Improve error message in case of invalid dynamic templates.

docs/reference/mapping/dynamic/templates.asciidoc

@@ -46,7 +46,8 @@ snippet may cause the update or validation of a dynamic template to fail under c
   the mapping snippet is considered valid. However, a validation error is returned at index time if a field matching
   the template is indexed as a different type. For example, configuring a dynamic template with no `match_mapping_type`
   is considered valid as string type, but if a field matching the dynamic template is indexed as a long, a validation
-  error is returned at index time.
+  error is returned at index time. It is recommended to configure the `match_mapping_type` to the expected JSON type or
+  configure the desired `type` in the mapping snippet.
 
 * If the `{name}` placeholder is used in the mapping snippet, validation is skipped when updating the dynamic
   template. This is because the field name is unknown at that time. Instead, validation occurs when the template is applied

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

@@ -31,6 +31,7 @@
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Iterator;
@@ -405,8 +406,10 @@ private static void validateDynamicTemplate(Mapper.TypeParser.ParserContext pars
 
         final boolean shouldEmitDeprecationWarning = parserContext.indexVersionCreated().onOrAfter(Version.V_7_7_0);
         if (dynamicTemplateInvalid && shouldEmitDeprecationWarning) {
-            String message = String.format(Locale.ROOT, "dynamic template [%s] has invalid content [%s]",
-                dynamicTemplate.getName(), Strings.toString(dynamicTemplate));
+            String format = "dynamic template [%s] has invalid content [%s], " +
+                "attempted to validate it with the following match_mapping_type: [%s]";
+            String message = String.format(Locale.ROOT, format, dynamicTemplate.getName(), Strings.toString(dynamicTemplate),
+                Arrays.toString(types));
 
             final String deprecationMessage;
             if (lastError != null) {

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

@@ -311,7 +311,8 @@ public void testIllegalDynamicTemplateUnknownFieldType() throws Exception {
         DocumentMapper mapper = mapperService.merge("type", new CompressedXContent(Strings.toString(mapping)), MergeReason.MAPPING_UPDATE);
         assertThat(mapper.mappingSource().toString(), containsString("\"type\":\"string\""));
         assertWarnings("dynamic template [my_template] has invalid content [{\"match_mapping_type\":\"string\",\"mapping\":{\"type\":" +
-            "\"string\"}}], caused by [No mapper found for type [string]]");
+            "\"string\"}}], attempted to validate it with the following match_mapping_type: [[string]], " +
+            "caused by [No mapper found for type [string]]");
     }
 
     public void testIllegalDynamicTemplateUnknownAttribute() throws Exception {
@@ -340,6 +341,7 @@ public void testIllegalDynamicTemplateUnknownAttribute() throws Exception {
         assertThat(mapper.mappingSource().toString(), containsString("\"foo\":\"bar\""));
         assertWarnings("dynamic template [my_template] has invalid content [{\"match_mapping_type\":\"string\",\"mapping\":{" +
             "\"foo\":\"bar\",\"type\":\"keyword\"}}], " +
+            "attempted to validate it with the following match_mapping_type: [[string]], " +
             "caused by [unknown parameter [foo] on mapper [__dynamic__my_template] of type [keyword]]");
     }
 
@@ -368,7 +370,8 @@ public void testIllegalDynamicTemplateInvalidAttribute() throws Exception {
         DocumentMapper mapper = mapperService.merge("type", new CompressedXContent(Strings.toString(mapping)), MergeReason.MAPPING_UPDATE);
         assertThat(mapper.mappingSource().toString(), containsString("\"analyzer\":\"foobar\""));
         assertWarnings("dynamic template [my_template] has invalid content [{\"match_mapping_type\":\"string\",\"mapping\":{" +
-            "\"analyzer\":\"foobar\",\"type\":\"text\"}}], caused by [analyzer [foobar] has not been configured in mappings]");
+            "\"analyzer\":\"foobar\",\"type\":\"text\"}}], attempted to validate it with the following match_mapping_type: [[string]], " +
+            "caused by [analyzer [foobar] has not been configured in mappings]");
     }
 
     public void testIllegalDynamicTemplateNoMappingType() throws Exception {
@@ -437,10 +440,14 @@ public void testIllegalDynamicTemplateNoMappingType() throws Exception {
             if (useMatchMappingType) {
                 assertWarnings("dynamic template [my_template] has invalid content [{\"match_mapping_type\":\"*\",\"mapping\":{" +
                     "\"foo\":\"bar\",\"type\":\"{dynamic_type}\"}}], " +
+                    "attempted to validate it with the following match_mapping_type: " +
+                    "[[object, string, long, double, boolean, date, binary]], " +
                     "caused by [unknown parameter [foo] on mapper [__dynamic__my_template] of type [binary]]");
             } else {
                 assertWarnings("dynamic template [my_template] has invalid content [{\"match\":\"string_*\",\"mapping\":{" +
                     "\"foo\":\"bar\",\"type\":\"{dynamic_type}\"}}], " +
+                    "attempted to validate it with the following match_mapping_type: " +
+                    "[[object, string, long, double, boolean, date, binary]], " +
                     "caused by [unknown parameter [foo] on mapper [__dynamic__my_template] of type [binary]]");
             }
         }