feat(Storage): Add x-goog-gcs-idempotency-token idempotency header

apis/Google.Cloud.Storage.V1/Google.Cloud.Storage.V1.Tests/StorageClientImplRetryTest.cs

@@ -491,5 +491,33 @@ Dictionary<string, string> ConvertHeader(string header) =>
                 header.Split(' ').ToDictionary(piece => piece.Split('/')[0], piece => piece.Split('/')[1]);
         }
         #endregion
+
+        #region Idempotency Header test case
+        [Fact]
+        public void IdempotencyHeaderIsSet()
+        {
+            var replayingMessageHandler = new ReplayingMessageHandler(RetryHandler.IdempotencyTokenHeader);
+            var service = new FakeStorageService(replayingMessageHandler);
+
+            var request = service.Buckets.Get("bucket");
+            service.ExpectRequest(request, HttpStatusCode.BadGateway);
+            service.ExpectRequest(request, HttpStatusCode.BadGateway);
+            service.ExpectRequest(request, new Bucket());
+
+            var client = new StorageClientImpl(service);
+            client.GetBucket("bucket");
+            service.Verify();
+
+            var actualHeaders = replayingMessageHandler.CapturedHeaders;
+
+            Assert.Equal(3, actualHeaders.Count);
+            Assert.All(actualHeaders, token => Assert.False(string.IsNullOrEmpty(token)));
+
+            var firstToken = actualHeaders.First();
+            // Just validate that it's a real GUID...
+            Guid.Parse(firstToken);
+            Assert.All(actualHeaders, token => Assert.Equal(firstToken, token));
+        }
+        #endregion
     }
 }

apis/Google.Cloud.Storage.V1/Google.Cloud.Storage.V1/RetryHandler.cs

@@ -35,6 +35,7 @@ internal sealed class RetryHandler : IHttpUnsuccessfulResponseHandler
         // For testing
         internal const string InvocationIdHeaderPart = "gccl-invocation-id";
         internal const string AttemptCountHeaderPart = "gccl-attempt-count";
+        internal const string IdempotencyTokenHeader = "x-goog-gcs-idempotency-token";
 
         private readonly RetryOptions _retryOptions;
         private readonly IScheduler _scheduler;
@@ -60,6 +61,7 @@ internal static void MarkAsRetriable<TResponse>(StorageBaseServiceRequest<TRespo
             // Note: we can't use ModifyRequest, as the x-goog-api-client header is added later by ConfigurableMessageHandler.
             // Additionally, that's only called once, and we may want to record the attempt number as well.
             request.AddExecuteInterceptor(InvocationIdInterceptor.Instance);
+            request.AddExecuteInterceptor(IdempotencyTokenInterceptor.Instance);
             request.AddUnsuccessfulResponseHandler(retryHandler);
         }
 
@@ -135,8 +137,26 @@ public Task InterceptAsync(HttpRequestMessage request, CancellationToken cancell
 
                 request.Headers.Remove(VersionHeaderBuilder.HeaderName);
                 request.Headers.Add(VersionHeaderBuilder.HeaderName, string.Join(" ", parts));
+                return Task.CompletedTask;
+            }
+        }
 
+        /// <summary>
+        /// Interceptor that automatically attaches a unique idempotency token 
+        /// to the x-goog-gcs-idempotency-token header.
+        /// </summary>
+        private sealed class IdempotencyTokenInterceptor : IHttpExecuteInterceptor
+        {
+            internal static IdempotencyTokenInterceptor Instance { get; } = new IdempotencyTokenInterceptor();
 
+            private IdempotencyTokenInterceptor() { }
+
+            public Task InterceptAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+            {
+                if (!request.Headers.Contains(IdempotencyTokenHeader))
+                {
+                    request.Headers.TryAddWithoutValidation(IdempotencyTokenHeader, Guid.NewGuid().ToString());
+                }
                 return Task.CompletedTask;
             }
         }