Suggest should ignore empty shards

From #3469.
When running suggest on empty shards, it raises an error like:

```
"failures" : [ {
      "status" : 400,
      "reason" : "ElasticSearchIllegalArgumentException[generator field [title] doesn't exist]"
    } ]
```

We should ignore empty shards.

Closes #3473.

src/main/java/org/elasticsearch/search/suggest/SuggestPhase.java

@@ -78,13 +78,17 @@ public Suggest execute(SuggestionSearchContext suggest, IndexReader reader) {
         try {
             CharsRef spare = new CharsRef(); // Maybe add CharsRef to CacheRecycler?
             final List<Suggestion<? extends Entry<? extends Option>>> suggestions = new ArrayList<Suggestion<? extends Entry<? extends Option>>>(suggest.suggestions().size());
+
             for (Map.Entry<String, SuggestionSearchContext.SuggestionContext> entry : suggest.suggestions().entrySet()) {
                 SuggestionSearchContext.SuggestionContext suggestion = entry.getValue();
                 Suggester<SuggestionContext> suggester = suggestion.getSuggester();
                 Suggestion<? extends Entry<? extends Option>> result = suggester.execute(entry.getKey(), suggestion, reader, spare);
-                assert entry.getKey().equals(result.name);
-                suggestions.add(result);
+                if (result != null) {
+                    assert entry.getKey().equals(result.name);
+                    suggestions.add(result);
+                }
             }
+
             return new Suggest(Suggest.Fields.SUGGEST, suggestions);
         } catch (IOException e) {
             throw new ElasticSearchException("I/O exception during suggest phase", e);

src/main/java/org/elasticsearch/search/suggest/Suggester.java

@@ -1,8 +1,8 @@
 /*
- * Licensed to ElasticSearch and Shay Banon under one
+ * Licensed to Elasticsearch (the "Author") under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information
- * regarding copyright ownership. ElasticSearch licenses this
+ * regarding copyright ownership. Author 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
@@ -16,21 +16,30 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+
 package org.elasticsearch.search.suggest;
-import java.io.IOException;
 
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.util.CharsRef;
-import org.elasticsearch.search.suggest.Suggest.Suggestion;
-import org.elasticsearch.search.suggest.Suggest.Suggestion.Entry;
-import org.elasticsearch.search.suggest.Suggest.Suggestion.Entry.Option;
 
-public interface Suggester<T extends SuggestionSearchContext.SuggestionContext> {
+import java.io.IOException;
+
+public abstract class Suggester<T extends SuggestionSearchContext.SuggestionContext> {
+
+    protected abstract Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>>
+        innerExecute(String name, T suggestion, IndexReader indexReader, CharsRef spare) throws IOException;
+
+    public abstract String[] names();
 
-    public Suggestion<? extends Entry<? extends Option>> execute(String name, T suggestion, IndexReader indexReader, CharsRef spare)
-            throws IOException;
+    public abstract SuggestContextParser getContextParser();
 
-    public String[] names();
+    public Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>>
+        execute(String name, T suggestion, IndexReader indexReader, CharsRef spare) throws IOException {
+        // #3469 We want to ignore empty shards
+        if (indexReader.numDocs() == 0) {
+            return null;
+        }
+        return innerExecute(name, suggestion, indexReader, spare);
+    }
 
-    public SuggestContextParser getContextParser();
 }

src/main/java/org/elasticsearch/search/suggest/completion/CompletionSuggester.java

@@ -31,9 +31,7 @@
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.text.StringText;
 import org.elasticsearch.index.mapper.core.CompletionFieldMapper;
-import org.elasticsearch.search.suggest.Suggest;
-import org.elasticsearch.search.suggest.SuggestContextParser;
-import org.elasticsearch.search.suggest.Suggester;
+import org.elasticsearch.search.suggest.*;
 import org.elasticsearch.search.suggest.completion.CompletionSuggestion.Entry.Option;
 
 import java.io.IOException;
@@ -42,14 +40,14 @@
 import java.util.List;
 import java.util.Map;
 
-public class CompletionSuggester implements Suggester<CompletionSuggestionContext> {
+public class CompletionSuggester extends Suggester<CompletionSuggestionContext> {
 
     private static final ScoreComparator scoreComparator = new ScoreComparator();
 
