fix: undefined index error in cache trait (#617)

Author: bshaffer

src/CacheTrait.php

@@ -95,7 +95,7 @@ private function getFullCacheKey($key)
             return null;
         }
 
-        $key = $this->cacheConfig['prefix'] . $key;
+        $key = ($this->cacheConfig['prefix'] ?? '') . $key;
 
         // ensure we do not have illegal characters
         $key = preg_replace('|[^a-zA-Z0-9_\.!]|', '', $key);

src/CredentialSource/AwsNativeSource.php

@@ -356,7 +356,7 @@ private static function hmacSign(string $key, string $msg): string
      */
     private static function utf8Encode(string $string): string
     {
-        return mb_convert_encoding($string, 'UTF-8', 'ISO-8859-1');
+        return (string) mb_convert_encoding($string, 'UTF-8', 'ISO-8859-1');
     }
 
     private static function getSignatureKey(

tests/Credentials/ImpersonatedServiceAccountCredentialsTest.php

@@ -98,6 +98,19 @@ public function testGetServiceAccountNameID()
         $this->assertEquals('1234567890987654321', $creds->getClientName());
     }
 
+    public function testGetCacheKey()
+    {
+        $creds = new ImpersonatedServiceAccountCredentials(self::SCOPE, [
+            'service_account_impersonation_url' => 'foo',
+            'source_credentials' => [
+                'type' => 'service_account',
+                'client_email' => '123',
+                'private_key' => 'abc'
+            ]
+        ]);
+        $this->assertEquals('foo123.scope1scope2', $creds->getCacheKey());
+    }
+
     public function testMissingImpersonationUriThrowsException()
     {
         $this->expectException(LogicException::class);