Enable setting client.path.prefix='/'

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

@@ -796,8 +796,10 @@ static URI buildUri(String pathPrefix, String path, Map<String, String> params)
         Objects.requireNonNull(path, "path must not be null");
         try {
             String fullPath;
-            if (pathPrefix != null) {
-                if (path.startsWith("/")) {
+            if (pathPrefix != null && pathPrefix.isEmpty() == false) {
+                if (pathPrefix.endsWith("/") && path.startsWith("/")) {
+                    fullPath = pathPrefix.substring(0, pathPrefix.length() - 1) + path;
+                } else if (pathPrefix.endsWith("/") || path.startsWith("/")) {
                     fullPath = pathPrefix + path;
                 } else {
                     fullPath = pathPrefix + "/" + path;

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

@@ -143,32 +143,33 @@ public RestClientBuilder setRequestConfigCallback(RequestConfigCallback requestC
      * For example, if this is set to "/my/path", then any client request will become <code>"/my/path/" + endpoint</code>.
      * <p>
      * In essence, every request's {@code endpoint} is prefixed by this {@code pathPrefix}. The path prefix is useful for when
-     * Elasticsearch is behind a proxy that provides a base path; it is not intended for other purposes and it should not be supplied in
-     * other scenarios.
+     * Elasticsearch is behind a proxy that provides a base path or a proxy that requires all paths to start with '/';
+     * it is not intended for other purposes and it should not be supplied in other scenarios.
      *
      * @throws NullPointerException if {@code pathPrefix} is {@code null}.
-     * @throws IllegalArgumentException if {@code pathPrefix} is empty, only '/', or ends with more than one '/'.
+     * @throws IllegalArgumentException if {@code pathPrefix} is empty, or ends with more than one '/'.
      */
     public RestClientBuilder setPathPrefix(String pathPrefix) {
         Objects.requireNonNull(pathPrefix, "pathPrefix must not be null");
         String cleanPathPrefix = pathPrefix;
 
+        if (cleanPathPrefix.isEmpty()) {
+            throw new IllegalArgumentException("pathPrefix must not be empty: [" + pathPrefix + "]");
+        }
+
         if (cleanPathPrefix.startsWith("/") == false) {
             cleanPathPrefix = "/" + cleanPathPrefix;
         }
 
         // best effort to ensure that it looks like "/base/path" rather than "/base/path/"
-        if (cleanPathPrefix.endsWith("/")) {
+        if (cleanPathPrefix.endsWith("/") && cleanPathPrefix.length() > 1) {
             cleanPathPrefix = cleanPathPrefix.substring(0, cleanPathPrefix.length() - 1);
 
             if (cleanPathPrefix.endsWith("/")) {
                 throw new IllegalArgumentException("pathPrefix is malformed. too many trailing slashes: [" + pathPrefix + "]");
             }
         }
 
-        if (cleanPathPrefix.isEmpty() || "/".equals(cleanPathPrefix)) {
-            throw new IllegalArgumentException("pathPrefix must not be empty or '/': [" + pathPrefix + "]");
-        }
 
         this.pathPrefix = cleanPathPrefix;
         return this;

client/rest/src/test/java/org/elasticsearch/client/RestClientBuilderTests.java

@@ -180,7 +180,6 @@ public void testSetPathPrefixNull() {
     }
 
     public void testSetPathPrefixEmpty() {
-        assertSetPathPrefixThrows("/");
         assertSetPathPrefixThrows("");
     }
 

client/rest/src/test/java/org/elasticsearch/client/RestClientTests.java

@@ -223,6 +223,26 @@ public void testBuildUriLeavesPathUntouched() {
             URI uri = RestClient.buildUri("/foo$bar", "/index/type/id", Collections.<String, String>emptyMap());
             assertEquals("/foo$bar/index/type/id", uri.getPath());
         }
+        {
+            URI uri = RestClient.buildUri("/", "/*", Collections.<String, String>emptyMap());
+            assertEquals("/*", uri.getPath());
+        }
+        {
+            URI uri = RestClient.buildUri("/", "*", Collections.<String, String>emptyMap());
+            assertEquals("/*", uri.getPath());
+        }
+        {
+            URI uri = RestClient.buildUri(null, "*", Collections.<String, String>emptyMap());
+            assertEquals("*", uri.getPath());
+        }
+        {
+            URI uri = RestClient.buildUri("", "*", Collections.<String, String>emptyMap());
+            assertEquals("*", uri.getPath());
+        }
+        {
+            URI uri = RestClient.buildUri(null, "/*", Collections.<String, String>emptyMap());
+            assertEquals("/*", uri.getPath());
+        }
         {
             URI uri = RestClient.buildUri(null, "/foo$bar/ty/pe/i/d", Collections.<String, String>emptyMap());
             assertEquals("/foo$bar/ty/pe/i/d", uri.getPath());

test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java

@@ -90,6 +90,7 @@ public abstract class ESRestTestCase extends ESTestCase {
     public static final String TRUSTSTORE_PASSWORD = "truststore.password";
     public static final String CLIENT_RETRY_TIMEOUT = "client.retry.timeout";
     public static final String CLIENT_SOCKET_TIMEOUT = "client.socket.timeout";
+    public static final String CLIENT_PATH_PREFIX = "client.path.prefix";
 
     /**
      * Convert the entity from a {@link Response} into a map of maps.
@@ -383,7 +384,11 @@ private void waitForClusterStateUpdatesToFinish() throws Exception {
      * Used to obtain settings for the REST client that is used to send REST requests.
      */
     protected Settings restClientSettings() {
-        return Settings.EMPTY;
+        Settings.Builder builder = Settings.builder();
+        if (System.getProperty("tests.rest.client_path_prefix") != null) {
+            builder.put(CLIENT_PATH_PREFIX, System.getProperty("tests.rest.client_path_prefix"));
+        }
+        return builder.build();
     }
 
     /**
@@ -454,6 +459,9 @@ protected static void configureClient(RestClientBuilder builder, Settings settin
             final TimeValue socketTimeout = TimeValue.parseTimeValue(socketTimeoutString, CLIENT_SOCKET_TIMEOUT);
             builder.setRequestConfigCallback(conf -> conf.setSocketTimeout(Math.toIntExact(socketTimeout.getMillis())));
         }
+        if (settings.hasValue(CLIENT_PATH_PREFIX)) {
+            builder.setPathPrefix(settings.get(CLIENT_PATH_PREFIX));
+        }
     }
 
     @SuppressWarnings("unchecked")

x-pack/qa/core-rest-tests-with-security/src/test/java/org/elasticsearch/xpack/security/CoreWithSecurityClientYamlTestSuiteIT.java

@@ -39,6 +39,7 @@ public static Iterable<Object[]> parameters() throws Exception {
     protected Settings restClientSettings() {
         String token = basicAuthHeaderValue(USER, new SecureString(PASS.toCharArray()));
         return Settings.builder()
+                .put(super.restClientSettings())
                 .put(ThreadContext.PREFIX + ".Authorization", token)
                 .build();
     }