+
     @Override
-    public Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>> execute(String name,
+    protected Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>> innerExecute(String name,
             CompletionSuggestionContext suggestionContext, IndexReader indexReader, CharsRef spare) throws IOException {
-
         if (suggestionContext.mapper() == null || !(suggestionContext.mapper() instanceof CompletionFieldMapper)) {
             throw new ElasticSearchException("Field [" + suggestionContext.getField() + "] is not a completion suggest field");
         }
@@ -70,7 +68,7 @@ public Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.S
                 Lookup lookup = lookupTerms.getLookup(suggestionContext.mapper(), suggestionContext);
                 List<Lookup.LookupResult> lookupResults = lookup.lookup(spare, false, suggestionContext.getSize());
                 for (Lookup.LookupResult res : lookupResults) {
-                    
+
                     final String key = res.key.toString();
                     final float score = res.value;
                     final Option value = results.get(key);
@@ -79,8 +77,8 @@ public Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.S
                                 : new BytesArray(res.payload));
                         results.put(key, option);
                     } else if (value.getScore() < score) {
-                            value.setScore(score);
-                            value.setPayload(res.payload == null ? null : new BytesArray(res.payload));
+                        value.setScore(score);
+                        value.setPayload(res.payload == null ? null : new BytesArray(res.payload));
                     }
                 }
             }

src/main/java/org/elasticsearch/search/suggest/phrase/PhraseSuggester.java

@@ -19,9 +19,6 @@
 package org.elasticsearch.search.suggest.phrase;
 
 
-import java.io.IOException;
-import java.util.List;
-
 import org.apache.lucene.analysis.TokenStream;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.search.spell.DirectSpellChecker;
@@ -37,7 +34,10 @@
 import org.elasticsearch.search.suggest.SuggestUtils;
 import org.elasticsearch.search.suggest.Suggester;
 
-public final class PhraseSuggester implements Suggester<PhraseSuggestionContext> {
+import java.io.IOException;
+import java.util.List;
+
+public final class PhraseSuggester extends Suggester<PhraseSuggestionContext> {
     private final BytesRef SEPARATOR = new BytesRef(" ");
 
     /*
@@ -49,7 +49,7 @@ public final class PhraseSuggester implements Suggester<PhraseSuggestionContext>
      *   - phonetic filters could be interesting here too for candidate selection
      */
     @Override
-    public Suggestion<? extends Entry<? extends Option>> execute(String name, PhraseSuggestionContext suggestion,
+    public Suggestion<? extends Entry<? extends Option>> innerExecute(String name, PhraseSuggestionContext suggestion,
             IndexReader indexReader, CharsRef spare) throws IOException {
         double realWordErrorLikelihood = suggestion.realworldErrorLikelyhood();
         List<PhraseSuggestionContext.DirectCandidateGenerator>  generators = suggestion.generators();

src/main/java/org/elasticsearch/search/suggest/term/TermSuggester.java

@@ -18,10 +18,6 @@
  */
 package org.elasticsearch.search.suggest.term;
 
-import java.io.IOException;
-import java.util.ArrayList;
-import java.util.List;
-
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.spell.DirectSpellChecker;
@@ -37,10 +33,14 @@
 import org.elasticsearch.search.suggest.Suggester;
 import org.elasticsearch.search.suggest.SuggestionSearchContext.SuggestionContext;
 
-public final class TermSuggester implements Suggester<TermSuggestionContext> {
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+public final class TermSuggester extends Suggester<TermSuggestionContext> {
 
     @Override
-    public TermSuggestion execute(String name, TermSuggestionContext suggestion, IndexReader indexReader, CharsRef spare) throws IOException {
+    public TermSuggestion innerExecute(String name, TermSuggestionContext suggestion, IndexReader indexReader, CharsRef spare) throws IOException {
         DirectSpellChecker directSpellChecker = SuggestUtils.getDirectSpellChecker(suggestion.getDirectSpellCheckerSettings());
 
         TermSuggestion response = new TermSuggestion(

src/test/java/org/elasticsearch/test/integration/search/suggest/CustomSuggester.java

@@ -23,10 +23,7 @@
 import org.elasticsearch.common.text.StringText;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.index.mapper.MapperService;
-import org.elasticsearch.search.suggest.Suggest;
-import org.elasticsearch.search.suggest.SuggestContextParser;
-import org.elasticsearch.search.suggest.Suggester;
-import org.elasticsearch.search.suggest.SuggestionSearchContext;
+import org.elasticsearch.search.suggest.*;
 
 import java.io.IOException;
 import java.util.Locale;
@@ -35,12 +32,12 @@
 /**
  *
  */
-public class CustomSuggester implements Suggester<CustomSuggester.CustomSuggestionsContext> {
+public class CustomSuggester extends Suggester<CustomSuggester.CustomSuggestionsContext> {
 
