Inhibit concurrent loading of deferred middleware

Author: vspinu

src/cider/nrepl.clj

@@ -12,6 +12,10 @@
   "Map of `delay`s holding deferred middleware handlers."
   (atom nil))
 
+(def LOCK
+  "Lock used to inhibit concurrent `require` of the middleware namespaces."
+  (Object.))
+
 (defn- resolve-or-fail [sym]
   (or (resolve sym)
       (throw (IllegalArgumentException. (format "Cannot resolve %s" sym)))))
@@ -25,8 +29,9 @@
         sym (symbol (name `~fn-name))]
     (swap! DELAYS assoc sym
            (delay
-             (require `~ns)
-             (resolve-or-fail `~fn-name)))
+             (locking LOCK
+               (require `~ns)
+               (resolve-or-fail `~fn-name))))
     `(@(get @DELAYS '~sym) ~handler ~msg)))
 
 (defmacro ^{:arglists '([name handler-fn descriptor]