test: start replacements once every reader is evaluating

A reader thread that first ran after the replacements finished saw the
done flag, returned zero evaluations and failed the test. Each reader
now evaluates at least once and the replacements wait for all four, so
they always overlap concurrent evaluations.

Author: lopezm1

src/test/java/co/featbit/server/ListTargetingTest.java

@@ -15,9 +15,11 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.IntStream;
 
@@ -154,24 +156,28 @@ void testConcurrentEvaluationsDuringReplacements() throws Exception {
         syncSegment(userIdRule("IsOneOf", first), Collections.emptyList(), Collections.emptyList());
 
         AtomicBoolean done = new AtomicBoolean();
+        CountDownLatch started = new CountDownLatch(4);
         ExecutorService readers = Executors.newFixedThreadPool(4);
         List<Future<Integer>> results = new ArrayList<>();
         try {
             for (int t = 0; t < 4; t++) {
                 results.add(readers.submit(() -> {
                     int evaluations = 0;
-                    while (!done.get()) {
+                    do {
                         for (int index : new int[]{0, size - 1, size, 2 * size - 1}) {
                             String reason = detail(index, false).getReason();
                             if (!reason.equals(REASON_RULE_MATCH) && !reason.equals(REASON_FALLTHROUGH)) {
                                 throw new AssertionError("user " + index + ": " + reason);
                             }
                             evaluations++;
                         }
-                    }
+                        started.countDown();
+                    } while (!done.get());
                     return evaluations;
                 }));
             }
+            // replace only once every reader is evaluating, so replacements overlap evaluations
+            assertTrue(started.await(10, TimeUnit.SECONDS));
             for (int i = 0; i < 200; i++) {
                 syncSegment(userIdRule("IsOneOf", i % 2 == 0 ? second : first), Collections.emptyList(), Collections.emptyList());
             }