[Java.time] Retain prefixed date pattern in formatter (elastic#48703)

JavaDateFormatter should keep the pattern with the prefixed 8 as it will be used for serialisation.
The stripped pattern should be used for the enclosed formatters.

closes elastic#48698

rest-api-spec/src/main/resources/rest-api-spec/test/search/180_locale_dependent_mapping.yml

@@ -0,0 +1,39 @@
+---
+"Test Index and Search locale dependent mappings / dates":
+  - skip:
+      version: " - 6.1.99"
+      reason: JDK9 only supports this with a special sysproperty added in 6.2.0
+  - do:
+      indices.create:
+          index: test_index
+          body:
+              settings:
+                  number_of_shards: 1
+              mappings:
+                  properties:
+                      date_field:
+                          type: date
+                          format: "8E, d MMM uuuu HH:mm:ss Z"
+                          locale: "de"
+  - do:
+      bulk:
+          refresh: true
+          body:
+              - '{"index": {"_index": "test_index", "_id": "1"}}'
+              - '{"date_field": "Mi, 06 Dez 2000 02:55:00 -0800"}'
+              - '{"index": {"_index": "test_index", "_id": "2"}}'
+              - '{"date_field": "Do, 07 Dez 2000 02:55:00 -0800"}'
+
+  - do:
+      search:
+          rest_total_hits_as_int: true
+          index: test_index
+          body: {"query" : {"range" : {"date_field" : {"gte": "Di, 05 Dez 2000 02:55:00 -0800", "lte": "Do, 07 Dez 2000 00:00:00 -0800"}}}}
+  - match: { hits.total: 1 }
+
+  - do:
+      search:
+          rest_total_hits_as_int: true
+          index: test_index
+          body: {"query" : {"range" : {"date_field" : {"gte": "Di, 05 Dez 2000 02:55:00 -0800", "lte": "Fr, 08 Dez 2000 00:00:00 -0800"}}}}
+  - match: { hits.total: 2 }

server/src/main/java/org/elasticsearch/common/time/DateFormatter.java

@@ -136,21 +136,23 @@ static DateFormatter forPattern(String input) {
             return Joda.forPattern(input);
         }
 
-        // dates starting with 8 will not be using joda but java time formatters
-        input = input.substring(1);
-
-        List<String> patterns = splitCombinedPatterns(input);
+        // support the 6.x BWC compatible way of parsing java 8 dates
+        String format = strip8Prefix(input);
+        List<String> patterns = splitCombinedPatterns(format);
         List<DateFormatter> formatters = patterns.stream()
                                                  .map(DateFormatters::forPattern)
                                                  .collect(Collectors.toList());
 
-        if (formatters.size() == 1) {
-            return formatters.get(0);
-        }
-
         return DateFormatters.merge(input, formatters);
     }
 
+    static String strip8Prefix(String input) {
+        if (input.startsWith("8")) {
+            return input.substring(1);
+        }
+        return input;
+    }
+
     static List<String> splitCombinedPatterns(String input) {
         List<String> patterns = new ArrayList<>();
         for (String pattern : Strings.delimitedListToStringArray(input, "||")) {

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

@@ -357,4 +357,21 @@ public DocumentMapper updateFieldType(Map<String, MappedFieldType> fullNameToFie
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         return mapping.toXContent(builder, params);
     }
+
+    @Override
+    public String toString() {
+        return "DocumentMapper{" +
+            "mapperService=" + mapperService +
+            ", type='" + type + '\'' +
+            ", typeText=" + typeText +
+            ", mappingSource=" + mappingSource +
+            ", mapping=" + mapping +
+            ", documentParser=" + documentParser +
+            ", fieldMappers=" + fieldMappers +
+            ", objectMappers=" + objectMappers +
+            ", hasNestedObjects=" + hasNestedObjects +
+            ", deleteTombstoneMetadataFieldMappers=" + Arrays.toString(deleteTombstoneMetadataFieldMappers) +
+            ", noopTombstoneMetadataFieldMappers=" + Arrays.toString(noopTombstoneMetadataFieldMappers) +
+            '}';
+    }
 }