Validate monitoring username at parse time

x-pack/plugin/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/exporter/http/HttpExporter.java

@@ -177,7 +177,46 @@ public Iterator<Setting<?>> settings() {
      */
     public static final Setting.AffixSetting<String> AUTH_USERNAME_SETTING =
             Setting.affixKeySetting("xpack.monitoring.exporters.","auth.username",
-                    (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope, Property.Filtered));
+                    (key) -> Setting.simpleString(
+                        key,
+                        new Setting.Validator<String>() {
+                            @Override
+                            public void validate(String password) {
+
+                            }
+
+                            @Override
+                            public void validate(String username, Map<Setting<?>, Object> settings) {
+                                final String namespace =
+                                    HttpExporter.AUTH_USERNAME_SETTING.getNamespace(
+                                        HttpExporter.AUTH_USERNAME_SETTING.getConcreteSetting(key));
+                                final String password =
+                                    (String) settings.get(AUTH_PASSWORD_SETTING.getConcreteSettingForNamespace(namespace));
+
+                                // password must be specified along with username for any auth
+                                if (Strings.isNullOrEmpty(username) == false) {
+                                    if (Strings.isNullOrEmpty(password)) {
+                                        throw new SettingsException(
+                                            "[" + AUTH_USERNAME_SETTING.getConcreteSettingForNamespace(namespace).getKey() + "] without [" +
+                                                AUTH_PASSWORD_SETTING.getConcreteSettingForNamespace(namespace).getKey() + "]");
+                                    }
+                                }
+                            }
+
+                            @Override
+                            public Iterator<Setting<?>> settings() {
+                                final String namespace =
+                                    HttpExporter.AUTH_USERNAME_SETTING.getNamespace(
+                                        HttpExporter.AUTH_USERNAME_SETTING.getConcreteSetting(key));
+                                final List<Setting<?>> settings = List.of(
+                                    HttpExporter.AUTH_PASSWORD_SETTING.getConcreteSettingForNamespace(namespace));
+                                return settings.iterator();
+                            }
+
+                        },
+                        Property.Dynamic,
+                        Property.NodeScope,
+                        Property.Filtered));
     /**
      * Password for basic auth.
      */

x-pack/plugin/monitoring/src/test/java/org/elasticsearch/xpack/monitoring/exporter/http/HttpExporterTests.java

@@ -238,6 +238,28 @@ public void testExporterWithPasswordButNoUsername() {
         assertThat(exception.getMessage(), equalTo(expected));
     }
 
+    public void testExporterWithUsernameButNoPassword() {
+        final String expected =
+            "[xpack.monitoring.exporters._http.auth.username] without [xpack.monitoring.exporters._http.auth.password]";
+        final String prefix = "xpack.monitoring.exporters._http";
+        final Settings settings = Settings.builder()
+            .put(prefix + ".type", HttpExporter.TYPE)
+            .put(prefix + ".host", "localhost:9200")
+            .put(prefix + ".auth.username", "_user")
+            .build();
+
+        final IllegalArgumentException e = expectThrows(
+            IllegalArgumentException.class,
+            () -> HttpExporter.AUTH_USERNAME_SETTING.getConcreteSetting(prefix + ".auth.username").get(settings));
+        assertThat(
+            e,
+            hasToString(
+                containsString("Failed to parse value [_user] for setting [xpack.monitoring.exporters._http.auth.username]")));
+
+        assertThat(e.getCause(), instanceOf(SettingsException.class));
+        assertThat(e.getCause(), hasToString(containsString(expected)));
+    }
+
     public void testExporterWithUnknownBlacklistedClusterAlerts() {
         final SSLIOSessionStrategy sslStrategy = mock(SSLIOSessionStrategy.class);
         when(sslService.sslIOSessionStrategy(any(Settings.class))).thenReturn(sslStrategy);