Report shard state changes better (#44419)

Today when the cluster health changes the `AllocationService` reports at most
ten shards that were started or failed, and always ends its message with `...`
suggesting that the list is truncated. This commit adjusts these messages to be
clearer about whether the list is truncated or not. When debug logging is
enabled the list is not truncated; if the list is truncated then its length is
logged, and if it is not truncated then no `...` is included in the message.

server/src/main/java/org/elasticsearch/cluster/routing/allocation/AllocationService.java

@@ -110,8 +110,9 @@ public class AllocationService {
         applyStartedShards(allocation, startedShards);
         gatewayAllocator.applyStartedShards(allocation, startedShards);
         reroute(allocation);
-        String startedShardsAsString = firstListElementsToCommaDelimitedString(startedShards, s -> s.shardId().toString());
-        return buildResultAndLogHealthChange(clusterState, allocation, "shards started [" + startedShardsAsString + "] ...");
+        String startedShardsAsString
+            = firstListElementsToCommaDelimitedString(startedShards, s -> s.shardId().toString(), logger.isDebugEnabled());
+        return buildResultAndLogHealthChange(clusterState, allocation, "shards started [" + startedShardsAsString + "]");
     }
 
     protected ClusterState buildResultAndLogHealthChange(ClusterState oldState, RoutingAllocation allocation, String reason) {
@@ -208,8 +209,9 @@ public class AllocationService {
         gatewayAllocator.applyFailedShards(allocation, failedShards);
 
         reroute(allocation);
-        String failedShardsAsString = firstListElementsToCommaDelimitedString(failedShards, s -> s.getRoutingEntry().shardId().toString());
-        return buildResultAndLogHealthChange(clusterState, allocation, "shards failed [" + failedShardsAsString + "] ...");
+        String failedShardsAsString
+            = firstListElementsToCommaDelimitedString(failedShards, s -> s.getRoutingEntry().shardId().toString(), logger.isDebugEnabled());
+        return buildResultAndLogHealthChange(clusterState, allocation, "shards failed [" + failedShardsAsString + "]");
     }
 
     /**
@@ -317,13 +319,14 @@ public class AllocationService {
      * @param <T>       The list element type.
      * @return A comma-separated string of the first few elements.
      */
-    private <T> String firstListElementsToCommaDelimitedString(List<T> elements, Function<T, String> formatter) {
+    static <T> String firstListElementsToCommaDelimitedString(List<T> elements, Function<T, String> formatter, boolean isDebugEnabled) {
         final int maxNumberOfElements = 10;
-        return elements
-                .stream()
-                .limit(maxNumberOfElements)
-                .map(formatter)
-                .collect(Collectors.joining(", "));
+        if (isDebugEnabled || elements.size() <= maxNumberOfElements) {
+            return elements.stream().map(formatter).collect(Collectors.joining(", "));
+        } else {
+            return elements.stream().limit(maxNumberOfElements).map(formatter).collect(Collectors.joining(", "))
+                + ", ... [" + elements.size() + " items in total]";
+        }
     }
 
     public CommandsResult reroute(final ClusterState clusterState, AllocationCommands commands, boolean explain, boolean retryFailed) {

server/src/test/java/org/elasticsearch/cluster/routing/allocation/AllocationServiceTests.java

@@ -0,0 +1,72 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.cluster.routing.allocation;
+
+import org.elasticsearch.test.ESTestCase;
+
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.not;
+
+public class AllocationServiceTests extends ESTestCase {
+
+    public void testFirstListElementsToCommaDelimitedStringReportsAllElementsIfShort() {
+        List<String> strings = IntStream.range(0, between(0, 10)).mapToObj(i -> randomAlphaOfLength(10)).collect(Collectors.toList());
+        assertAllElementsReported(strings, randomBoolean());
+    }
+
+    public void testFirstListElementsToCommaDelimitedStringReportsAllElementsIfDebugEnabled() {
+        List<String> strings = IntStream.range(0, between(0, 100)).mapToObj(i -> randomAlphaOfLength(10)).collect(Collectors.toList());
+        assertAllElementsReported(strings, true);
+    }
+
+    private void assertAllElementsReported(List<String> strings, boolean isDebugEnabled) {
+        final String abbreviated = AllocationService.firstListElementsToCommaDelimitedString(strings, Function.identity(), isDebugEnabled);
+        for (String string : strings) {
+            assertThat(abbreviated, containsString(string));
+        }
+        assertThat(abbreviated, not(containsString("...")));
+    }
+
+    public void testFirstListElementsToCommaDelimitedStringReportsFirstElementsIfLong() {
+        List<String> strings = IntStream.range(0, between(11, 100)).mapToObj(i -> randomAlphaOfLength(10))
+            .distinct().collect(Collectors.toList());
+        final String abbreviated = AllocationService.firstListElementsToCommaDelimitedString(strings, Function.identity(), false);
+        for (int i = 0; i < strings.size(); i++) {
+            if (i < 10) {
+                assertThat(abbreviated, containsString(strings.get(i)));
+            } else {
+                assertThat(abbreviated, not(containsString(strings.get(i))));
+            }
+        }
+        assertThat(abbreviated, containsString("..."));
+        assertThat(abbreviated, containsString("[" + strings.size() + " items in total]"));
+    }
+
+    public void testFirstListElementsToCommaDelimitedStringUsesFormatterNotToString() {
+        List<String> strings = IntStream.range(0, between(1, 100)).mapToObj(i -> "original").collect(Collectors.toList());
+        final String abbreviated = AllocationService.firstListElementsToCommaDelimitedString(strings, s -> "formatted", randomBoolean());
+        assertThat(abbreviated, containsString("formatted"));
+        assertThat(abbreviated, not(containsString("original")));
+    }
+}