Set current index on first call of config/index

Fix the "orphaned indicies" problem described in riemann#957.

Allows a Riemann config to make multiple calls to riemann.config/index
with predictable results, both on initial load and after a reload.

Without this change, multiple calls are OK after reload or explicit
apply, but create orphan indices on initial load.

Closes: riemann#957

src/riemann/config.clj

@@ -231,22 +231,39 @@
  (reduce conj (:streams @next-core) things))))
 
 (defn index
- "Set the index used by this core. Returns the index."
+ "Create a new index and add it to the core and next core as needed.
+
+ If the current core has no index, add the new index to it.
+
+ If the next core has no index, add the current core's index (when equivalent)
+ or add the new index (when not equivalent).
+
+ In practice, because Riemann currently has only one index type (NBHM), only a
+ single index will ever be created. It will be preserved across all future
+ calls to `index`.
+
+ Returns the index."
  [& opts]
  (locking core
- (let [index (:index @core)
+ (let [current-index (:index @core)
+ ; Create a new index, later we'll decide whether it needs to be added
+ ; to the current core, next core, or both.
  ; Note that we need to wrap the *current* core's pubsub; the next
  ; core's pubsub module will be discarded in favor of the current one
  ; when core transition takes place.
- index' (-> (apply riemann.index/index opts)
- (core/wrap-index (:pubsub @core)))
- ; If the new index is equivalent to the old one, preserve the old
- ; one.
- index' (if (service/equiv? index index')
- index
- index')]
- (swap! next-core assoc :index index')
- index')))
+ new-index (-> (apply riemann.index/index opts)
+ (core/wrap-index (:pubsub @core)))
+
+ ; If the new index is equivalent to the old one, preserve the old one.
+ chosen-index (if (service/equiv? current-index new-index)
+ current-index
+ new-index)]
+
+ ; If the current core has no index, add the newly created one.
+ (when (nil? current-index) (swap! core assoc :index chosen-index))
+ ; Always add the new index to the next core, ready for core transition.
+ (swap! next-core assoc :index chosen-index)
+ chosen-index)))
 
 (defn update-index
  "Updates the given index with all events received. Also publishes to the

test/riemann/config_test.clj

@@ -130,10 +130,12 @@
 (deftest index-test
  (let [i (index)]
  (testing "index creation helper creates the index properly"
- (is (satisfies? Index i))
- (is (= i (:index @next-core)))
- (is (nil? (:index @core))))
- (testing "index is applied to the core properly during initial load"
+ (is (satisfies? Index i)))
+ (testing "index creation helper sets the index of the current core"
+ (is (= i (:index @core))))
+ (testing "index creation helper sets the index of the next core"
+ (is (= i (:index @next-core))))
+ (testing "current core index is preserved after apply"
  (apply!)
  (is (identical? i (:index @core))))
  (testing "we have the proper reference to the index after a reload"