;
+        }
+      }
+
+      if (dirMatch !== '') {
+        const dirMapping = pkgExports[dirMatch];
+        const remainder = mappingKey.slice(dirMatch.length);
+        const expectedPrefix = path.resolve(basePath, dirMapping);
+        const resolved = path.resolve(expectedPrefix, remainder);
+        if (resolved.startsWith(expectedPrefix)) {
+          return resolved;
+        }
+      }
+      throw new ERR_PATH_NOT_EXPORTED(basePath, mappingKey);
+    }
+  }
+
+  return path.resolve(nmPath, request);
+}
+
 Module._findPath = function(request, paths, isMain) {
-  if (path.isAbsolute(request)) {
+  const absoluteRequest = path.isAbsolute(request);
+  if (absoluteRequest) {
     paths = [''];
   } else if (!paths || paths.length === 0) {
     return false;
@@ -322,7 +402,7 @@ Module._findPath = function(request, paths, isMain) {
     // Don't search further if path doesn't exist
     const curPath = paths[i];
     if (curPath && stat(curPath) < 1) continue;
-    var basePath = path.resolve(curPath, request);
+    var basePath = resolveExports(curPath, request, absoluteRequest);
     var filename;
 
     var rc = stat(basePath);

src/module_wrap.cc

@@ -856,7 +856,7 @@ Maybe<URL> PackageExportsResolve(Environment* env,
   std::string msg = "Package exports for '" +
       URL(".", pjson_url).ToFilePath() + "' do not define a '" + pkg_subpath +
       "' subpath, imported from " + base.ToFilePath();
-  node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
+  node::THROW_ERR_PATH_NOT_EXPORTED(env, msg.c_str());
   return Nothing<URL>();
 }
 

src/node_errors.h

@@ -53,6 +53,7 @@ void PrintErrorString(const char* format, ...);
   V(ERR_MISSING_PLATFORM_FOR_WORKER, Error)                                  \
   V(ERR_MODULE_NOT_FOUND, Error)                                             \
   V(ERR_OUT_OF_RANGE, RangeError)                                            \
+  V(ERR_PATH_NOT_EXPORTED, Error)                                            \
   V(ERR_SCRIPT_EXECUTION_INTERRUPTED, Error)                                 \
   V(ERR_SCRIPT_EXECUTION_TIMEOUT, Error)                                     \
   V(ERR_STRING_TOO_LONG, Error)                                              \

src/node_file.cc

@@ -872,7 +872,9 @@ static void InternalModuleReadJSON(const FunctionCallbackInfo<Value>& args) {
   }
 
   const size_t size = offset - start;
-  if (size == 0 || size == SearchString(&chars[start], size, "\"main\"")) {
+  if (size == 0 || (
+    size == SearchString(&chars[start], size, "\"main\"") &&
+    size == SearchString(&chars[start], size, "\"exports\""))) {
     return;
   } else {
     Local<String> chars_string =

src/node_options.cc

@@ -319,6 +319,7 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             "experimental ES Module support and caching modules",
             &EnvironmentOptions::experimental_modules,
             kAllowedInEnvironment);
+  Implies("--experimental-modules", "--experimental-exports");
   AddOption("--experimental-wasm-modules",
             "experimental ES Module support for webassembly modules",
             &EnvironmentOptions::experimental_wasm_modules,

test/es-module/test-esm-exports.mjs

@@ -1,4 +1,4 @@
-// Flags: --experimental-modules --experimental-exports
+// Flags: --experimental-modules
 
 import { mustCall } from '../common/index.mjs';
 import { ok, strictEqual } from 'assert';