Incorporate host in request after authenticate() call (#282)

## Changes

Notebook native auth was not working because the API client accessed the
host before the credential provider had a chance to set it. This change
postpones reading the host until after the `config.authenticate()` call.

Fixes #230.

## Tests

* The new API client test adds coverage for this case.
* I manually confirmed that notebook native auth now works out of the
box on DBR.

databricks-sdk-java/src/main/java/com/databricks/sdk/core/ApiClient.java

@@ -231,11 +231,10 @@ private <T> T execute(Request in, Class<T> target) throws IOException {
   }
 
   private Response getResponse(Request in) {
-    in.withUrl(config.getHost() + in.getUrl());
-    return executeInner(in);
+    return executeInner(in, in.getUrl());
   }
 
-  private Response executeInner(Request in) {
+  private Response executeInner(Request in, String path) {
     RetryStrategy retryStrategy = retryStrategyPicker.getRetryStrategy(in);
     int attemptNumber = 0;
     while (true) {
@@ -247,6 +246,10 @@ private Response executeInner(Request in) {
       // Authenticate the request. Failures should not be retried.
       in.withHeaders(config.authenticate());
 
+      // Prepend host to URL only after config.authenticate().
+      // This call may configure the host (e.g. in case of notebook native auth).
+      in.withUrl(config.getHost() + path);
+
       // Set User-Agent with auth type info, which is available only
       // after the first invocation to config.authenticate()
       String userAgent = String.format("%s auth/%s", UserAgent.asString(), config.getAuthType());

databricks-sdk-java/src/test/java/com/databricks/sdk/core/ApiClientTest.java

@@ -42,26 +42,24 @@ public boolean equals(Object o) {
     }
   }
 
-  private ApiClient getApiClient(Request request, List<ResponseProvider> responses) {
+  private ApiClient getApiClient(
+      DatabricksConfig config, Request request, List<ResponseProvider> responses) {
     DummyHttpClient hc = new DummyHttpClient();
     for (ResponseProvider response : responses) {
       hc.with(request, response);
     }
+    return new ApiClient(config.setHttpClient(hc), new FakeTimer());
+  }
+
+  private ApiClient getApiClient(Request request, List<ResponseProvider> responses) {
     String host = request.getUri().getScheme() + "://" + request.getUri().getHost();
     DatabricksConfig config =
-        new DatabricksConfig()
-            .setHttpClient(hc)
-            .setHost(host)
-            .setCredentialsProvider(new DummyCredentialsProvider());
-    return new ApiClient(config, new FakeTimer());
+        new DatabricksConfig().setHost(host).setCredentialsProvider(new DummyCredentialsProvider());
+    return getApiClient(config, request, responses);
   }
 
   private <T> void runApiClientTest(
-      Request request,
-      List<ResponseProvider> responses,
-      Class<? extends T> clazz,
-      T expectedResponse) {
-    ApiClient client = getApiClient(request, responses);
+      ApiClient client, Request request, Class<? extends T> clazz, T expectedResponse) {
     T response;
     if (request.getMethod().equals(Request.GET)) {
       response = client.GET(request.getUri().getPath(), clazz, Collections.emptyMap());
@@ -73,6 +71,15 @@ private <T> void runApiClientTest(
     assertEquals(response, expectedResponse);
   }
 
+  private <T> void runApiClientTest(
+      Request request,
+      List<ResponseProvider> responses,
+      Class<? extends T> clazz,
+      T expectedResponse) {
+    ApiClient client = getApiClient(request, responses);
+    runApiClientTest(client, request, clazz, expectedResponse);
+  }
+
   private void runFailingApiClientTest(
       Request request, List<ResponseProvider> responses, Class<?> clazz, String expectedMessage) {
     DatabricksException exception =
@@ -347,6 +354,39 @@ void retryUnknownHostException() {
         new MyEndpointResponse().setKey("value"));
   }
 
+  class HostPopulatingCredentialsProvider implements CredentialsProvider {
+    private final String host;
+    private final CredentialsProvider parent;
+
+    public HostPopulatingCredentialsProvider(String host) {
+      this.host = host;
+      this.parent = new DummyCredentialsProvider();
+    }
+
+    @Override
+    public String authType() {
+      return parent.authType();
+    }
+
+    @Override
+    public HeaderFactory configure(DatabricksConfig config) {
+      config.setHost(this.host);
+      return parent.configure(config);
+    }
+  }
+
+  @Test
+  void populateHostFromCredentialProvider() {
+    Request req = getBasicRequest();
+    DatabricksConfig config =
+        new DatabricksConfig()
+            .setCredentialsProvider(new HostPopulatingCredentialsProvider("http://my.host"));
+    ApiClient client =
+        getApiClient(config, req, Collections.singletonList(getSuccessResponse(req)));
+    runApiClientTest(
+        client, req, MyEndpointResponse.class, new MyEndpointResponse().setKey("value"));
+  }
+
   @Test
   void testGetBackoffFromRetryAfterHeader() {
     Request req = getBasicRequest();