NPE happens on short prefix query on a field with index_prefixes

[mattjarnevic]

Running ElasticSearch version 7.17.5 Running latest release (7.2.1) of elasticsearch-rails and elasticsearch-model

When a prefix query run on a field with index_prefixes, if the length of characters in the query is [min_chars of the index_prefixes option] - 1, an error is responded. The root cause seems to be NullPointerException.

To Reproduce Steps to reproduce the behavior:

1. Create an index which has a field with index_prefixes option; its min_chars is implicitly 2.

   PUT test
   {
   "mappings": {
   "properties": {
     "t": {
       "type": "text",
       "index_prefixes": {}
     }
   }
   }
   }

2. Search by prefix query with one character on the field.

   GET test/_search
   {"query":{"prefix":{"t": "a"}}}
   An error is responded. It says the root cause is NullPointerException.
   {
   "error" : {
   "root_cause" : [
     {
       "type" : "query_shard_exception",
       "reason" : "failed to create query: Cannot invoke \"Object.hashCode()\" because \"this.rewriteMethod\" is null",
       "index" : "test",
       "index_uuid" : "XPqfSP1xT7WJt9Sxzt65qg"
     }
   ],
   "type" : "search_phase_execution_exception",
   "reason" : "all shards failed",
   "phase" : "query",
   "grouped" : true,
   "failed_shards" : [
     {
       "shard" : 0,
       "index" : "test",
       "node" : "QA35rulpSza20Wsx6rNSEg",
       "reason" : {
         "type" : "query_shard_exception",
         "reason" : "failed to create query: Cannot invoke \"Object.hashCode()\" because \"this.rewriteMethod\" is null",
         "index" : "test",
         "index_uuid" : "XPqfSP1xT7WJt9Sxzt65qg",
         "caused_by" : {
           "type" : "null_pointer_exception",
           "reason" : "Cannot invoke \"Object.hashCode()\" because \"this.rewriteMethod\" is null"
         }
       }
     }
   ]
   },
   "status" : 400
   }

   simple_query_string also fails.

   # fails with the same error
   GET test/_search
   {"query":{"simple_query_string":{"fields":["t"], "query":"a*"}}}

   Expected behavior Run without errors. I know it is not efficient.

This issue was found in OpenSearch and has a very detailed explanation of the issue along with a fix. Copied the details over for the basics of when this occurs. https://github.com/opensearch-project/OpenSearch/pull/2879