 
     // This is a pretty dumb implementation which returns the original text + fieldName + custom config option + 12 or 123
     @Override
-    public Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>> execute(String name, CustomSuggestionsContext suggestion, IndexReader indexReader, CharsRef spare) throws IOException {
+    public Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>> innerExecute(String name, CustomSuggestionsContext suggestion, IndexReader indexReader, CharsRef spare) throws IOException {
         // Get the suggestion context
         String text = suggestion.getText().utf8ToString();
 

src/test/java/org/elasticsearch/test/integration/search/suggest/SuggestSearchTests.java

@@ -240,6 +240,9 @@ public void testEmpty() throws Exception {
                         .put(SETTING_NUMBER_OF_REPLICAS, 0))
                 .execute().actionGet();
         client().admin().cluster().prepareHealth("test").setWaitForGreenStatus().execute().actionGet();
+        client().prepareIndex("test", "type1", "1")
+                .setSource(XContentFactory.jsonBuilder().startObject().field("foo", "bar").endObject()).execute().actionGet();
+        client().admin().indices().prepareRefresh().execute().actionGet();
 
         Suggest suggest = searchSuggest(client(), termSuggestion("test").suggestMode("always") // Always, otherwise the results can vary between requests.
                                 .text("abcd")
@@ -1078,4 +1081,51 @@ public void testDifferentShardSize() throws Exception {
         assertThat(suggest.getSuggestion("simple").getEntries().get(0).getOptions().size(), equalTo(3));
     }
 
+    @Test // see #3469
+    public void testEmptyShards() throws IOException, InterruptedException {
+        Builder builder = ImmutableSettings.builder();
+        builder.put("index.number_of_shards", 5).put("index.number_of_replicas", 0);
+        builder.put("index.analysis.analyzer.suggest.tokenizer", "standard");
+        builder.putArray("index.analysis.analyzer.suggest.filter", "standard", "lowercase", "shingler");
+        builder.put("index.analysis.filter.shingler.type", "shingle");
+        builder.put("index.analysis.filter.shingler.min_shingle_size", 2);
+        builder.put("index.analysis.filter.shingler.max_shingle_size", 5);
+        builder.put("index.analysis.filter.shingler.output_unigrams", true);
+
+        XContentBuilder mapping = XContentFactory.jsonBuilder().startObject().startObject("type1")
+                .startObject("properties")
+                .startObject("name")
+                .field("type", "multi_field")
+                .field("path", "just_name")
+                .startObject("fields")
+                .startObject("name")
+                .field("type", "string")
+                .field("analyzer", "suggest")
+                .endObject()
+                .endObject()
+                .endObject()
+                .endObject()
+                .endObject().endObject();
+        client().admin().indices().prepareDelete().execute().actionGet();
+        client().admin().indices().prepareCreate("test").setSettings(builder.build()).addMapping("type1", mapping).execute().actionGet();
+        client().admin().cluster().prepareHealth("test").setWaitForGreenStatus().execute().actionGet();
+        client().prepareIndex("test", "type2", "1")
+                .setSource(XContentFactory.jsonBuilder().startObject().field("foo", "bar").endObject()).execute().actionGet();
+        client().prepareIndex("test", "type2", "2")
+                .setSource(XContentFactory.jsonBuilder().startObject().field("foo", "bar").endObject()).execute().actionGet();
+        client().prepareIndex("test", "type1", "1")
+                .setSource(XContentFactory.jsonBuilder().startObject().field("name", "Just testing the suggestions api").endObject()).execute().actionGet();
+        client().prepareIndex("test", "type1", "2")
+                .setSource(XContentFactory.jsonBuilder().startObject().field("name", "An other title").endObject()).execute().actionGet();
+        client().admin().indices().prepareRefresh().execute().actionGet();
+
+        SearchRequestBuilder suggestBuilder = client().prepareSearch().setSearchType(SearchType.COUNT);
+        suggestBuilder.setSuggestText("tetsting sugestion");
+        suggestBuilder.addSuggestion(phraseSuggestion("did_you_mean").field("name").maxErrors(5.0f));
+        SearchResponse searchResponse = suggestBuilder.execute().actionGet();
+
+        ElasticsearchAssertions.assertNoFailures(searchResponse);
+        ElasticsearchAssertions.assertSuggestion(searchResponse.getSuggest(), 0, 0, "did_you_mean", "testing suggestions");
+    }
+
 }