* added execute() wrapper to generalize command execution in downloader

 * added Composer\Util\Svn::doAuthDance() to ask for credentials in interactive sessions

src/Composer/Downloader/SvnDownloader.php

@@ -21,6 +21,11 @@ use Composer\Util\Svn as SvnUtil;
  */
 class SvnDownloader extends VcsDownloader
 {
+    /**
+     * @var bool $useAuth
+     */
+    protected $useAuth = false;
+
     /**
      * @var \Composer\Util\Svn $util
      */
@@ -39,7 +44,7 @@ class SvnDownloader extends VcsDownloader
         $command = $util->getCommand("svn co", sprintf("%s/%s", $url, $ref), $path);
 
         $this->io->write("    Checking out ".$package->getSourceReference());
-        $this->process->execute($command);
+        $this->execute($command, $util);
     }
 
     /**
@@ -54,7 +59,7 @@ class SvnDownloader extends VcsDownloader
         $command = $util->getCommand("svn switch", sprintf("%s/%s", $url, $ref));
 
         $this->io->write("    Checking out " . $ref);
-        $this->process->execute(sprintf('cd %s && %s', $path, $command));
+        $this->execute(sprintf('cd %s && %s', $path, $command), $util);
     }
 
     /**
@@ -68,6 +73,49 @@ class SvnDownloader extends VcsDownloader
         }
     }
 
+    /**
+     * Wrap {@link \Composer\Util\ProcessExecutor::execute().
+     *
+     * @param string  $cmd
+     * @param SvnUtil $util
+     *
+     * @return string
+     */
+    protected function execute($command, SvnUtil $util)
+    {
+        $status = $this->process->execute($command, $output);
+        if ($status == 0) {
+            return $output;
+        }
+
+        // this could be any failure, since SVN exits with 1 always
+
+        if (empty($output)) {
+            $output = $this->process->getErrorOutput();
+        }
+
+        if (!$this->io->isInteractive()) {
+            return $output;
+        }
+
+        // the error is not auth-related
+        if (strpos($output, 'authorization failed:') === false) {
+            return $output;
+        }
+
+        // no authorization has been detected so far
+        if (!$this->useAuth) {
+            $this->useAuth = $util->doAuthDance()->hasAuth();
+            $credentials   = $util->getCredentialString();
+
+            // restart the process
+            $output = $this->execute($command . ' ' . $credentials, $util);
+        } else {
+            $this->io->write("Authorization failed: {$command}");
+        }
+        return $output;
+    }
+
     /**
      * This is heavy - recreating Util often.
      *