.../EntityResource/EntityResourceTestBase.php      | 141 ++++++++++-----------
 1 file changed, 69 insertions(+), 72 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index b12c4a1..421d666 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -89,52 +89,61 @@ public function setUp() {
   abstract protected function getExpectedNormalizedEntity();
 
   public function testGet() {
+    // The URL and Guzzle request options that will be used in this test. The
+    // request options will be modified/expanded throughout this test:
+    // - to first test all mistakes a developer might make, and assert that the
+    //   error responses provide a good DX
+    // - to eventually result in a well-formed request that succeeds.
+    $url = $this->getUrl();
+    $request_options = [];
+
     // @todo test what happens before provisioning. Edge case: node.
 
     $this->setUpAuthorization('GET');
-
-    // Provision REST resource.
     $this->provisionEntityResource();
 
-    /** @var \Psr\Http\Message\ResponseInterface $response */
     $has_canonical_url = $this->entity->hasLinkTemplate('canonical');
-    $url = $has_canonical_url ? $this->entity->toUrl() : Url::fromUri('base:entity/' . static::$entityType . '/' . $this->entity->id());
 
-    // Requesting without the appropriate ?_format query argument.
-    try {
-      $response = $this->httpClient->get($url->toString(), $this->getAuthenticationRequestOptions());
-    } catch (ClientException $e) {
-      $response = $e->getResponse();
-    }
-    // When the anonymous user accesses a canonical (HTML) route, we won't get a
-    // 406, but a 200, and that is appropriate when no format is specified.
-    if ($has_canonical_url && !$this->account) {
+    // DX: 406 when ?_format is missing, except when requesting a canonical HTML
+    // route.
+    $response = $this->request('GET', $url, $request_options);
+    if ($has_canonical_url && !static::$auth) {
       $this->assertSame(200, $response->getStatusCode());
     }
     else {
       $this->assert406Response($response);
     }
 
+
     $url->setOption('query', ['_format' => static::$format]);
 
-    // Verify that the entity exists: use a HEAD request.
-    $response = $this->httpClient->head($url->toString(), $this->getAuthenticationRequestOptions());
-    $this->assertSame(200, $response->getStatusCode());
+
+    // DX: forgetting authentication: authentication provider-specific error
+    // response.
+    if (static::$auth) {
+      $response = $this->request('GET', $url, $request_options);
+      $this->verifyResponseWhenMissingAuthentication($response);
+    }
+
+
+    $request_options = array_merge_recursive($request_options, $this->getAuthenticationRequestOptions());
+
+
+    // 200 for well-formed HEAD request.
+    $response = $this->request('HEAD', $url, $request_options);
+    $this->assertResourceResponse(200, '', $response);
     if (!$this->account) {
       $this->assertSame(['MISS'], $response->getHeader('X-Drupal-Cache'));
     }
     else {
       $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
     }
-    $this->assertSame('', (string)$response->getBody());
     $head_headers = $response->getHeaders();
 
-    // Now do a GET request. And because of the preceding HEAD request, this
-    // should be a Page Cache hit.
-    $response = $this->httpClient->get($url->toString(), $this->getAuthenticationRequestOptions());
-    $this->assertSame(200, $response->getStatusCode());
-    $this->assertSame([static::$mimeType], $response->getHeader('Content-Type'));
-    if (!$this->account) {
+    // 200 for well-formed GET request. Page Cache hit because of HEAD request.
+    $response = $this->request('GET', $url, $request_options);
+    $this->assertResourceResponse(200, FALSE, $response);
+    if (!static::$auth) {
       $this->assertSame(['HIT'], $response->getHeader('X-Drupal-Cache'));
     }
     else {
@@ -160,59 +169,47 @@ public function testGet() {
     }
     $this->assertSame($get_headers, $head_headers);
 
-    if ($this->account) {
-      // Try to read the entity without the necessary authentication.
-      try {
-        $this->httpClient->get($url->toString());
-        $this->fail('No error response for missing authentication!');
-      }
-      catch (ClientException $e) {
-        $this->verifyResponseWhenMissingAuthentication($e->getResponse());
-      }
-    }
 
