Make it possible to set preserve-paths in dependency extras.

README.md

@@ -16,7 +16,7 @@ Simply install the plugin with composer: `composer require drupal-composer/prese
 
 ## Configuration
 
-For configuring the paths you need to set `preserve-paths` within the `extra` of your root `composer.json`.
+For configuring the paths you need to set `preserve-paths` within the `extra` of your `composer.json`.
 
 ```json
 {
@@ -31,6 +31,24 @@ For configuring the paths you need to set `preserve-paths` within the `extra` of
 }
 ```
 
+In case you need to set the `preserve-paths` option in a dependent package, you must also set `preserve-paths-as-dependency` in the `composer.json` of the dependency.
+
+```json
+{
+    "extra": {
+        "preserve-paths-as-dependency": true,
+        "preserve-paths": [
+          "web/sites/all/modules/contrib",
+          "web/sites/all/themes/contrib",
+          "web/sites/all/libraries",
+          "web/sites/all/drush"
+        ]
+      }
+}
+```
+
+You can disable this setting in your root `composer.json` by setting `preserve-paths-ignore-dependencies` to true.
+
 ## Example
 
 An example composer.json using [composer/installers](https://packagist.org/packages/composer/installers):

src/PluginWrapper.php

@@ -64,7 +64,7 @@ public function prePackage(PackageEvent $event)
 
         $preserver = new PathPreserver(
             $paths,
-            $this->getPreservePaths(),
+            $this->getPreservePaths($packages),
             $this->composer->getConfig()->get('cache-dir'),
             $this->filesystem,
             $this->io
@@ -158,13 +158,47 @@ protected function getUniqueNameFromPackages(array $packages)
     }
 
     /**
-     * Get preserve paths from root configuration.
+     * Get preserve paths.
+     *
+     * @param \Composer\Package\PackageInterface[] $packages
      *
      * @return string[]
      */
-    protected function getPreservePaths()
+    protected function getPreservePaths($packages)
     {
         $extra = $this->composer->getPackage()->getExtra();
+        $paths = array();
+
+        /*
+         * Root package allows that preserve paths from dependencies are also evaluated.
+         * Negative check as this feature is developed for situations where the root package
+         * can not be adjusted, but paths must be preserved.
+         */
+        if (!isset($extra['preserve-paths-ignore-dependencies']) || !$extra['preserve-paths-ignore-dependencies']) {
+            foreach ($packages as $package) {
+                $paths = array_merge($paths, $this->extractPathsFromExtra($package->getExtra(), false));
+            }
+        }
+
+        $paths = array_unique(array_merge($paths, $this->extractPathsFromExtra($extra)));
+
+        return $this->absolutePaths($paths);
+    }
+
+    /**
+     * Extract the paths from the extra data.
+     *
+     * @param array $extra
+     * @param boolean $root
+     *
+     * @return string[]
+     */
+    protected function extractPathsFromExtra($extra, $root = true)
+    {
+        // Package does not explicitly allow to preserve paths if used as dependency.
+        if (!$root && (!isset($extra['preserve-paths-as-dependency']) || $extra['preserve-paths-as-dependency'] != true)) {
+            return array();
+        }
 
         if (!isset($extra['preserve-paths'])) {
             $paths = array();
@@ -174,7 +208,7 @@ protected function getPreservePaths()
             $paths = array_values((array) $extra['preserve-paths']);
         }
 
-        return $this->absolutePaths($paths);
+        return $paths;
     }
 
     /**