Refactor how symlinks are discovered in local-cli, support scoped modules

Summary:
This PR refactors the symlink finding logic in local-cli in order to support nested symlinked modules as well as symlinked scoped NPM modules.

Run tests, or try project with `npm link`'ed or `yarn link`'ed dependencies.
Closes https://github.com/facebook/react-native/pull/15776

Reviewed By: cpojer

Differential Revision: D5823008

Pulled By: jeanlauliac

fbshipit-source-id: f2daeceef37bed2f8a136a0a5918730f9832884c

local-cli/util/findSymlinkedModules.js

@@ -0,0 +1,110 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @format
+ * @flow
+ */
+
+const path = require('path');
+const fs = require('fs');
+
+/**
+ * Find symlinked modules inside "node_modules."
+ *
+ * Naively, we could just perform a depth-first search of all folders in
+ * node_modules, recursing when we find a symlink.
+ *
+ * We can be smarter than this due to our knowledge of how npm/Yarn lays out
+ * "node_modules" / how tools that build on top of npm/Yarn (such as Lerna)
+ * install dependencies.
+ *
+ * Starting from a given root node_modules folder, this algorithm will look at
+ * both the top level descendants of the node_modules folder or second level
+ * descendants of folders that start with "@" (which indicates a scoped
+ * package). If any of those folders is a symlink, it will recurse into the
+ * link, and perform the same search in the linked folder.
+ *
+ * The end result should be a list of all resolved module symlinks for a given
+ * root.
+ */
+module.exports = function findSymlinkedModules(
+  projectRoot: string,
+  ignoredRoots?: Array<string> = [],
+) {
+  const timeStart = Date.now();
+  const nodeModuleRoot = path.join(projectRoot, 'node_modules');
+  const resolvedSymlinks = findModuleSymlinks(nodeModuleRoot, [
+    ...ignoredRoots,
+    projectRoot,
+  ]);
+  const timeEnd = Date.now();
+
+  console.log(
+    `Scanning folders for symlinks in ${nodeModuleRoot} (${timeEnd -
+      timeStart}ms)`,
+  );
+
+  return resolvedSymlinks;
+};
+
+function findModuleSymlinks(
+  modulesPath: string,
+  ignoredPaths: Array<string> = [],
+): Array<string> {
+  if (!fs.existsSync(modulesPath)) {
+    return [];
+  }
+
+  // Find module symlinks
+  const moduleFolders = fs.readdirSync(modulesPath);
+  const symlinks = moduleFolders.reduce((links, folderName) => {
+    const folderPath = path.join(modulesPath, folderName);
+    const maybeSymlinkPaths = [];
+    if (folderName.startsWith('@')) {
+      const scopedModuleFolders = fs.readdirSync(folderPath);
+      maybeSymlinkPaths.push(
+        ...scopedModuleFolders.map(name => path.join(folderPath, name)),
+      );
+    } else {
+      maybeSymlinkPaths.push(folderPath);
+    }
+    return links.concat(resolveSymlinkPaths(maybeSymlinkPaths, ignoredPaths));
+  }, []);
+
+  // For any symlinks found, look in _that_ modules node_modules directory
+  // and find any symlinked modules
+  const nestedSymlinks = symlinks.reduce(
+    (links, symlinkPath) =>
+      links.concat(
+        // We ignore any found symlinks or anything from the ignored list,
+        // to prevent infinite recursion
+        findModuleSymlinks(path.join(symlinkPath, 'node_modules'), [
+          ...ignoredPaths,
+          ...symlinks,
+        ]),
+      ),
+    [],
+  );
+
+  return [...new Set([...symlinks, ...nestedSymlinks])];
+}
+
+function resolveSymlinkPaths(maybeSymlinkPaths, ignoredPaths) {
+  return maybeSymlinkPaths.reduce((links, maybeSymlinkPath) => {
+    if (fs.lstatSync(maybeSymlinkPath).isSymbolicLink()) {
+      const resolved = path.resolve(
+        path.dirname(maybeSymlinkPath),
+        fs.readlinkSync(maybeSymlinkPath),
+      );
+      if (ignoredPaths.indexOf(resolved) === -1 && fs.existsSync(resolved)) {
+        links.push(resolved);
+      }
+    }
+    return links;
+  }, []);
+}