Add cache-files-ttl setting, and docs for the cache

doc/04-schema.md

@@ -596,6 +596,10 @@ The following options are supported:
 * **notify-on-install:** Defaults to `true`. Composer allows repositories to
   define a notification URL, so that they get notified whenever a package from
   that repository is installed. This option allows you to disable that behaviour.
+* **cache-files-ttl:** Defaults to `15552000` (6 months). Composer caches all
+  dist (zip, tar, ..) packages that it downloads. Those are purged after six
+  months of being unused by default. This option allows you to tweak this
+  duration (in seconds) or disable it completely by setting it to 0.
 
 Example:
 

src/Composer/Command/ConfigCommand.php

@@ -184,6 +184,8 @@ EOT
         // handle config values
         $uniqueConfigValues = array(
             'process-timeout' => array('is_numeric', 'intval'),
+            'cache-ttl' => array('is_numeric', 'intval'),
+            'cache-files-ttl' => array('is_numeric', 'intval'),
             'vendor-dir' => array('is_string', function ($val) { return $val; }),
             'bin-dir' => array('is_string', function ($val) { return $val; }),
             'notify-on-install' => array(

src/Composer/Config.php

@@ -120,6 +120,16 @@ class Config
 
                 return rtrim($this->process(getenv($env) ?: $this->config[$key]), '/\\');
 
+            case 'cache-ttl':
+                return (int) $this->config[$key];
+
+            case 'cache-files-ttl':
+                if (isset($this->config[$key])) {
+                    return (int) $this->config[$key];
+                }
+
+                return (int) $this->config['cache-ttl'];
+
             case 'home':
                 return rtrim($this->process($this->config[$key]), '/\\');
 

src/Composer/Factory.php

@@ -240,7 +240,10 @@ class Factory
      */
     public function createDownloadManager(IOInterface $io, Config $config)
     {
-        $cache = new Cache($io, $config->get('home').'/cache.files/', 'a-z0-9_./');
+        $cache = null;
+        if ($config->get('cache-files-ttl') > 0) {
+            $cache = new Cache($io, $config->get('home').'/cache.files/', 'a-z0-9_./');
+        }
 
         $dm = new Downloader\DownloadManager();
         $dm->setDownloader('git', new Downloader\GitDownloader($io, $config));