AutoFollowIT should not rely on assertBusy but should use latches instead (#49141)

AutoFollowIT relies on assertBusy() calls to wait for a given number of 
leader indices to be created but this is prone to failures on CI. Instead, 
we should use latches to indicate when auto-follow patterns must be 
paused and resumed.

Author: tlrx

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/AutoFollowIT.java

@@ -36,6 +36,7 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -499,6 +500,9 @@ public void testPauseAndResumeWithMultipleAutoFollowPatterns() throws Exception
 
         final AtomicBoolean running = new AtomicBoolean(true);
         final AtomicInteger leaderIndices = new AtomicInteger(0);
+        final CountDownLatch latchThree = new CountDownLatch(3);
+        final CountDownLatch latchSix = new CountDownLatch(6);
+        final CountDownLatch latchNine = new CountDownLatch(9);
 
         // start creating new indices on the remote cluster
         final Thread createNewLeaderIndicesThread = new Thread(() -> {
@@ -513,6 +517,9 @@ public void testPauseAndResumeWithMultipleAutoFollowPatterns() throws Exception
                     } else {
                         Thread.sleep(200L);
                     }
+                    latchThree.countDown();
+                    latchSix.countDown();
+                    latchNine.countDown();
                 } catch (Exception e) {
                     throw new AssertionError(e);
                 }
@@ -521,23 +528,29 @@ public void testPauseAndResumeWithMultipleAutoFollowPatterns() throws Exception
         createNewLeaderIndicesThread.start();
 
         // wait for 3 leader indices to be created on the remote cluster
-        assertBusy(() -> assertThat(leaderIndices.get(), greaterThanOrEqualTo(3)));
-        assertBusy(() -> assertThat(getAutoFollowStats().getNumberOfSuccessfulFollowIndices(), greaterThanOrEqualTo(3L)));
+        latchThree.await(30L, TimeUnit.SECONDS);
+        assertThat(leaderIndices.get(), greaterThanOrEqualTo(3));
+        assertBusy(() -> assertThat(getAutoFollowStats().getNumberOfSuccessfulFollowIndices(), greaterThanOrEqualTo(3L)),
+            30L, TimeUnit.SECONDS);
 
         // now pause some random patterns
         pausedAutoFollowerPatterns.forEach(this::pauseAutoFollowPattern);
         assertBusy(() -> autoFollowPatterns.forEach(pattern ->
-            assertThat(getAutoFollowPattern(pattern).isActive(), equalTo(pausedAutoFollowerPatterns.contains(pattern) == false))));
+            assertThat(getAutoFollowPattern(pattern).isActive(), equalTo(pausedAutoFollowerPatterns.contains(pattern) == false))),
+            30L, TimeUnit.SECONDS);
 
         // wait for more leader indices to be created on the remote cluster
-        assertBusy(() -> assertThat(leaderIndices.get(), greaterThanOrEqualTo(6)));
+        latchSix.await(30L, TimeUnit.SECONDS);
+        assertThat(leaderIndices.get(), greaterThanOrEqualTo(6));
 
         // resume auto follow patterns
         pausedAutoFollowerPatterns.forEach(this::resumeAutoFollowPattern);
-        assertBusy(() -> autoFollowPatterns.forEach(pattern -> assertTrue(getAutoFollowPattern(pattern).isActive())));
+        assertBusy(() -> autoFollowPatterns.forEach(pattern -> assertTrue(getAutoFollowPattern(pattern).isActive())),
+            30L, TimeUnit.SECONDS);
 
         // wait for more leader indices to be created on the remote cluster
-        assertBusy(() -> assertThat(leaderIndices.get(), greaterThanOrEqualTo(9)));
+        latchNine.await(30L, TimeUnit.SECONDS);
+        assertThat(leaderIndices.get(), greaterThanOrEqualTo(9));
         assertBusy(() -> assertThat(getAutoFollowStats().getNumberOfSuccessfulFollowIndices(), greaterThanOrEqualTo(9L)),
             30L, TimeUnit.SECONDS);