Backport: Trigger log with no hits (#827)

* Backport: Trigger log with no hits

* Bump up nrtsearch version

Author: fragosoluana

build.gradle

@@ -26,7 +26,7 @@ java {
 }
 
 allprojects {
-    version = '0.42.3'
+    version = '0.42.4'
     group = 'com.yelp.nrtsearch'
 }
 

src/main/java/com/yelp/nrtsearch/server/luceneserver/SearchHandler.java

@@ -52,6 +52,7 @@
 import com.yelp.nrtsearch.server.utils.ObjectToCompositeFieldTransformer;
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
 import java.util.List;
@@ -335,6 +336,14 @@ public SearchResponse handle(IndexState indexState, SearchRequest searchRequest)
   private void fetchFields(SearchContext searchContext)
       throws IOException, ExecutionException, InterruptedException {
     if (searchContext.getResponseBuilder().getHitsBuilderList().isEmpty()) {
+      // call log even when there is no hits.
+      // HitsLogger implementation should decide what to log or not when there is no hits.
+      if (searchContext.getFetchTasks().getHitsLoggerFetchTask() != null) {
+        searchContext
+            .getFetchTasks()
+            .getHitsLoggerFetchTask()
+            .processAllHits(searchContext, Collections.emptyList());
+      }
       return;
     }
 

src/test/java/com/yelp/nrtsearch/server/luceneserver/logging/HitsLoggerTest.java

@@ -467,6 +467,39 @@ public void testLoggingWithZeroHitsToLog() {
     assertEquals(1, response.getHitsCount());
   }
 
+  @Test
+  public void testLoggingWithZeroHits() {
+    SearchRequest request =
+        SearchRequest.newBuilder()
+            .setTopHits(1)
+            .setStartHit(0)
+            .setIndexName(DEFAULT_TEST_INDEX)
+            .addRetrieveFields("doc_id")
+            .setQuery(
+                Query.newBuilder()
+                    .setTermQuery(
+                        TermQuery.newBuilder()
+                            .setField("vendor_name")
+                            .setTextValue("vendor_no_hits")
+                            .build())
+                    .build())
+            .setLoggingHits(
+                LoggingHits.newBuilder()
+                    .setName("custom_logger")
+                    .setHitsToLog(1)
+                    .setParams(
+                        Struct.newBuilder()
+                            .putFields(
+                                "external_value", Value.newBuilder().setStringValue("abc").build()))
+                    .build())
+            .build();
+    SearchResponse response = getGrpcServer().getBlockingStub().search(request);
+
+    assertEquals("LOGGED  {external_value=abc}", HitsLoggerTest.logMessage);
+    assertEquals(0, response.getTotalHits().getValue());
+    assertEquals(0, response.getHitsCount());
+  }
+
   @Test
   public void testLoggingTimeTaken() {
     SearchRequest request =