Bug 1409876 - Refuse to perform a VCS checkout into a cache root; r=dustin

This seems like a reasonable thing to enforce.

MozReview-Commit-ID: 3BZQSkwRYeN

--HG--
extra : rebase_source : 8dae62edb35202da0f0e90ddec3eacb212ada371

taskcluster/docker/recipes/run-task

@@ -471,6 +471,14 @@ def main(args):
         if not checkout:
             return
 
+        # The checkout path becomes the working directory. Since there are
+        # special cache files in the cache's root directory and working
+        # directory purging could blow them away, disallow this scenario.
+        if os.path.exists(os.path.join(checkout, '.cacherequires')):
+            print('error: cannot perform vcs checkout into cache root: %s' %
+                  checkout)
+            sys.exit(1)
+
         # Ensure the directory for the source checkout exists.
         try:
             os.makedirs(os.path.dirname(checkout))