Short-circuit select if there's any error (#61)

Author: adamw

channels/src/main/java/com/softwaremill/jox/Select.java

@@ -100,6 +100,13 @@ public static <U> Object selectOrClosed(SelectClause<? extends U>... clauses) th
 
     @SafeVarargs
     private static <U> Object doSelectOrClosed(SelectClause<? extends U>... clauses) throws InterruptedException {
+        // short-circuiting if any of the channels is in error; otherwise, we might have selected a clause, for which
+        // a value was available immediately - even though a channel for a clause appearing later was in error
+        var anyError = getAnyChannelInError(clauses);
+        if (anyError != null) {
+            return anyError;
+        }
+
         // check that the clause doesn't refer to a channel that is already used in a different clause
         var allRendezvous = verifyChannelsUnique_getAreAllRendezvous(clauses);
 
@@ -132,6 +139,20 @@ private static boolean verifyChannelsUnique_getAreAllRendezvous(SelectClause<?>[
         return allRendezvous;
     }
 
+    private static ChannelError getAnyChannelInError(SelectClause<?>[] clauses) {
+        for (var clause : clauses) {
+            var ch = clause.getChannel();
+            if (ch != null) {
+                // if a channel is in error, closedForSend() will return that information
+                var closedForSend = clause.getChannel().closedForSend();
+                if (closedForSend instanceof ChannelError ce) {
+                    return ce;
+                }
+            }
+        }
+        return null;
+    }
+
     public static <T> SelectClause<T> defaultClause(T value) {
         return defaultClause(() -> value);
     }

channels/src/test/java/com/softwaremill/jox/SelectReceiveTest.java

@@ -222,4 +222,21 @@ void testBufferExpandedWhenSelecting() throws InterruptedException {
     void testSelectFromNone() throws InterruptedException {
         assertEquals(new ChannelDone(), selectOrClosed());
     }
+
+    @Test
+    public void testSelect_immediate_withError() throws InterruptedException {
+        // given
+        Channel<String> ch1 = new Channel<>(2);
+        ch1.send("x");
+
+        var e = new RuntimeException("boom!");
+        Channel<String> ch2 = new Channel<>(2);
+        ch2.error(e);
+
+        // when
+        var result = selectOrClosed(ch1.receiveClause(), ch2.receiveClause());
+
+        // then
+        assertEquals(new ChannelError(e), result);
+    }
 }