YAML config file parsing behaviour changed between 5.x and master

[spinscale]

The root of this change is #22225

The following configuration file works in 5.x (tested with a 5.4 snapshot from https://oss.sonatype.org/content/repositories/snapshots/org/elasticsearch/distribution/zip/elasticsearch/5.4.0-SNAPSHOT/), but throws an exception in master, tested with a snapshot from https://oss.sonatype.org/content/repositories/snapshots/org/elasticsearch/distribution/zip/elasticsearch/6.0.0-alpha1-SNAPSHOT/

node:
  name: node1

node:
  attr:
    rack: r1

This stacktrace is shown on master

Exception in thread "main" SettingsException[Failed to load settings from /tmp/test-nesting/elasticsearch-6.0.0-alpha1-SNAPSHOT/config/elasticsearch.yml]; nested: JsonParseException[Duplicate field 'node'
...
; line: 4, column: 5]];
	at org.elasticsearch.node.InternalSettingsPreparer.prepareEnvironment(InternalSettingsPreparer.java:104)
	at org.elasticsearch.cli.EnvironmentAwareCommand.createEnv(EnvironmentAwareCommand.java:63)
	at org.elasticsearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:58)
	at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:122)
	at org.elasticsearch.cli.Command.main(Command.java:88)
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:91)
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:84)
Caused by: com.fasterxml.jackson.core.JsonParseException: Duplicate field 'node'
 at [Source: node:
  name: node1

node:
  attr:
    rack: r1
...
; line: 4, column: 5]
	at com.fasterxml.jackson.core.json.JsonReadContext._checkDup(JsonReadContext.java:204)
	at com.fasterxml.jackson.core.json.JsonReadContext.setCurrentName(JsonReadContext.java:198)
	at com.fasterxml.jackson.dataformat.yaml.YAMLParser.nextToken(YAMLParser.java:372)
	at org.elasticsearch.common.xcontent.json.JsonXContentParser.nextToken(JsonXContentParser.java:52)
	at org.elasticsearch.common.settings.loader.XContentSettingsLoader.serializeObject(XContentSettingsLoader.java:105)
	at org.elasticsearch.common.settings.loader.XContentSettingsLoader.load(XContentSettingsLoader.java:75)
	at org.elasticsearch.common.settings.loader.XContentSettingsLoader.load(XContentSettingsLoader.java:52)
	at org.elasticsearch.common.settings.loader.YamlSettingsLoader.load(YamlSettingsLoader.java:50)
	at org.elasticsearch.common.settings.Settings$Builder.loadFromStream(Settings.java:1046)
	at org.elasticsearch.common.settings.Settings$Builder.loadFromPath(Settings.java:1035)
	at org.elasticsearch.node.InternalSettingsPreparer.prepareEnvironment(InternalSettingsPreparer.java:102)
	... 6 more

I do consider this a good thing, just something we should be aware of that needs to be kept in mind when upgrading (part of the upgrade docs, as it is hard to warn about before the upgrade I suppose).

Feel free to close without comment if not considered an issue.

[jasontedor]

I think it's fine as this is a good change, and the effort to work around this is minimal. I think that it just needs to be documented so I opened #24022.