swarming: Add server-side implementation for supplemental bot_config

appengine/swarming/server/bot_groups_config.py

Index: appengine/swarming/server/bot_groups_config.py
diff --git a/appengine/swarming/server/bot_groups_config.py b/appengine/swarming/server/bot_groups_config.py
index 962a36087159b45a6dc360a5e357410a9ba8def0..547c7c0a6ff850f6195f12f91b4d83907f617d5e 100644
--- a/appengine/swarming/server/bot_groups_config.py
+++ b/appengine/swarming/server/bot_groups_config.py
@@ -7,6 +7,7 @@
 import collections
 import hashlib
 import logging
+import os
 import re
 
 from components import auth
@@ -47,6 +48,10 @@ BotGroupConfig = collections.namedtuple('BotGroupConfig', [
   # dimension from that set, the list of value for it will be empty. Key and
   # values are unicode strings.
   'dimensions',
+
+  # Name of the supplemental bot_config.py to inject to the bot during
+  # handshake.
+  'bot_config_script',
 ])
 
 
@@ -71,7 +76,8 @@ _DEFAULT_BOT_GROUPS = _BotGroups(
         require_service_account=None,
         ip_whitelist=auth.BOTS_IP_WHITELIST,
         owners=(),
-        dimensions={}))
+        dimensions={},
+        bot_config_script=''))
 
 
 def _gen_version(fields):
@@ -145,7 +151,8 @@ def _bot_group_proto_to_tuple(msg, trusted_dimensions):
     require_service_account=auth_cfg.require_service_account,
     ip_whitelist=auth_cfg.ip_whitelist,
     owners=tuple(msg.owners),
-    dimensions={k: sorted(v) for k, v in dimensions.iteritems()})
+    dimensions={k: sorted(v) for k, v in dimensions.iteritems()},
+    bot_config_script=msg.bot_config_script)
 
 
 def _expand_bot_id_expr(expr):
@@ -392,6 +399,16 @@ def validate_settings(cfg, ctx):
         if not local_config.validate_flat_dimension(dim):
           ctx.error('bad dimension %r', dim)
 
+      # Validate 'bot_config_script': the supplemental bot_config.py.
+      if entry.bot_config_script:
+        # Another check confirms that the script itself is valid python.
+        if not entry.bot_config_script.endswith('.py'):
+          ctx.error('Invalid bot_config_script name')
+        if os.path.basename(entry.bot_config_script) != entry.bot_config_script:
+          ctx.error('Invalid bot_config_script name')
+        # We can't validate that the file exists here. It'll generate an HTTP
+        # 500 at handshake time.
+
   # Now verify bot_id_prefix is never a prefix of other prefix. It causes
   # ambiguities.
   for smaller, s_idx in bot_id_prefixes.iteritems():