CCACHE-65 potential fix for cache stampede

Signed-off-by: Sean Corfield <sean@corfield.org>

Author: seancorfield

.clj-kondo/config.edn

@@ -0,0 +1 @@
+{:lint-as {clojure.core.cache/defcache clojure.core/defrecord}}

README.md

@@ -173,6 +173,8 @@ Developer Information
 Change Log
 ====================
 
+* Release 1.2.next in progress
+  * [CCACHE-65](http://clojure.atlassian.net/browse/CCACHE-65) Use `delay` in `lookup-or-miss` to avoid cache-stampede.
 * Release 1.1.234 on 2024-02-19
   * Update parent pom and `data.priority-map` versions
 * Release 1.0.225 on 2021-12-06

src/main/clojure/clojure/core/cache/wrapped.clj

@@ -29,9 +29,9 @@
 
   Reads from the current version of the atom."
   ([cache-atom e]
-   (c/lookup @cache-atom e))
+   (force (c/lookup @cache-atom e)))
   ([cache-atom e not-found]
-   (c/lookup @cache-atom e not-found)))
+   (force (c/lookup @cache-atom e not-found))))
 
 (def ^{:private true} default-wrapper-fn #(%1 %2))
 
@@ -51,23 +51,23 @@
   ([cache-atom e wrap-fn value-fn]
    (let [d-new-value (delay (wrap-fn value-fn e))]
      (loop [n 0
-            v (c/lookup (swap! cache-atom
-                               c/through-cache
+            v (force (c/lookup (swap! cache-atom
+                                      c/through-cache
+                                      e
+                                      default-wrapper-fn
+                                      (fn [_] d-new-value))
                                e
-                               default-wrapper-fn
-                               (fn [_] @d-new-value))
-                        e
-                        ::expired)]
+                               ::expired))]
        (when (< n 10)
          (if (= ::expired v)
            (recur (inc n)
-                  (c/lookup (swap! cache-atom
-                                   c/through-cache
+                  (force (c/lookup (swap! cache-atom
+                                          c/through-cache
+                                          e
+                                          default-wrapper-fn
+                                          (fn [_] d-new-value))
                                    e
-                                   default-wrapper-fn
-                                   (fn [_] @d-new-value))
-                            e
-                            ::expired))
+                                   ::expired)))
            v))))))
 
 (defn has?

src/test/clojure/clojure/core/cache/wrapped_test.clj

@@ -8,6 +8,7 @@
 
 (ns clojure.core.cache.wrapped-test
   (:require [clojure.core.cache.wrapped :as c]
+            [clojure.core.cache :as cache]
             [clojure.test :refer [deftest is]]))
 
 (deftest basic-wrapped-test
@@ -40,3 +41,27 @@
           (recur (+ 1 n)))))
     (println "ttl test completed" limit "calls in"
              (- (System/currentTimeMillis) start) "ms")))
+
+(deftest cache-stampede
+  (let [thread-count 100
+        cache-atom (-> {}
+                       (cache/ttl-cache-factory :ttl 120000)
+                       (cache/lu-cache-factory :threshold 100)
+                       (atom))
+        latch (java.util.concurrent.CountDownLatch. thread-count)
+        invocations-counter (atom 0)
+        values (atom [])]
+    (dotimes [_ thread-count]
+      (.start (Thread. (fn []
+                         (swap! values conj
+                                (c/lookup-or-miss cache-atom "my-key"
+                                                  (fn [_]
+                                                    (swap! invocations-counter inc)
+                                                    (Thread/sleep 3000)
+                                                    "some value")))
+                         (.countDown latch)))))
+
+    (.await latch)
+    (is (= 1 (deref invocations-counter)))
+    (doseq [v @values]
+      (is (= "some value" v)))))