[9.0] Restore V8 REST compatibility around highlight force_source parameter (#124873)

docs/changelog/124873.yaml

@@ -0,0 +1,5 @@
+pr: 124873
+summary: Restore V8 REST compatibility around highlight `force_source` parameter
+area: Highlighting
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/search/fetch/subphase/highlight/AbstractHighlighterBuilder.java

@@ -17,6 +17,7 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.core.RestApiVersion;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.Rewriteable;
 import org.elasticsearch.search.fetch.subphase.highlight.HighlightBuilder.BoundaryScannerType;
@@ -64,6 +65,8 @@ public abstract class AbstractHighlighterBuilder<HB extends AbstractHighlighterB
     public static final ParseField TYPE_FIELD = new ParseField("type");
     public static final ParseField FRAGMENTER_FIELD = new ParseField("fragmenter");
     public static final ParseField NO_MATCH_SIZE_FIELD = new ParseField("no_match_size");
+    public static final ParseField FORCE_SOURCE_FIELD = new ParseField("force_source").withAllDeprecated()
+        .forRestApiVersion(restApiVersion -> restApiVersion == RestApiVersion.V_8);
     public static final ParseField PHRASE_LIMIT_FIELD = new ParseField("phrase_limit");
     public static final ParseField OPTIONS_FIELD = new ParseField("options");
     public static final ParseField HIGHLIGHT_QUERY_FIELD = new ParseField("highlight_query");
@@ -666,6 +669,7 @@ static <HB extends AbstractHighlighterBuilder<HB>> BiFunction<XContentParser, HB
         parser.declareString(HB::highlighterType, TYPE_FIELD);
         parser.declareString(HB::fragmenter, FRAGMENTER_FIELD);
         parser.declareInt(HB::noMatchSize, NO_MATCH_SIZE_FIELD);
+        parser.declareBoolean((builder, value) -> {}, FORCE_SOURCE_FIELD);  // force_source is ignored
         parser.declareInt(HB::phraseLimit, PHRASE_LIMIT_FIELD);
         parser.declareInt(HB::maxAnalyzedOffset, MAX_ANALYZED_OFFSET_FIELD);
         parser.declareObject(HB::options, (XContentParser p, Void c) -> {

server/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/HighlightBuilderTests.java

@@ -18,6 +18,8 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.common.util.set.Sets;
+import org.elasticsearch.common.xcontent.LoggingDeprecationHandler;
+import org.elasticsearch.core.RestApiVersion;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexVersion;
@@ -46,6 +48,7 @@
 import org.elasticsearch.xcontent.XContentFactory;
 import org.elasticsearch.xcontent.XContentParseException;
 import org.elasticsearch.xcontent.XContentParser;
+import org.elasticsearch.xcontent.XContentParserConfiguration;
 import org.elasticsearch.xcontent.XContentType;
 import org.elasticsearch.xcontent.json.JsonXContent;
 import org.junit.AfterClass;
@@ -607,6 +610,36 @@ public void testOrderSerialization() throws Exception {
         }
     }
 
+    public void testForceSourceRemovedInV9() throws IOException {
+        String highlightJson = """
+            { "fields" : { }, "force_source" : true }
+            """;
+
+        XContentParserConfiguration config = XContentParserConfiguration.EMPTY.withRegistry(xContentRegistry())
+            .withDeprecationHandler(LoggingDeprecationHandler.INSTANCE)
+            .withRestApiVersion(RestApiVersion.V_9);
+        try (XContentParser parser = JsonXContent.jsonXContent.createParser(config, highlightJson)) {
+            XContentParseException xContentParseException = expectThrows(
+                XContentParseException.class,
+                () -> HighlightBuilder.fromXContent(parser)
+            );
+            assertThat(xContentParseException.getMessage(), containsString("unknown field [force_source]"));
+        }
+    }
+
+    public void testForceSourceV8Comp() throws IOException {
+        String highlightJson = """
+            { "fields" : { }, "force_source" : true }
+            """;
+        XContentParserConfiguration config = XContentParserConfiguration.EMPTY.withRegistry(xContentRegistry())
+            .withDeprecationHandler(LoggingDeprecationHandler.INSTANCE)
+            .withRestApiVersion(RestApiVersion.V_8);
+        try (XContentParser parser = JsonXContent.jsonXContent.createParser(config, highlightJson)) {
+            HighlightBuilder.fromXContent(parser);
+            assertWarnings("Deprecated field [force_source] used, this field is unused and will be removed entirely");
+        }
+    }
+
     protected static XContentBuilder toXContent(HighlightBuilder highlight, XContentType contentType) throws IOException {
         XContentBuilder builder = XContentFactory.contentBuilder(contentType);
         if (randomBoolean()) {