cros_sdk: remove old SDK tarballs

BUG=chromium-os:27785
TEST=cros_sdk # see old tarballs pwned

Change-Id: I13a7b76f116aa2d9649542fa215f14b5e1a53603
Reviewed-on: https://gerrit.chromium.org/gerrit/18316
Tested-by: Zdenek Behan <zbehan@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index 7c91ecd..c9276a3 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -85,8 +85,18 @@
 
 def FetchRemoteTarball(url):
   """Fetches a tarball given by url, and place it in sdk/."""
-  tarball_dest = os.path.join(SDK_DIR,
-      os.path.basename(urlparse.urlparse(url).path))
+  tarball_name = os.path.basename(urlparse.urlparse(url).path)
+  tarball_dest = os.path.join(SDK_DIR, tarball_name)
+
+  # Cleanup old tarballs.
+  files_to_delete = [f for f in os.listdir(SDK_DIR) if f != tarball_name]
+  if files_to_delete:
+    print 'Cleaning up old tarballs: ' + str(files_to_delete)
+    for f in files_to_delete:
+      f_path = os.path.join(SDK_DIR, f)
+      # Only delete regular files that belong to us.
+      if os.path.isfile(f_path) and os.stat(f_path).st_uid == os.getuid():
+        os.remove(f_path)
 
   print 'Downloading sdk: "%s"' % url
   cmd = ['curl', '-f', '--retry', '5', '-L', '-y', '30',