node.name=${HOSTNAME} in the environment variable could not be resolved

[maomao905]

Elasticsearch Version

8.6.0

Installed Plugins

no installed plugins

Java Version

19.0.1

OS Version

macOS Monterey 12.3.1

Problem Description

I use the environment variable node.name=$HOSTNAME to dynamically set the node name.
It is fine until Elasticsearch version 7.17.7.
When I upgraded to 8.6.0, Elasticsearch failed to start.
The problem is HOSTNAME in the environment variable could not be resolved when starting up Elasticsearch.
The expected behavior is $HOSTNAME should be replaced by the hostname of the running Elasticsearch machine.

Steps to Reproduce

docker-compose up es will reproduce the problem.

• docker-compose.yml
  $${HOSTNAME} is escaped and should be interpreted as ${HOSTNAME} in Elasticsearch.
  I checked that there is no problem with version 7.17.7 using the image docker.elastic.co/elasticsearch/elasticsearch:7.17.7.

version: '3'
services:
  es:
    image: docker.elastic.co/elasticsearch/elasticsearch:8.6.0
    ports:
      - "9200:9200"
    environment:
      - discovery.type=single-node
      - node.name=$${HOSTNAME}
      - node.roles=master,data
      - ES_JAVA_OPTS=-Xms425m -Xmx425m

I confirmed that the same problem occurs in AWS ECS.

Logs (if relevant)

Exception in thread "main" java.lang.IllegalArgumentException: Could not resolve placeholder 'HOSTNAME'
	at org.elasticsearch.common.settings.PropertyPlaceholder.parseStringValue(PropertyPlaceholder.java:102)
	at org.elasticsearch.common.settings.PropertyPlaceholder.replacePlaceholders(PropertyPlaceholder.java:57)
	at org.elasticsearch.common.settings.Settings$Builder.replacePropertyPlaceholders(Settings.java:1296)
	at org.elasticsearch.node.InternalSettingsPreparer.prepareEnvironment(InternalSettingsPreparer.java:56)
	at org.elasticsearch.common.cli.EnvironmentAwareCommand.createEnv(EnvironmentAwareCommand.java:110)
	at org.elasticsearch.common.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:54)
	at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:85)
	at org.elasticsearch.cli.Command.main(Command.java:50)
	at org.elasticsearch.launcher.CliToolLauncher.main(CliToolLauncher.java:64)

[maomao905]

By the way,
According to the document, the default node name is the hostname of the machine.

The node name defaults to the hostname of the machine when Elasticsearch starts.

https://www.elastic.co/guide/en/elasticsearch/reference/7.17/important-settings.html#node-name

Therefore, there is no need to set ${HOSTNAME} in the environment variable.

But, I actually set the node.name with the prefix of the node role like data.${HOSTNAME}, client.${HOSTNAME}.
That's why we want to specify it by the environment variable.

Alternatively, node.name could be specified in the elasticsearch.yml. But the environment variable is more flexible and does not need an elasticsearch.yml file for each node role.

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[mark-vieira]

This is basically the same problem as reported in #90823. The issue here is that we used to allow variable substitution in environment variables with other environment variables, although this was untested, and undocumented, and now this no longer works.

@rjernst Should we treat this as a regression? The previous issue was closed because the original submitter found a workaround but I don't think that would work for this case.