Remove discovery-file plugin (#33257)

In #33241 we moved the file-based discovery functionality to core
Elasticsearch, but preserved the `discovery-file` plugin, and support for the
existing location of the `unicast_hosts.txt` file, for BWC reasons. This commit
completes the removal of this plugin.

docs/plugins/discovery.asciidoc

@@ -21,10 +21,6 @@ The Azure Classic discovery plugin uses the Azure Classic API for unicast discov
 
 The Google Compute Engine discovery plugin uses the GCE API for unicast discovery.
 
-<<discovery-file,File-based discovery>>::
-
-The File-based discovery plugin allows providing the unicast hosts list through a dynamically updatable file.
-
 [float]
 ==== Community contributed discovery plugins
 
@@ -38,5 +34,3 @@ include::discovery-ec2.asciidoc[]
 include::discovery-azure-classic.asciidoc[]
 
 include::discovery-gce.asciidoc[]
-
-include::discovery-file.asciidoc[]

docs/reference/cat/plugins.asciidoc

@@ -23,7 +23,6 @@ U7321H6 analysis-stempel        {version} The Stempel (Polish) Analysis plugin i
 U7321H6 analysis-ukrainian      {version} The Ukrainian Analysis plugin integrates the Lucene UkrainianMorfologikAnalyzer into elasticsearch.
 U7321H6 discovery-azure-classic {version} The Azure Classic Discovery plugin allows to use Azure Classic API for the unicast discovery mechanism
 U7321H6 discovery-ec2           {version} The EC2 discovery plugin allows to use AWS API for the unicast discovery mechanism.
-U7321H6 discovery-file          {version} Discovery file plugin enables unicast discovery from hosts stored in a file.
 U7321H6 discovery-gce           {version} The Google Compute Engine (GCE) Discovery plugin allows to use GCE API for the unicast discovery mechanism.
 U7321H6 ingest-attachment       {version} Ingest processor that uses Apache Tika to extract contents
 U7321H6 ingest-geoip            {version} Ingest processor that uses looksup geo data based on ip adresses using the Maxmind geo database

docs/reference/migration/migrate_7_0/plugins.asciidoc

@@ -22,4 +22,12 @@ See {plugins}/repository-gcs-client.html#repository-gcs-client[Google Cloud Stor
 ==== Analysis Plugin changes
 
 * The misspelled helper method `requriesAnalysisSettings(AnalyzerProvider<T> provider)` has been
-renamed to `requiresAnalysisSettings`
+renamed to `requiresAnalysisSettings`
+
+==== File-based discovery plugin
+
+* This plugin has been removed since its functionality is now part of
+Elasticsearch and requires no plugin. The location of the hosts file has moved
+from `$ES_PATH_CONF/file-discovery/unicast_hosts.txt` to
+`$ES_PATH_CONF/unicast_hosts.txt`. See <<file-based-hosts-provider, the
+file-based hosts provider documentation>> for further information.

qa/vagrant/src/test/resources/packaging/tests/module_and_plugin_test_cases.bash

@@ -224,10 +224,6 @@ fi
     install_and_check_plugin discovery ec2 aws-java-sdk-core-*.jar
 }
 
-@test "[$GROUP] install discovery-file plugin" {
-    install_and_check_plugin discovery file
-}
-
 @test "[$GROUP] install ingest-attachment plugin" {
     # we specify the version on the poi-3.17.jar so that the test does
     # not spuriously pass if the jar is missing but the other poi jars
@@ -364,10 +360,6 @@ fi
     remove_plugin discovery-ec2
 }
 
-@test "[$GROUP] remove discovery-file plugin" {
-    remove_plugin discovery-file
-}
-
 @test "[$GROUP] remove ingest-attachment plugin" {
     remove_plugin ingest-attachment
 }

server/src/main/java/org/elasticsearch/discovery/zen/FileBasedUnicastHostsProvider.java

@@ -49,40 +49,28 @@ public class FileBasedUnicastHostsProvider extends AbstractComponent implements
     public static final String UNICAST_HOSTS_FILE = "unicast_hosts.txt";
 
     private final Path unicastHostsFilePath;
-    private final Path legacyUnicastHostsFilePath;
 
     public FileBasedUnicastHostsProvider(Settings settings, Path configFile) {
         super(settings);
         this.unicastHostsFilePath = configFile.resolve(UNICAST_HOSTS_FILE);
-        this.legacyUnicastHostsFilePath = configFile.resolve("discovery-file").resolve(UNICAST_HOSTS_FILE);
     }
 
     private List<String> getHostsList() {
         if (Files.exists(unicastHostsFilePath)) {
-            return readFileContents(unicastHostsFilePath);
-        }
-
-        if (Files.exists(legacyUnicastHostsFilePath)) {
-            deprecationLogger.deprecated("Found dynamic hosts list at [{}] but this path is deprecated. This list should be at [{}] " +
-                "instead. Support for the deprecated path will be removed in future.", legacyUnicastHostsFilePath, unicastHostsFilePath);
-            return readFileContents(legacyUnicastHostsFilePath);
+            try (Stream<String> lines = Files.lines(unicastHostsFilePath)) {
+                return lines.filter(line -> line.startsWith("#") == false) // lines starting with `#` are comments
+                    .collect(Collectors.toList());
+            } catch (IOException e) {
+                logger.warn(() -> new ParameterizedMessage("failed to read file [{}]", unicastHostsFilePath), e);
+                return Collections.emptyList();
+            }
         }
 
         logger.warn("expected, but did not find, a dynamic hosts list at [{}]", unicastHostsFilePath);
 
         return Collections.emptyList();
     }
 
-    private List<String> readFileContents(Path path) {
-        try (Stream<String> lines = Files.lines(path)) {
-            return lines.filter(line -> line.startsWith("#") == false) // lines starting with `#` are comments
-                .collect(Collectors.toList());
-        } catch (IOException e) {
-            logger.warn(() -> new ParameterizedMessage("failed to read file [{}]", unicastHostsFilePath), e);
-            return Collections.emptyList();
-        }
-    }
-
     @Override
     public List<TransportAddress> buildDynamicHosts(HostsResolver hostsResolver) {
         final List<TransportAddress> transportAddresses = hostsResolver.resolveHosts(getHostsList(), 1);