[releng] block_retired: Document race condition

[Till Maas]

commit 1088dd3dd7703579b15d7862bdf84ad6d274e980
Author: Till Maas <opensource at till.name>
Date:   Fri Nov 21 23:26:36 2014 +0100

    block_retired: Document race condition

 scripts/block_retired.py |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/scripts/block_retired.py b/scripts/block_retired.py
index e6058f2..c6f6bbd 100755
--- a/scripts/block_retired.py
+++ b/scripts/block_retired.py
@@ -169,6 +169,8 @@ def block_package(packages, branch="master", staging=False):
 
     # Untag builds first due to koji/mash bug:
     # https://fedorahosted.org/koji/ticket/299
+    # FIXME: This introduces a theoretical race condition when a package is
+    # built after all builds were untagged and before the package is blocked
     if epel_build_tag:
         run_koji(["untag-build", "--all", tag] + packages)