-    // Try to read the entity in an unsupported format.
-    try {
-      $url->setOption('query', ['_format' => 'non_existing_format']);
-      $this->httpClient->get($url->toString(), $this->getAuthenticationRequestOptions());
-      $this->fail('No 406 response received.');
-    }
-    catch (ClientException $e) {
-      $response = $e->getResponse();
-      $this->assert406Response($response);
-      // @todo this works fine locally, but on testbot it comes back with 'text/plain; charset=UTF-8'. WTF.
+    $url->setOption('query', ['_format' => 'non_existing_format']);
+
+
+    // DX: 406 when requesting unsupported format.
+    $response = $this->request('GET', $url, $request_options);
+    $this->assert406Response($response);
+    // @todo this works fine locally, but on testbot it comes back with 'text/plain; charset=UTF-8'. WTF.
 //      $this->assertSame(['text/html; charset=UTF-8'], $response->getHeader('Content-Type'));
-    }
 
-    // Repeat the same request, but this time with the format's MIME type in the
-    // Accept header. This time we should get a response with the expected error
-    // MIME type.
-    try {
-      $options = $this->getAuthenticationRequestOptions();
-      $options['headers']['Accept'] = static::$mimeType;
-      $url->setOption('query', ['_format' => 'non_existing_format']);
-      $this->httpClient->get($url->toString(), $options);
-      $this->fail('No 406 response received.');
-    }
-    catch (ClientException $e) {
-      $response = $e->getResponse();
-      $this->assert406Response($response);
-      $this->assertSame([static::$expectedErrorMimeType], $response->getHeader('Content-Type'));
-    }
 
-    // Try to read an entity that does not exist.
-    $non_existing_entity_url = Url::fromRoute('rest.entity.' . static::$entityType . '.GET.' . static::$format);
-    $non_existing_entity_url->setRouteParameter(static::$entityType, 987654321);
-    $non_existing_entity_url->setOption('query', ['_format' => static::$format]);
-    try {
-      $this->httpClient->get($non_existing_entity_url->toString());
-      $this->fail('No 404 response received.');
-    }
-    catch (ClientException $e) {
-      $path = str_replace('987654321', '{' . static::$entityType . '}', $non_existing_entity_url->setAbsolute()->setOptions(['base_url' => '', 'query' => []])->toString());
-      $message = 'The "' . static::$entityType . '" parameter was not converted for the path "' . $path . '" (route name: "rest.entity.' . static::$entityType . '.GET.' . static::$format . '")';
-      $this->assertResourceErrorResponse(404, $message, $e->getResponse());
-    }
+    $request_options[RequestOptions::HEADERS]['Accept'] = static::$mimeType;
+
+
+    // DX: 406 when requesting unsupported format but specifying Accept header.
+    $response = $this->request('GET', $url, $request_options);
+    $this->assert406Response($response);
+    $this->assertSame([static::$expectedErrorMimeType], $response->getHeader('Content-Type'));
+
+
+    $url = Url::fromRoute('rest.entity.' . static::$entityType . '.GET.' . static::$format);
+    $url->setRouteParameter(static::$entityType, 987654321);
+    $url->setOption('query', ['_format' => static::$format]);
+
+
+    // DX: 404 when GETting non-existing entity.
+    $response = $this->request('GET', $url, $request_options);
+    $path = str_replace('987654321', '{' . static::$entityType . '}', $url->setAbsolute()->setOptions(['base_url' => '', 'query' => []])->toString());
+    $message = 'The "' . static::$entityType . '" parameter was not converted for the path "' . $path . '" (route name: "rest.entity.' . static::$entityType . '.GET.' . static::$format . '")';
+    $this->assertResourceErrorResponse(404, $message, $response);
+  }
+
+  /**
+   * Gets an entity resource's GET/PATCH/DELETE URL.
+   *
+   * @return \Drupal\Core\Url
+   *   The URL to GET/PATCH/DELETE.
+   */
+  protected function getUrl() {
+    $has_canonical_url = $this->entity->hasLinkTemplate('canonical');
+    return $has_canonical_url ? $this->entity->toUrl() : Url::fromUri('base:entity/' . static::$entityType . '/' . $this->entity->id());
   }
 
   public function testPost() {