Critical bugfix: (where) evaluated its children every time.

When I added (else) blocks to (where) in 0.1.3, I inadvertently
introduced a bug which caused children of (where) to be evaluated
*every* time. This breaks all stateful streams like ddt, rate, etc. It's
a straightforward fix. I've also added a test explicitly confirming
that (by) evaluates its children at the correct times.

Future changes to macros will be accompanied by careful
time-of-evaluation tests. This stuff is surprisingly tricky to do right.

src/riemann/streams.clj

@@ -484,7 +484,7 @@
                 (let [dt (- (:time event) (:time prev-event))]
                   (when-not (zero? dt)
                     (let [diff (/ (- m (:metric prev-event)) dt)]
-                  (call-rescue (assoc event :metric diff) args))))))))))))
+                      (call-rescue (assoc event :metric diff) args))))))))))))
 
 (defn rate
   "Take the sum of every event over interval seconds and divide by the interval
@@ -1035,10 +1035,12 @@
   [expr & children]
   (let [p (where-rewrite expr)
         [true-kids else-kids] (where-partition-clauses children)]
-    `(fn [event#]
-       (if (let [~'event event#] ~p)
-         (call-rescue event# ~true-kids)
-         (call-rescue event# ~else-kids)))))
+    `(let [true-kids# ~true-kids
+           else-kids# ~else-kids]
+       (fn [event#]
+         (if (let [~'event event#] ~p)
+           (call-rescue event# true-kids#)
+           (call-rescue event# else-kids#))))))
 
 (defn update-index
   "Updates the given index with all events received."

test/riemann/test/streams.clj

@@ -265,6 +265,16 @@
            (is (= @a ["cat" "badger"]))
            (is (= @b ["dog" nil]))))
 
+(deftest where-child-evaluated-once
+         ; Where should evaluate its children exactly once.
+         (let [x (atom 0)
+               s (where true (do (swap! x inc) identity))]
+           (is (= @x 1))
+           (s {:service "test"})
+           (is (= @x 1))
+           (s {:service "test"})
+           (is (= @x 1))))
+
 (deftest default-kv
          (let [r (ref nil)
                s (default :service "foo" (register r))]