Execute system call only once even if the node is not found

src/Provider/Node/SystemNodeProvider.php

@@ -25,7 +25,7 @@ class SystemNodeProvider implements NodeProviderInterface
     /**
      * Returns the system node ID
      *
-     * @return string System node ID as a hexadecimal string
+     * @return string|false System node ID as a hexadecimal string, or false if it is not found
      */
     public function getNode()
     {
@@ -40,6 +40,7 @@ class SystemNodeProvider implements NodeProviderInterface
 
         // Search the ifconfig output for all MAC addresses and return
         // the first one found
+        $node = false;
         if (preg_match_all($pattern, $this->getIfconfig(), $matches, PREG_PATTERN_ORDER)) {
             $node = $matches[1][0];
             $node = str_replace(':', '', $node);

tests/src/Provider/Node/SystemNodeProviderTest.php

@@ -71,6 +71,20 @@ class SystemNodeProviderTest extends TestCase
         $this->assertEquals('AABBCCDDEEFF', $node);
     }
 
+    public function testGetNodeWillNotExecuteSystemCallIfFailedFirstTime()
+    {
+        $provider = $this->getMockBuilder('Ramsey\Uuid\Provider\Node\SystemNodeProvider')
+            ->setMethods(['getIfconfig'])
+            ->getMock();
+
+        $provider->expects($this->once())
+            ->method('getIfconfig')
+            ->willReturn('some string that does not match the mac address');
+
+        $provider->getNode();
+        $provider->getNode();
+    }
+
     public function osCommandDataProvider()
     {
         return [