setting default similarity of index to default makes index unusable

[grmblfrz]

Elasticsearch version: 5.4.0

Plugins installed: [analysis-icu, bundle, scoring-scripts (locally developed)]

JVM version (java -version):

openjdk version "1.8.0_121"
OpenJDK Runtime Environment (build 1.8.0_121-8u121-b13-1~bpo8+1-b13)
OpenJDK 64-Bit Server VM (build 25.121-b13, mixed mode)

OS version (uname -a if on a Unix-like system):
Linux 3.10.0 (in docker container)

Description of the problem including expected versus actual behavior:
I accidently set the default similarity of an index to 'default'. Now its impossible to open the index or change the faulty setting. It should be possible to recover from this mistake.
Steps to reproduce:

curl -XDELETE 'http://localhost:9200/similaritybug?pretty'
{ 
  "acknowledged" : true
}
curl -XPUT 'http://localhost:9200/similaritybug?pretty'
{ 
  "acknowledged" : true,
  "shards_acknowledged" : true
}
curl -XPOST 'http://localhost:9200/similaritybug/_close?pretty'
{ 
  "acknowledged" : true
}
curl -XPUT 'http://localhost:9200/similaritybug/_settings?pretty' -d '{ 
  "settings": { 
    "index": { 
      "similarity": { 
        "default": { 
          "type": "default" 
        } 
      } 
    } 
  } 
}'
{ 
  "acknowledged" : true
}
curl -XPOST 'http://localhost:9200/similaritybug/_open?pretty'
{ 
  "error" : {
    "root_cause" : [
      { 
        "type" : "exception",
        "reason" : "Failed to verify index [similaritybug/CvQX4fZRTwiLNPVqT8eHUg]"
      }
    ],
    "type" : "exception",
    "reason" : "Failed to verify index [similaritybug/CvQX4fZRTwiLNPVqT8eHUg]",
    "caused_by" : {
      "type" : "illegal_argument_exception",
      "reason" : "Unknown Similarity type [default] for [default]"
    }
  },
  "status" : 500
}
curl -XPOST 'http://localhost:9200/similaritybug/_close?pretty'
{ 
  "acknowledged" : true
}
curl -XPUT http://localhost:9200/similaritybug/_settings\?pretty -d '{ 
  "settings": { 
    "index": { 
      "similarity": { 
        "default": { 
          "type": "BM25" 
        } 
      } 
    } 
  } 
}'
{ 
  "error" : {
    "root_cause" : [
      { 
        "type" : "illegal_argument_exception",
        "reason" : "Unknown Similarity type [default] for [default]"
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "Unknown Similarity type [default] for [default]"
  },
  "status" : 400
}
curl -XPOST 'http://localhost:9200/similaritybug/_open?pretty'
{ 
  "error" : {
    "root_cause" : [
      { 
        "type" : "exception",
        "reason" : "Failed to verify index [similaritybug/CvQX4fZRTwiLNPVqT8eHUg]"
      }
    ],
    "type" : "exception",
    "reason" : "Failed to verify index [similaritybug/CvQX4fZRTwiLNPVqT8eHUg]",
    "caused_by" : {
      "type" : "illegal_argument_exception",
      "reason" : "Unknown Similarity type [default] for [default]"
    }
  },
  "status" : 500
}

[jimczi]

@grmblfrz this is a bug indeed.
The settings update should have failed the first time. This is fixed in 5.4.1 (the next bugfix release of 5.4):
#24487