Restore version dependent logic from CompletionFieldMapper (#119102)

This has been removed previously with #113011 but needs to be added back
for v7 read-only compatibility.

server/src/main/java/org/elasticsearch/index/mapper/CompletionFieldMapper.java

@@ -18,9 +18,11 @@
 import org.apache.lucene.search.suggest.document.RegexCompletionQuery;
 import org.apache.lucene.search.suggest.document.SuggestField;
 import org.elasticsearch.common.ParsingException;
+import org.elasticsearch.common.logging.DeprecationCategory;
 import org.elasticsearch.common.unit.Fuzziness;
 import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.analysis.AnalyzerScope;
 import org.elasticsearch.index.analysis.NamedAnalyzer;
 import org.elasticsearch.index.query.SearchExecutionContext;
@@ -207,11 +209,29 @@ public CompletionFieldMapper build(MapperBuilderContext context) {
 
         private void checkCompletionContextsLimit() {
             if (this.contexts.getValue() != null && this.contexts.getValue().size() > COMPLETION_CONTEXTS_LIMIT) {
-                throw new IllegalArgumentException(
-                    "Limit of completion field contexts [" + COMPLETION_CONTEXTS_LIMIT + "] has been exceeded"
-                );
+                if (indexVersionCreated.onOrAfter(IndexVersions.V_8_0_0)) {
+                    throw new IllegalArgumentException(
+                        "Limit of completion field contexts [" + COMPLETION_CONTEXTS_LIMIT + "] has been exceeded"
+                    );
+                } else {
+                    deprecationLogger.warn(
+                        DeprecationCategory.MAPPINGS,
+                        "excessive_completion_contexts",
+                        "You have defined more than ["
+                            + COMPLETION_CONTEXTS_LIMIT
+                            + "] completion contexts"
+                            + " in the mapping for field ["
+                            + leafName()
+                            + "]. "
+                            + "The maximum allowed number of completion contexts in a mapping will be limited to "
+                            + "["
+                            + COMPLETION_CONTEXTS_LIMIT
+                            + "] starting in version [8.0]."
+                    );
+                }
             }
         }
+
     }
 
     public static final Set<String> ALLOWED_CONTENT_FIELD_NAMES = Set.of(

server/src/test/java/org/elasticsearch/index/mapper/CompletionFieldMapperTests.java

@@ -34,6 +34,7 @@
 import org.elasticsearch.common.util.BigArrays;
 import org.elasticsearch.core.CheckedConsumer;
 import org.elasticsearch.index.IndexSettings;
+import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.analysis.AnalyzerScope;
 import org.elasticsearch.index.analysis.IndexAnalyzers;
 import org.elasticsearch.index.analysis.NamedAnalyzer;
@@ -60,6 +61,7 @@
 import java.util.Set;
 import java.util.function.Function;
 
+import static org.elasticsearch.index.mapper.CompletionFieldMapper.COMPLETION_CONTEXTS_LIMIT;
 import static org.elasticsearch.xcontent.XContentFactory.jsonBuilder;
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.containsString;
@@ -757,7 +759,7 @@ public void testLimitOfContextMappings() throws Throwable {
             .startObject("suggest")
             .field("type", "completion")
             .startArray("contexts");
-        for (int i = 0; i < CompletionFieldMapper.COMPLETION_CONTEXTS_LIMIT + 1; i++) {
+        for (int i = 0; i < COMPLETION_CONTEXTS_LIMIT + 1; i++) {
             mappingBuilder.startObject();
             mappingBuilder.field("name", Integer.toString(i));
             mappingBuilder.field("type", "category");
@@ -769,7 +771,7 @@ public void testLimitOfContextMappings() throws Throwable {
         MapperParsingException e = expectThrows(MapperParsingException.class, () -> createDocumentMapper(fieldMapping(b -> {
             b.field("type", "completion");
             b.startArray("contexts");
-            for (int i = 0; i < CompletionFieldMapper.COMPLETION_CONTEXTS_LIMIT + 1; i++) {
+            for (int i = 0; i < COMPLETION_CONTEXTS_LIMIT + 1; i++) {
                 b.startObject();
                 b.field("name", Integer.toString(i));
                 b.field("type", "category");
@@ -779,8 +781,29 @@ public void testLimitOfContextMappings() throws Throwable {
         })));
         assertTrue(
             e.getMessage(),
-            e.getMessage()
-                .contains("Limit of completion field contexts [" + CompletionFieldMapper.COMPLETION_CONTEXTS_LIMIT + "] has been exceeded")
+            e.getMessage().contains("Limit of completion field contexts [" + COMPLETION_CONTEXTS_LIMIT + "] has been exceeded")
+        );
+
+        // test pre-8 deprecation warnings
+        createDocumentMapper(IndexVersions.V_7_0_0, fieldMapping(b -> {
+            b.field("type", "completion");
+            b.startArray("contexts");
+            for (int i = 0; i < COMPLETION_CONTEXTS_LIMIT + 1; i++) {
+                b.startObject();
+                b.field("name", Integer.toString(i));
+                b.field("type", "category");
+                b.endObject();
+            }
+            b.endArray();
+        }));
+        assertCriticalWarnings(
+            "You have defined more than ["
+                + COMPLETION_CONTEXTS_LIMIT
+                + "] completion contexts"
+                + " in the mapping for field [field]. The maximum allowed number of completion contexts in a mapping will be limited to "
+                + "["
+                + COMPLETION_CONTEXTS_LIMIT
+                + "] starting in version [8.0]."
         );
     }