Add buffer to chunk size to prevent risk of oversize post requests (e…

…lastic#48595) (elastic#49892)

x-pack/legacy/plugins/file_upload/public/util/size_limited_chunking.js

@@ -5,16 +5,23 @@
  */
 import { MAX_BYTES } from '../../common/constants/file_import';
 
+// MAX_BYTES is a good guideline for splitting up posts, but this logic
+// occasionally sizes chunks so closely to the limit, that the remaining content
+// of a post (besides features) tips it over the max. Adding a 2MB buffer
+// to ensure this doesn't happen
+const CHUNK_BUFFER = 2097152;
+
 // Add data elements to chunk until limit is met
-export function sizeLimitedChunking(dataArr, maxChunkSize = MAX_BYTES) {
+export function sizeLimitedChunking(dataArr, maxByteSize = MAX_BYTES - CHUNK_BUFFER) {
   let chunkSize = 0;
+
   return dataArr.reduce((accu, el) => {
     const featureByteSize = (
       new Blob([JSON.stringify(el)], { type: 'application/json' })
     ).size;
-    if (featureByteSize > maxChunkSize) {
-      throw `Some features exceed maximum chunk size of ${maxChunkSize}`;
-    } else if (chunkSize + featureByteSize < maxChunkSize) {
+    if (featureByteSize > maxByteSize) {
+      throw `Some features exceed maximum chunk size of ${maxByteSize}`;
+    } else if (chunkSize + featureByteSize < maxByteSize) {
       const lastChunkRef = accu.length - 1;
       chunkSize += featureByteSize;
       accu[lastChunkRef].push(el);