Elasticsearch version (bin/elasticsearch --version): 7.8.0

Plugins installed: []

JVM version (java -version):
openjdk 14.0.1 2020-04-14
OpenJDK Runtime Environment AdoptOpenJDK (build 14.0.1+7)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 14.0.1+7, mixed mode, sharing)

OS version (uname -a if on a Unix-like system):
Linux ubuntu 4.15.0-106-generic #107-Ubuntu SMP Thu Jun 4 11:27:52 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior:
Running 7.8.0 official docker image. Put an index template that using one component template. Restart the container.
Use the same configuration to update the existed component template. You will get "updating component template [x] results in invalid composable template [x] after templates are merged"

Steps to reproduce:
1.Start a es7.8.0 nodel with docker-compose.
compose file:
version: '3'
services:
es_master:
image: elasticsearch:7.8.0
container_name: es_master
restart: always
environment:
- cluster.name=shotecorps
- network.host=192.168.55.3
- http.port=9200
- transport.port=9300
- node.name=master
- node.remote_cluster_client=false
- node.ml=false
- discovery.seed_hosts=192.168.55.3:9300
- cluster.initial_master_nodes=master1
- "ES_JAVA_OPTS=-Xms2g -Xmx2g"
- "action.auto_create_index=-write-,-act-,-read-,"
- "node.attr.hot=true"
- "node.attr.warm=true"
- path.repo=/usr/share/elasticsearch/data/backup
- bootstrap.memory_lock=true
ports:
- "9300:9300"
volumes:
- /opt/elasticsearch/config/:/usr/share/elasticsearch/config/
- /opt/elasticsearch/data/:/usr/share/elasticsearch/data/
- /opt/elasticsearch/log/:/usr/share/elasticsearch/logs/
logging:
driver: "json-file"
options:
max-size: "100m"
network_mode: "host"
kibana:
image: kibana:7.8.0
container_name: kibana
restart: always
container_name: kibana
restart: always
environment:
- ELASTICSEARCH_URL=http://192.168.55.3:9200
- ELASTICSEARCH_HOSTS=http://192.168.55.3:9200
ports:
- "5601:5601"

2. put component template
   PUT _component_template/a
   {
   "template": {
   "settings": {
   "index": {
   "codec": "default",
   "routing": {
   "allocation": {
   "require": {
   "hot": "true"
   }
   }
   },
   "refresh_interval": "60s",
   "number_of_shards": "1",
   "number_of_replicas": "0"
   }
   },
   "mappings": {
   "dynamic": "false"
   }
   }
   }

3. put index template
   PUT _index_template/template_2
   {
   "index_patterns": ["test*"],
   "template": {
   "mappings": {
   "properties": {
   "host_name": {
   "type": "keyword"
   }
   }
   }
   },
   "priority": 10,
   "composed_of": ["a"]
   }

(Now you can update the component/index template, or create new index template that uses the existed component template successfully)

4. kill es container and restart it
   shotecorps@ubuntu:/tmp$ docker ps
   CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
   1743ca1ce44b kibana:7.8.0 "/usr/local/bin/dumb…" 5 minutes ago Up 5 minutes 0.0.0.0:5601->5601/tcp kibana
   56a923fdf7c2 elasticsearch:7.8.0 "/tini -- /usr/local…" 5 minutes ago Up 5 minutes es_master
   shotecorps@ubuntu:/tmp$ docker rm 56a923fdf7c2 -f
   56a923fdf7c2
   shotecorps@ubuntu:/tmp$ docker ps
   CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
   1743ca1ce44b kibana:7.8.0 "/usr/local/bin/dumb…" 5 minutes ago Up 5 minutes 0.0.0.0:5601->5601/tcp kibana
   shotecorps@ubuntu:/tmp$ docker-compose -f docker-compose.yml up -d
   kibana is up-to-date
   Creating es_master ... done

5. After es started, update old component template will get error response.
   And you cannot put new index template that uses the old component template either.

PUT _component_template/a
{
"template": {
"settings": {
"index": {
"codec": "default",
"routing": {
"allocation": {
"require": {
"hot": "true"
}
}
},
"refresh_interval": "60s",
"number_of_shards": "1",
"number_of_replicas": "0"
}
},
"mappings": {
"dynamic": "false"
}
}
}

{
"error" : {
"root_cause" : [
{
"type" : "illegal_argument_exception",
"reason" : "updating component template [a] results in invalid composable template [template_2] after templates are merged"
}
],
"type" : "illegal_argument_exception",
"reason" : "updating component template [a] results in invalid composable template [template_2] after templates are merged",
"caused_by" : {
"type" : "illegal_argument_exception",
"reason" : "invalid composite mappings for [template_2]",
"caused_by" : {
"type" : "illegal_state_exception",
"reason" : "invalid mapping definition, expected a single map underneath [_doc] but it was: [{properties={host_name={type=keyword}}}]"
}
}
},
"status" : 400
}

PUT _index_template/template_3
{
"index_patterns": ["new*"],
"template": {
"mappings": {
"properties": {
"host_name": {
"type": "keyword"
}
}
}
},
"priority": 10,
"composed_of": ["a"]
}
{
"error" : {
"root_cause" : [
{
"type" : "illegal_argument_exception",
"reason" : "composable template [template_3] template after composition with component templates [a] is invalid"
}
],
"type" : "illegal_argument_exception",
"reason" : "composable template [template_3] template after composition with component templates [a] is invalid",
"caused_by" : {
"type" : "illegal_argument_exception",
"reason" : "invalid composite mappings for [template_3]",
"caused_by" : {
"type" : "illegal_state_exception",
"reason" : "invalid mapping definition, expected a single map underneath [_doc] but it was: [{dynamic=false}]"
}
}
},
"status" : 400
}