From d4f992a3597172fa4dcf5e6b83feb5c51a79f360 Mon Sep 17 00:00:00 2001
From: Dave Ray <daveray@gmail.com>
Date: Thu, 25 Jul 2013 21:31:07 -0700
Subject: [PATCH] Expose current HystrixCommand to fns

@ohpauleez noticed that the current HystrixCommand was inaccessible from
:fallback-fn. This change introduces a dynamic var, *command* that
exposes the currently running HystrixCommand instance so it can be
interrogated as needed. At some point, if the dynamic var doesn't work
out or proves error-prone or unwieldy, we may introduce alternative
functions that explicitly pass the function as the first arg.
---
 .../main/clojure/com/netflix/hystrix/core.clj | 27 ++++++++++++++++---
 .../clojure/com/netflix/hystrix/core_test.clj | 25 +++++++++++++++++
 2 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/hystrix-contrib/hystrix-clj/src/main/clojure/com/netflix/hystrix/core.clj b/hystrix-contrib/hystrix-clj/src/main/clojure/com/netflix/hystrix/core.clj
index 2484b3431..d88987035 100644
--- a/hystrix-contrib/hystrix-clj/src/main/clojure/com/netflix/hystrix/core.clj
+++ b/hystrix-contrib/hystrix-clj/src/main/clojure/com/netflix/hystrix/core.clj
@@ -277,6 +277,21 @@
 
 ;################################################################################
 
+(def ^:dynamic *command*
+  "A dynamic var which is bound to the HystrixCommand instance during execution of
+  :run-fn and :fallback-fn.
+
+  It's occasionally useful, especially for fallbacks, to base the result on the state of
+  the comand. The fallback might vary based on whether it was triggered by an application
+  error versus a timeout.
+
+  Note: As always with dynamic vars be careful about scoping. This binding only holds for
+  the duration of the :run-fn or :fallback-fn.
+  "
+  nil)
+
+;################################################################################
+
 (defmacro with-request-context
   "Executes body within a new Hystrix Context.
 
@@ -616,10 +631,14 @@
     (when (not (instance? HystrixCommand$Setter setter))
       (throw (IllegalStateException. (str ":init-fn didn't return HystrixCommand$Setter instance"))))
     (proxy [HystrixCommand] [^HystrixCommand$Setter setter]
-      (run []         (apply run-fn args))
-      (getFallback [] (if fallback-fn
-                        (apply fallback-fn args)
-                        (throw (UnsupportedOperationException. "No :fallback-fn provided"))))
+      (run []
+        (binding [*command* this]
+          (apply run-fn args)))
+      (getFallback []
+        (if fallback-fn
+          (binding [*command* this]
+            (apply fallback-fn args))
+          (throw (UnsupportedOperationException. "No :fallback-fn provided"))))
       (getCacheKey [] (if cache-key-fn
                         (apply cache-key-fn args))))))
 
diff --git a/hystrix-contrib/hystrix-clj/src/test/clojure/com/netflix/hystrix/core_test.clj b/hystrix-contrib/hystrix-clj/src/test/clojure/com/netflix/hystrix/core_test.clj
index 1da742058..d23408a8b 100644
--- a/hystrix-contrib/hystrix-clj/src/test/clojure/com/netflix/hystrix/core_test.clj
+++ b/hystrix-contrib/hystrix-clj/src/test/clojure/com/netflix/hystrix/core_test.clj
@@ -172,6 +172,31 @@
         (is (= "hello-world" (.get qc) @qc))
         (is (.isDone qc))))))
 
+(deftest test-this-command-binding
+  (let [base-def {:type :command
+                  :group-key :test-this-command-binding-group
+                  :command-key :test-this-command-binding
+                  }]
+    (testing "this is bound while :run-fn is executing"
+      (let [captured (atom nil)
+            command-def (normalize (assoc base-def
+                                          :run-fn (fn []
+                                                    (reset! captured *command*))))
+            command (instantiate command-def)]
+        (.execute command)
+        (is (identical? command @captured))))
+
+
+    (testing "this is bound while :fallback-fn is executing"
+      (let [captured (atom nil)
+            command-def (normalize (assoc base-def
+                                          :run-fn (fn [] (throw (Exception. "FALLBACK!")))
+                                          :fallback-fn (fn [] (reset! captured *command*))
+                                          ))
+            command (instantiate command-def)]
+        (.execute command)
+        (is (identical? command @captured))))))
+
 (deftest test-collapser
   ; These atoms are only for testing. In real life, collapser functions should *never*
   ; have side effects.