ASYNC-255 Add guard in alts for write op with nil message

Author: puredanger

src/main/clojure/cljs/core/async.cljs

@@ -180,6 +180,12 @@
   (let [flag (alt-flag)
         ports (vec ports) ;; ensure vector for indexed nth
         n (count ports)
+        _ (loop [i 0] ;; check for invalid write op
+            (when (< i n)
+              (let [port (nth ports i)]
+                (when (vector? port)
+                  (assert (some? (port 1)) "can't put nil on channel")))
+              (recur (unchecked-inc i))))
         idxs (random-array n)
         priority (:priority opts)
         ret

src/main/clojure/clojure/core/async.clj

@@ -280,6 +280,12 @@ to catch and handle."
   (let [flag (alt-flag)
         ports (vec ports) ;; ensure vector for indexed nth
         n (count ports)
+        _ (loop [i 0] ;; check for invalid write op
+            (when (< i n)
+              (let [port (nth ports i)]
+                (when (vector? port)
+                  (assert (some? (port 1)) "can't put nil on channel")))
+              (recur (unchecked-inc i))))
         ^ints idxs (random-array n)
         priority (:priority opts)
         ret

src/test/clojure/clojure/core/async_test.clj

@@ -465,3 +465,10 @@
              :ok
              (catch AssertionError e
                :ko))))))
+
+(deftest test-alts-put-nil-invalid
+  (is
+    (thrown? AssertionError
+      (let [c1 (a/chan)
+            c2 (a/chan)]
+        (a/alts!! [c1 [c2 nil]])))))