Add specific test for serializing all mapping parameter values (#61844)

This commit adds a test to MapperTestCase that explicitly checks that a mapper can
serialize all its default values, and that this serialization can then be re-parsed. Note that
the test is disabled for non-parametrized mappers as their serialization may in some cases
output parameters that are not accepted. Gradually moving all mappers to parametrized
form will address this.

The commit also contains a fix to keyword mappers, which were not correctly serializing
the similarity parameter; this partially addresses #61563. It also enables `null` as a 
value for `null_value` on `scaled_float`, as a follow-up to #61798

modules/mapper-extras/src/main/java/org/elasticsearch/index/mapper/ScaledFloatFieldMapper.java

@@ -92,7 +92,7 @@ public static class Builder extends ParametrizedFieldMapper.Builder {
                 }
             });
         private final Parameter<Double> nullValue = new Parameter<>("null_value", false, () -> null,
-            (n, c, o) -> XContentMapValues.nodeDoubleValue(o), m -> toType(m).nullValue);
+            (n, c, o) -> o == null ? null : XContentMapValues.nodeDoubleValue(o), m -> toType(m).nullValue).acceptsNull();
 
         private final Parameter<Map<String, String>> meta = Parameter.metaParam();
 

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

@@ -99,7 +99,9 @@ public static class Builder extends ParametrizedFieldMapper.Builder {
         private final Parameter<Boolean> hasNorms
             = Parameter.boolParam("norms", false, m -> toType(m).fieldType.omitNorms() == false, false);
         private final Parameter<SimilarityProvider> similarity = new Parameter<>("similarity", false, () -> null,
-            (n, c, o) -> TypeParsers.resolveSimilarity(c, n, o.toString()), m -> toType(m).similarity);
+            (n, c, o) -> TypeParsers.resolveSimilarity(c, n, o), m -> toType(m).similarity)
+            .setSerializer((b, f, v) -> b.field(f, v == null ? null : v.name()), v -> v == null ? null : v.name())
+            .acceptsNull();
 
         private final Parameter<String> normalizer
             = Parameter.stringParam("normalizer", false, m -> toType(m).normalizerName, "default");

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

@@ -398,8 +398,11 @@ public static List<String> parseCopyFields(Object propNode) {
         return copyFields;
     }
 
-    public static SimilarityProvider resolveSimilarity(Mapper.TypeParser.ParserContext parserContext, String name, String value) {
-        SimilarityProvider similarityProvider = parserContext.getSimilarity(value);
+    public static SimilarityProvider resolveSimilarity(Mapper.TypeParser.ParserContext parserContext, String name, Object value) {
+        if (value == null) {
+            return null;    // use default
+        }
+        SimilarityProvider similarityProvider = parserContext.getSimilarity(value.toString());
         if (similarityProvider == null) {
             throw new MapperParsingException("Unknown Similarity type [" + value + "] for field [" + name + "]");
         }

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

@@ -250,6 +250,18 @@ public void testEnableNorms() throws IOException {
         assertEquals(0, fieldNamesFields.length);
     }
 
+    public void testConfigureSimilarity() throws IOException {
+        MapperService mapperService = createMapperService(
+            fieldMapping(b -> b.field("type", "keyword").field("similarity", "boolean"))
+        );
+        MappedFieldType ft = mapperService.documentMapper().fieldTypes().get("field");
+        assertEquals("boolean", ft.getTextSearchInfo().getSimilarity().name());
+
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class,
+            () -> merge(mapperService, fieldMapping(b -> b.field("type", "keyword").field("similarity", "BM25"))));
+        assertThat(e.getMessage(), containsString("Cannot update parameter [similarity] from [boolean] to [BM25]"));
+    }
+
     public void testNormalizer() throws IOException {
         DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "keyword").field("normalizer", "lowercase")));
         ParsedDocument doc = mapper.parse(source(b -> b.field("field", "AbC")));

test/framework/src/main/java/org/elasticsearch/index/mapper/FieldMapperTestCase2.java

@@ -228,4 +228,9 @@ private XContentBuilder mappingsToJson(ToXContent builder, boolean includeDefaul
         ToXContent.Params params = includeDefaults ? new ToXContent.MapParams(Map.of("include_defaults", "true")) : ToXContent.EMPTY_PARAMS;
         return mapping(b -> builder.toXContent(b, params));
     }
+
+    @Override
+    public void testMinimalToMaximal() {
+        assumeFalse("`include_defaults` includes unsupported properties in non-parametrized mappers", false);
+    }
 }

test/framework/src/main/java/org/elasticsearch/index/mapper/MapperServiceTestCase.java

@@ -30,6 +30,7 @@
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.compress.CompressedXContent;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.common.xcontent.XContentType;
@@ -63,6 +64,8 @@ public abstract class MapperServiceTestCase extends ESTestCase {
 
     protected static final Settings SETTINGS = Settings.builder().put("index.version.created", Version.CURRENT).build();
 
+    protected static final ToXContent.Params INCLUDE_DEFAULTS = new ToXContent.MapParams(Map.of("include_defaults", "true"));
+
     protected Collection<? extends Plugin> getPlugins() {
         return emptyList();
     }

test/framework/src/main/java/org/elasticsearch/index/mapper/MapperTestCase.java

@@ -64,6 +64,18 @@ public final void testMinimalSerializesToItself() throws IOException {
         assertParseMinimalWarnings();
     }
 
+    // TODO make this final once we remove FieldMapperTestCase2
+    public void testMinimalToMaximal() throws IOException {
+        XContentBuilder orig = JsonXContent.contentBuilder().startObject();
+        createMapperService(fieldMapping(this::minimalMapping)).documentMapper().mapping().toXContent(orig, INCLUDE_DEFAULTS);
+        orig.endObject();
+        XContentBuilder parsedFromOrig = JsonXContent.contentBuilder().startObject();
+        createMapperService(orig).documentMapper().mapping().toXContent(parsedFromOrig, INCLUDE_DEFAULTS);
+        parsedFromOrig.endObject();
+        assertEquals(Strings.toString(orig), Strings.toString(parsedFromOrig));
+        assertParseMinimalWarnings();
+    }
+
     protected void assertParseMinimalWarnings() {
         // Most mappers don't emit any warnings
     }