Return undefined if the found node_modules directory is not writa…

…ble (#19)

index.js

@@ -1,9 +1,19 @@
 'use strict';
 const path = require('path');
+const fs = require('fs');
 const commonDir = require('commondir');
 const pkgDir = require('pkg-dir');
 const makeDir = require('make-dir');
 
+const isWritable = path => {
+	try {
+		fs.accessSync(path, fs.constants.W_OK);
+		return true;
+	} catch (_) {
+		return false;
+	}
+};
+
 module.exports = (options = {}) => {
 	const {name} = options;
 	let directory = options.cwd;
@@ -17,6 +27,10 @@ module.exports = (options = {}) => {
 	directory = pkgDir.sync(directory);
 
 	if (directory) {
+		if (!isWritable(path.join(directory, 'node_modules'))) {
+			return undefined;
+		}
+
 		directory = path.join(directory, 'node_modules', '.cache', name);
 
 		if (directory && options.create) {

readme.md

@@ -45,7 +45,7 @@ findCacheDir({name: 'unicorns'});
 
 ### findCacheDir(options?)
 
-Finds the cache directory using the supplied options. The algorithm tries to find a `package.json` file, searching every parent directory of the `cwd` specified (or implied from other options). It returns a `string` containing the absolute path to the cache directory, or `undefined` if `package.json` was never found.
+Finds the cache directory using the supplied options. The algorithm tries to find a `package.json` file, searching every parent directory of the `cwd` specified (or implied from other options). It returns a `string` containing the absolute path to the cache directory, or `undefined` if `package.json` was never found or if the `node_modules` directory is unwritable.
 
 #### options