Report errors properly when git driver fails to write in the cache, refs #482

src/Composer/Repository/Vcs/GitDriver.php

@@ -38,6 +38,13 @@ class GitDriver extends VcsDriver
         } else {
             $this->repoDir = $this->config->get('home') . '/cache.git/' . preg_replace('{[^a-z0-9.]}i', '-', $this->url) . '/';
 
+            $fs = new Filesystem();
+            $fs->ensureDirectoryExists(dirname($this->repoDir));
+
+            if (!is_writable(dirname($this->repoDir))) {
+                throw new \RuntimeException('Can not clone '.$this->url.' to access package information. The "'.dirname($this->repoDir).'" directory is not writable by the current user.');
+            }
+
             // update the repo if it is a valid git repository
             if (is_dir($this->repoDir) && 0 === $this->process->execute('git remote', $output, $this->repoDir)) {
                 if (0 !== $this->process->execute('git remote update --prune origin', $output, $this->repoDir)) {
@@ -45,7 +52,6 @@ class GitDriver extends VcsDriver
                 }
             } else {
                 // clean up directory and do a fresh clone into it
-                $fs = new Filesystem();
                 $fs->removeDirectory($this->repoDir);
 
                 // added in git 1.7.1, prevents prompting the user