Fix crash when the only index in config is head (#325)

Author: droserasprout

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 
+* index: Fixed crash when the only index in config is `head`.
 * index: Fixed fetching originations during initial sync.
 
 ## 5.0.3 - 2022-05-04

src/dipdup/config.py

@@ -40,6 +40,7 @@
 
 from dipdup.datasources.metadata.enums import MetadataNetwork
 from dipdup.datasources.subscription import BigMapSubscription
+from dipdup.datasources.subscription import HeadSubscription
 from dipdup.datasources.subscription import OriginationSubscription
 from dipdup.datasources.subscription import Subscription
 from dipdup.datasources.subscription import TransactionSubscription
@@ -1395,9 +1396,8 @@ def _resolve_index_subscriptions(self, index_config: IndexConfigT) -> None:
                 address, path = big_map_handler_config.contract_config.address, big_map_handler_config.path
                 index_config.subscriptions.add(BigMapSubscription(address=address, path=path))
 
-        # NOTE: HeadSubscription is always enabled
         elif isinstance(index_config, HeadIndexConfig):
-            pass
+            index_config.subscriptions.add(HeadSubscription())
 
         else:
             raise NotImplementedError(f'Index kind `{index_config.kind}` is not supported')