Don't synchronize MapperService.doMerge for preflight checks (#109934)

We don't need to synchronize if we are not actually updating the mapping
field. This is needlessly slowing down dynamic mapping updates and
blocking the write pool.

Author: original-brownbear

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

@@ -560,18 +560,25 @@ public DocumentMapper merge(String type, CompressedXContent mappingSource, Merge
         return doMerge(type, reason, mappingSourceAsMap);
     }
 
-    private synchronized DocumentMapper doMerge(String type, MergeReason reason, Map<String, Object> mappingSourceAsMap) {
+    private DocumentMapper doMerge(String type, MergeReason reason, Map<String, Object> mappingSourceAsMap) {
         Mapping incomingMapping = parseMapping(type, reason, mappingSourceAsMap);
-        Mapping mapping = mergeMappings(this.mapper, incomingMapping, reason, this.indexSettings);
         // TODO: In many cases the source here is equal to mappingSource so we need not serialize again.
         // We should identify these cases reliably and save expensive serialization here
-        DocumentMapper newMapper = newDocumentMapper(mapping, reason, mapping.toCompressedXContent());
         if (reason == MergeReason.MAPPING_AUTO_UPDATE_PREFLIGHT) {
-            return newMapper;
+            // only doing a merge without updating the actual #mapper field, no need to synchronize
+            Mapping mapping = mergeMappings(this.mapper, incomingMapping, MergeReason.MAPPING_AUTO_UPDATE_PREFLIGHT, this.indexSettings);
+            return newDocumentMapper(mapping, MergeReason.MAPPING_AUTO_UPDATE_PREFLIGHT, mapping.toCompressedXContent());
+        } else {
+            // synchronized concurrent mapper updates are guaranteed to set merged mappers derived from the mapper value previously read
+            // TODO: can we even have concurrent updates here?
+            synchronized (this) {
+                Mapping mapping = mergeMappings(this.mapper, incomingMapping, reason, this.indexSettings);
+                DocumentMapper newMapper = newDocumentMapper(mapping, reason, mapping.toCompressedXContent());
+                this.mapper = newMapper;
+                assert assertSerialization(newMapper, reason);
+                return newMapper;
+            }
         }
-        this.mapper = newMapper;
-        assert assertSerialization(newMapper, reason);
-        return newMapper;
     }
 
     private DocumentMapper newDocumentMapper(Mapping mapping, MergeReason reason, CompressedXContent mappingSource) {