Remove discovery-file plugin

In elastic#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/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.

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);

server/src/test/java/org/elasticsearch/discovery/zen/FileBasedUnicastHostsProviderTests.java

@@ -52,11 +52,9 @@
 
 public class FileBasedUnicastHostsProviderTests extends ESTestCase {
 
-    private boolean legacyLocation;
     private ThreadPool threadPool;
     private ExecutorService executorService;
     private MockTransportService transportService;
-    private Path configPath;
 
     @Before
     public void setUp() throws Exception {
@@ -108,24 +106,12 @@ public void testBuildDynamicNodes() throws Exception {
         assertEquals(9300, nodes.get(2).getPort());
     }
 
-    public void testBuildDynamicNodesLegacyLocation() throws Exception {
-        legacyLocation = true;
-        testBuildDynamicNodes();
-        assertDeprecatedLocationWarning();
-    }
-
     public void testEmptyUnicastHostsFile() throws Exception {
         final List<String> hostEntries = Collections.emptyList();
         final List<TransportAddress> addresses = setupAndRunHostProvider(hostEntries);
         assertEquals(0, addresses.size());
     }
 
-    public void testEmptyUnicastHostsFileLegacyLocation() throws Exception {
-        legacyLocation = true;
-        testEmptyUnicastHostsFile();
-        assertDeprecatedLocationWarning();
-    }
-
     public void testUnicastHostsDoesNotExist() {
         final Settings settings = Settings.builder().put(Environment.PATH_HOME_SETTING.getKey(), createTempDir()).build();
         final FileBasedUnicastHostsProvider provider = new FileBasedUnicastHostsProvider(settings, createTempDir().toAbsolutePath());
@@ -141,12 +127,6 @@ public void testInvalidHostEntries() throws Exception {
         assertEquals(0, addresses.size());
     }
 
-    public void testInvalidHostEntriesLegacyLocation() throws Exception {
-        legacyLocation = true;
-        testInvalidHostEntries();
-        assertDeprecatedLocationWarning();
-    }
-
     public void testSomeInvalidHostEntries() throws Exception {
         final List<String> hostEntries = Arrays.asList("192.168.0.1:9300:9300", "192.168.0.1:9301");
         final List<TransportAddress> addresses = setupAndRunHostProvider(hostEntries);
@@ -155,41 +135,21 @@ public void testSomeInvalidHostEntries() throws Exception {
         assertEquals(9301, addresses.get(0).getPort());
     }
 
-    public void testSomeInvalidHostEntriesLegacyLocation() throws Exception {
-        legacyLocation = true;
-        testSomeInvalidHostEntries();
-        assertDeprecatedLocationWarning();
-    }
-
     // sets up the config dir, writes to the unicast hosts file in the config dir,
     // and then runs the file-based unicast host provider to get the list of discovery nodes
     private List<TransportAddress> setupAndRunHostProvider(final List<String> hostEntries) throws IOException {
         final Path homeDir = createTempDir();
         final Settings settings = Settings.builder()
             .put(Environment.PATH_HOME_SETTING.getKey(), homeDir)
             .build();
-        if (randomBoolean()) {
-            configPath = homeDir.resolve("config");
-        } else {
-            configPath = createTempDir();
-        }
-        final Path discoveryFilePath = legacyLocation ? configPath.resolve("discovery-file") : configPath;
-        Files.createDirectories(discoveryFilePath);
-        final Path unicastHostsPath = discoveryFilePath.resolve(UNICAST_HOSTS_FILE);
-        try (BufferedWriter writer = Files.newBufferedWriter(unicastHostsPath)) {
+        final Path configPath = randomBoolean() ? homeDir.resolve("config") : createTempDir();
+        Files.createDirectories(configPath);
+        try (BufferedWriter writer = Files.newBufferedWriter(configPath.resolve(UNICAST_HOSTS_FILE))) {
             writer.write(String.join("\n", hostEntries));
         }
 
         return new FileBasedUnicastHostsProvider(settings, configPath).buildDynamicHosts((hosts, limitPortCounts) ->
             UnicastZenPing.resolveHostsLists(executorService, logger, hosts, limitPortCounts, transportService,
                 TimeValue.timeValueSeconds(10)));
     }
-
-    private void assertDeprecatedLocationWarning() {
-        assertWarnings("Found dynamic hosts list at [" +
-            configPath.resolve("discovery-file").resolve(UNICAST_HOSTS_FILE) +
-            "] but this path is deprecated. This list should be at [" +
-            configPath.resolve(UNICAST_HOSTS_FILE) +
-            "] instead. Support for the deprecated path will be removed in future.");
-    }
 }