Add Graphite Web HTTP request timeout option

If the Graphite Web server is unreachable, all requests for frontend
pages containing graphs hang until the backend HTTP request times out,
resulting in a very poor UX.

The Guzzle documentation states that the default behaviour is to wait
indefinitely, however in our testing the cURL handler has an internal
default of 30 seconds:

https://docs.guzzlephp.org/en/stable/request-options.html#timeout

This commit makes the HTTP request timeout configurable and sets a
reasonable default of 10 seconds.

application/forms/Config/BackendForm.php

@@ -53,6 +53,16 @@ class BackendForm extends ConfigForm
                     'label'         => $this->translate('Connect insecurely'),
                     'description'   => $this->translate('Check this to not verify the remote\'s TLS certificate')
                 ]
+            ],
+            [
+                'number',
+                'graphite_timeout',
+                [
+                    'label'         => $this->translate('Request timeout'),
+                    'description'   => $this->translate('The timeout for HTTP requests to Graphite Web'),
+                    'min'           => 0,
+                    'placeholder'   => 10
+                ]
             ]
         ]);
     }

library/Graphite/Graphing/GraphingTrait.php

@@ -73,6 +73,7 @@ trait GraphingTrait
                     ->setUser($graphite->user)
                     ->setPassword($graphite->password)
                     ->setInsecure((bool) $graphite->insecure)
+                    ->setTimeout(isset($graphite->timeout) ? intval($graphite->timeout) : 10)
             );
         }
 

library/Graphite/Graphing/GraphiteWebClient.php

@@ -40,6 +40,13 @@ class GraphiteWebClient
      */
     protected $insecure = false;
 
+    /**
+     * Timeout for every Graphite Web HTTP request
+     *
+     * @var ?int
+     */
+    protected $timeout;
+
     /**
      * HTTP client
      *
@@ -79,9 +86,12 @@ class GraphiteWebClient
         // TODO(ak): keep connections alive (TCP handshakes are a bit expensive and TLS handshakes are very expensive)
         return (string) $this->httpClient->send(
             new Request($method, $this->completeUrl($url)->getAbsoluteUrl(), $headers, $body),
-            ['curl' => [
-                CURLOPT_SSL_VERIFYPEER => ! $this->insecure
-            ]]
+            [
+                'curl' => [
+                    CURLOPT_SSL_VERIFYPEER => ! $this->insecure
+                ],
+                'timeout' => $this->timeout ?? 10
+            ]
         )->getBody();
     }
 
@@ -195,4 +205,28 @@ class GraphiteWebClient
 
         return $this;
     }
+
+    /**
+     * Get the HTTP request timeout
+     *
+     * @return ?int
+     */
+    public function getTimeout(): ?int
+    {
+        return $this->timeout;
+    }
+
+    /**
+     * Set the HTTP request timeout
+     *
+     * @param ?int $timeout
+     *
+     * @return $this
+     */
+    public function setTimeout(?int $timeout): self
+    {
+        $this->timeout = $timeout;
+
+        return $this;
+    }
 }