Resolve symlinked modules correctly (#92)

Some systems (e.g. rush, pnpm) use symlinks to create a recursive dependency graph
instead of relying on the hoisting aspect of the node module resolution algorithm (like npm and yarn do)
in these cases, we want to resolve to the real path of a module, so that the tree structure below
only ever tries to run the `x` module once (rather than once on each module that depends on it)

- node_modules
  - a
    - node_modules
      - symlink to x
  - b
    - node_modules
      - symlink to x

Co-authored-by: Ryan Tsao <ryan.j.tsao@gmail.com>

.travis.yml

@@ -1,7 +1,5 @@
 language: node_js
 node_js:
-    - "0.8"
-    - "0.10"
-    - "4"
-    - "5"
-    - "6"
+  - "8"
+  - "10"
+  - "12"

index.js

@@ -206,7 +206,7 @@ function build_resolve_opts(opts, base) {
     return opts;
 }
 
-function resolve(id, opts, cb) {
+function resolve(id, opts, callback) {
 
     // opts.filename
     // opts.paths
@@ -216,6 +216,24 @@ function resolve(id, opts, cb) {
     opts = opts || {};
     opts.filename = opts.filename || '';
 
+    var cb = function(err, path, pkg) {
+        fs.stat(path, function(notPath) {
+            if (notPath) {
+                callback(err, path, pkg);
+            }
+            else {
+                fs.realpath(path, function(notReal, real) {
+                    if (notReal) {
+                        callback(err, path, pkg);
+                    }
+                    else {
+                        callback(err, real, pkg);
+                    }
+                });
+            }
+        });
+    }
+
     var base = path.dirname(opts.filename);
 
     if (opts.basedir) {

package.json

@@ -8,7 +8,7 @@
     "empty.js"
   ],
   "scripts": {
-    "test": "mocha --reporter list test/*.js"
+    "test": "node scripts/setup-symlinks.js && mocha --reporter list test/*.js"
   },
   "repository": {
     "type": "git",

scripts/setup-symlinks.js

@@ -0,0 +1,10 @@
+var fs = require('fs');
+
+try {
+  fs.mkdirSync(__dirname + '/../test/fixtures/node_modules/linker/node_modules');
+} catch (e) {}
+process.chdir(__dirname + '/../test/fixtures/node_modules/linker/node_modules');
+try {
+  fs.unlinkSync('linked');
+} catch (e) {}
+fs.symlinkSync('../../../linked', 'linked', 'dir');

test/fixtures/linked/index.js

@@ -0,0 +1 @@
+// dummy

test/modules.js

@@ -319,3 +319,25 @@ test('not fail on accessing path name defined in Object.prototype', function (do
         done();
     });
 });
+
+test('respect symlinks', function (done) {
+    // some systems (e.g. rush, pnpm) use symlinks to create a recursive dependency graph
+    // instead of relying on the hoisting aspect of the node module resolution algorithm (like npm and yarn do)
+    // in these cases, we want to resolve to the real path of a module, so that the tree structure below
+    // only ever tries to run the `x` module once (rather than once on each module that depends on it)
+    //
+    // - node_modules
+    //   - a
+    //     - node_modules
+    //       - symlink to x
+    //   - b
+    //     - node_modules
+    //       - symlink to x
+    //
+    resolve('linked', { paths: [ fixtures_dir + '/linker/node_modules' ], package: { main: 'fixtures' } }, function(err, path, pkg) {
+        assert.ifError(err);
+        assert.equal(path, require.resolve('./fixtures/linked/index'));
+        assert.strictEqual(pkg, undefined);
+        done();
+    });
+});