Add hash-based staged rollout to build executors

Fixes #1882

buildman/manager/executor.py

@@ -9,6 +9,7 @@ import trollius
 import datetime
 import release
 import socket
+import hashlib
 
 from jinja2 import FileSystemLoader, Environment
 from trollius import coroutine, From, Return, get_event_loop
@@ -68,11 +69,21 @@ class BuilderExecutor(object):
 
   def allowed_for_namespace(self, namespace):
     """ Returns true if this executor can be used for builds in the given namespace. """
-    namespace_whitelist = self.executor_config.get('NAMESPACE_WHITELIST')
-    if namespace_whitelist is not None:
-      return namespace in namespace_whitelist
 
-    return True
+    # Check for an explicit namespace whitelist.
+    namespace_whitelist = self.executor_config.get('NAMESPACE_WHITELIST')
+    if namespace_whitelist is not None and namespace in namespace_whitelist:
+      return True
+
+    # Check for a staged rollout percentage. If found, we hash the namespace and, if it is found
+    # in the first X% of the character space, we allow this executor to be used.
+    staged_rollout = self.executor_config.get('STAGED_ROLLOUT')
+    if staged_rollout is not None:
+      bucket = int(hashlib.sha256(namespace).hexdigest()[-2:], 16)
+      return bucket < (256 * staged_rollout)
+
+    # If there are no restrictions in place, we are free to use this executor.
+    return staged_rollout is None and namespace_whitelist is None
 
   @property
   def minimum_retry_threshold(self):