Clean up allowPartialSearchResults serialization (#37911)

When serializing allowPartialSearchResults to the shards through ShardSearchTransportRequest, we use an optional boolean field, though
the corresponding instance member is declared `boolean` which can never
be null. We also have an assert to verify that the incoming search
request provides a non-null value for the flag, and a comment explaining
that null should be considered a bug.

This commit makes the allowPartialSearchResults method in
ShardSearchRequest return a `boolean` rather than a `Boolean` and
changes the serialization from optional to non optional, in a bw comp manner.

server/src/main/java/org/elasticsearch/search/internal/ShardSearchLocalRequest.java

@@ -170,7 +170,7 @@ public class ShardSearchLocalRequest implements ShardSearchRequest {
     }
 
     @Override
-    public Boolean allowPartialSearchResults() {
+    public boolean allowPartialSearchResults() {
         return allowPartialSearchResults;
     }
 
@@ -216,7 +216,9 @@ public class ShardSearchLocalRequest implements ShardSearchRequest {
         nowInMillis = in.readVLong();
         requestCache = in.readOptionalBoolean();
         clusterAlias = in.readOptionalString();
-        if (in.getVersion().onOrAfter(Version.V_6_3_0)) {
+        if (in.getVersion().onOrAfter(Version.V_7_0_0)) {
+            allowPartialSearchResults = in.readBoolean();
+        } else if (in.getVersion().onOrAfter(Version.V_6_3_0)) {
             allowPartialSearchResults = in.readOptionalBoolean();
         }
         if (in.getVersion().onOrAfter(Version.V_6_4_0)) {
@@ -244,7 +246,9 @@ public class ShardSearchLocalRequest implements ShardSearchRequest {
         }
         out.writeOptionalBoolean(requestCache);
         out.writeOptionalString(clusterAlias);
-        if (out.getVersion().onOrAfter(Version.V_6_3_0)) {
+        if (out.getVersion().onOrAfter(Version.V_7_0_0)) {
+            out.writeBoolean(allowPartialSearchResults);
+        } else if (out.getVersion().onOrAfter(Version.V_6_3_0)) {
             out.writeOptionalBoolean(allowPartialSearchResults);
         }
         if (asKey == false) {
@@ -295,5 +299,4 @@ public class ShardSearchLocalRequest implements ShardSearchRequest {
             }
         }
     }
-
 }

server/src/main/java/org/elasticsearch/search/internal/ShardSearchRequest.java

@@ -69,7 +69,7 @@ public interface ShardSearchRequest {
 
     Boolean requestCache();
 
-    Boolean allowPartialSearchResults();
+    boolean allowPartialSearchResults();
 
     Scroll scroll();
 

server/src/main/java/org/elasticsearch/search/internal/ShardSearchTransportRequest.java

@@ -150,7 +150,7 @@ public class ShardSearchTransportRequest extends TransportRequest implements Sha
     }
 
     @Override
-    public Boolean allowPartialSearchResults() {
+    public boolean allowPartialSearchResults() {
         return shardSearchLocalRequest.allowPartialSearchResults();
     }
 

server/src/test/java/org/elasticsearch/index/SearchSlowLogTests.java

@@ -114,8 +114,8 @@ public class SearchSlowLogTests extends ESSingleNodeTestCase {
                 }
 
                 @Override
-                public Boolean allowPartialSearchResults() {
-                    return null;
+                public boolean allowPartialSearchResults() {
+                    return true;
                 }
 
                 @Override

server/src/test/java/org/elasticsearch/search/internal/ShardSearchTransportRequestTests.java

@@ -40,6 +40,7 @@ import org.elasticsearch.index.query.RandomQueryBuilder;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.indices.InvalidAliasNameException;
 import org.elasticsearch.search.AbstractSearchTestCase;
+import org.elasticsearch.test.VersionUtils;
 
 import java.io.IOException;
 
@@ -76,6 +77,19 @@ public class ShardSearchTransportRequestTests extends AbstractSearchTestCase {
         assertEquals(deserializedRequest.getAliasFilter(), shardSearchTransportRequest.getAliasFilter());
         assertEquals(deserializedRequest.indexBoost(), shardSearchTransportRequest.indexBoost(), 0.0f);
         assertEquals(deserializedRequest.getClusterAlias(), shardSearchTransportRequest.getClusterAlias());
+        assertEquals(shardSearchTransportRequest.allowPartialSearchResults(), deserializedRequest.allowPartialSearchResults());
+    }
+
+    public void testAllowPartialResultsSerializationPre7_0_0() throws IOException {
+        Version version = VersionUtils.randomVersionBetween(random(), Version.V_6_0_0, VersionUtils.getPreviousVersion(Version.V_7_0_0));
+        ShardSearchTransportRequest shardSearchTransportRequest = createShardSearchTransportRequest();
+        ShardSearchTransportRequest deserializedRequest =
+            copyWriteable(shardSearchTransportRequest, namedWriteableRegistry, ShardSearchTransportRequest::new, version);
+        if (version.before(Version.V_6_3_0)) {
+            assertFalse(deserializedRequest.allowPartialSearchResults());
+        } else {
+            assertEquals(shardSearchTransportRequest.allowPartialSearchResults(), deserializedRequest.allowPartialSearchResults());
+        }
     }
 
     private ShardSearchTransportRequest createShardSearchTransportRequest() throws IOException {

server/src/test/java/org/elasticsearch/search/slice/SliceBuilderTests.java

@@ -163,8 +163,8 @@ public class SliceBuilderTests extends ESTestCase {
         }
 
         @Override
-        public Boolean allowPartialSearchResults() {
-            return null;
+        public boolean allowPartialSearchResults() {
+            return true;
         }
 
         @Override