Fixes infinite loop in CredentialProvider (#1705)

* Fixes infinite loop in CredentialProvider if InstanceProfileProvider throws

* Updated test name to be more meaningful

* Added changelog document

.changes/nextrelease/credential_provider_loop_fix.json

@@ -0,0 +1,7 @@
+[
+    {
+        "type": "bugfix",
+        "category": "Credentials",
+        "description": "Fixes a failure loop if InstanceProfileProvider fails."
+    }
+]

src/Credentials/CredentialProvider.php

@@ -158,6 +158,11 @@ public static function memoize(callable $provider)
                     }
                     // Refresh the result and forward the promise.
                     return $result = $provider();
+                })
+                ->otherwise(function($reason) use (&$result) {
+                    // Cleanup rejected promise.
+                    $result = null;
+                    return new Promise\RejectedPromise($reason);
                 });
         };
     }

tests/Credentials/CredentialProviderTest.php

@@ -325,6 +325,19 @@ public function testMemoizes()
         $this->assertEquals(1, $called);
     }
 
+    public function testMemoizesCleansUpOnError()
+    {
+        $called = 0;
+        $f = function () use (&$called) {
+            $called++;
+            return Promise\rejection_for('Error');
+        };
+        $p = CredentialProvider::memoize($f);
+        $p()->wait(false);
+        $p()->wait(false);
+        $this->assertEquals(2, $called);
+    }
+
     public function testCallsDefaultsCreds()
     {
         $k = getenv(CredentialProvider::ENV_KEY);