Reinstate unicast_hosts.txt

Author: DaveCTurner

docs/plugins/discovery-file.asciidoc

@@ -2,9 +2,13 @@
 === File-Based Discovery Plugin
 
 The functionality provided by the `discovery-file` plugin is now available in
-Elasticsearch without requiring a plugin. This plugin still exists for backwards
-compatibility, but it no longer does anything and will be removed in a future
-version.
+Elasticsearch without requiring a plugin. This plugin still exists to ensure
+backwards compatibility, but it will be removed in a future version.
+
+On installation, this plugin creates a file at
+`$ES_PATH_CONF/discovery-file/unicast_hosts.txt` that comprises comments that
+describe how to use it. It is preferable not to install this plugin and instead
+to create this file, and its containing directory, using standard tools.
 
 :plugin_name: discovery-file
 include::install_remove.asciidoc[]

plugins/discovery-file/build.gradle

@@ -26,6 +26,12 @@ esplugin {
     classname 'org.elasticsearch.discovery.file.FileBasedDiscoveryPlugin'
 }
 
+bundlePlugin {
+    from('config/discovery-file') {
+        into 'config'
+    }
+}
+
 task setupSeedNodeAndUnicastHostsFile(type: DefaultTask) {
     mustRunAfter(precommit)
 }

plugins/discovery-file/config/discovery-file/unicast_hosts.txt

@@ -0,0 +1,20 @@
+# The unicast_hosts.txt file contains the list of unicast hosts to connect to
+# for pinging during the discovery process, when using the file-based discovery
+# mechanism.  This file should contain one entry per line, where an entry is a
+# host/port combination.  The host and port should be separated by a `:`.  If
+# the port is left off, a default port of 9300 is assumed.  For example, if the
+# cluster has three nodes that participate in the discovery process:
+# (1) 66.77.88.99 running on port 9300 (2) 66.77.88.100 running on port 9305
+# and (3) 66.77.88.99 running on port 10005, then this file should contain the
+# following text:
+#
+#10.10.10.5
+#10.10.10.6:9305
+#10.10.10.5:10005
+#
+# For IPv6 addresses, make sure to put a bracket around the host part of the address,
+# for example: [2001:cdba:0000:0000:0000:0000:3257:9652]:9301 (where 9301 is the port).
+#
+# NOTE: all lines starting with a `#` are comments, and comments must exist
+# on lines of their own (i.e. comments cannot begin in the middle of a line)
+#