Validate proxy base path at parse time (#47912)

client/rest/src/main/java/org/elasticsearch/client/RestClientBuilder.java

@@ -135,6 +135,11 @@ public final class RestClientBuilder {
      * @throws IllegalArgumentException if {@code pathPrefix} is empty, or ends with more than one '/'.
      */
     public RestClientBuilder setPathPrefix(String pathPrefix) {
+        this.pathPrefix = cleanPathPrefix(pathPrefix);
+        return this;
+    }
+
+    public static String cleanPathPrefix(String pathPrefix) {
         Objects.requireNonNull(pathPrefix, "pathPrefix must not be null");
 
         if (pathPrefix.isEmpty()) {
@@ -154,10 +159,7 @@ public final class RestClientBuilder {
                 throw new IllegalArgumentException("pathPrefix is malformed. too many trailing slashes: [" + pathPrefix + "]");
             }
         }
-
-
-        this.pathPrefix = cleanPathPrefix;
-        return this;
+        return cleanPathPrefix;
     }
 
     /**

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

@@ -197,7 +197,20 @@ public class HttpExporter extends Exporter {
      */
     public static final Setting.AffixSetting<String> PROXY_BASE_PATH_SETTING =
             Setting.affixKeySetting("xpack.monitoring.exporters.","proxy.base_path",
-                    (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope));
+                    (key) -> Setting.simpleString(
+                        key,
+                        value -> {
+                            if (Strings.isNullOrEmpty(value) == false) {
+                                try {
+                                    RestClientBuilder.cleanPathPrefix(value);
+                                } catch (RuntimeException e) {
+                                    Setting<?> concreteSetting = HttpExporter.PROXY_BASE_PATH_SETTING.getConcreteSetting(key);
+                                    throw new SettingsException("[" + concreteSetting.getKey() + "] is malformed [" + value + "]", e);
+                                }
+                            }
+                        },
+                        Property.Dynamic,
+                        Property.NodeScope));
     /**
      * A boolean setting to enable or disable sniffing for extra connections.
      */

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

@@ -279,6 +279,29 @@ public class HttpExporterTests extends ESTestCase {
         new HttpExporter(config, sslService, threadContext).close();
     }
 
+    public void testExporterWithInvalidProxyBasePath() throws Exception {
+        final String prefix = "xpack.monitoring.exporters._http";
+        final String settingName = ".proxy.base_path";
+        final String settingValue = "z//";
+        final String expected = "[" + prefix + settingName + "] is malformed [" + settingValue + "]";
+        final Settings settings = Settings.builder()
+            .put(prefix + ".type", HttpExporter.TYPE)
+            .put(prefix + ".host", "localhost:9200")
+            .put(prefix + settingName, settingValue)
+            .build();
+
+        final IllegalArgumentException e = expectThrows(
+            IllegalArgumentException.class,
+            () -> HttpExporter.PROXY_BASE_PATH_SETTING.getConcreteSetting(prefix + settingName).get(settings));
+        assertThat(
+            e,
+            hasToString(
+                containsString("Failed to parse value [" + settingValue + "] for setting [" + prefix + settingName + "]")));
+
+        assertThat(e.getCause(), instanceOf(SettingsException.class));
+        assertThat(e.getCause(), hasToString(containsString(expected)));
+    }
+
     public void testCreateRestClient() throws IOException {
         final SSLIOSessionStrategy sslStrategy = mock(SSLIOSessionStrategy.class);