feat(storage): introduce core feature adoption tracking bitmask and options

google/cloud/storage/google_cloud_cpp_storage.bzl

@@ -61,6 +61,7 @@ google_cloud_cpp_storage_hdrs = [
     "internal/curl/request_builder.h",
     "internal/default_object_acl_requests.h",
     "internal/empty_response.h",
+    "internal/feature_tracker.h",
     "internal/generate_message_boundary.h",
     "internal/generic_object_request.h",
     "internal/generic_request.h",
@@ -173,6 +174,7 @@ google_cloud_cpp_storage_srcs = [
     "internal/crc32c.cc",
     "internal/default_object_acl_requests.cc",
     "internal/empty_response.cc",
+    "internal/feature_tracker.cc",
     "internal/generate_message_boundary.cc",
     "internal/generic_stub_adapter.cc",
     "internal/generic_stub_factory.cc",

google/cloud/storage/google_cloud_cpp_storage.cmake

@@ -95,6 +95,8 @@ add_library(
     internal/default_object_acl_requests.h
     internal/empty_response.cc
     internal/empty_response.h
+    internal/feature_tracker.cc
+    internal/feature_tracker.h
     internal/generate_message_boundary.cc
     internal/generate_message_boundary.h
     internal/generic_object_request.h
@@ -437,6 +439,7 @@ if (BUILD_TESTING)
         internal/const_buffer_test.cc
         internal/crc32c_test.cc
         internal/default_object_acl_requests_test.cc
+        internal/feature_tracker_test.cc
         internal/generate_message_boundary_test.cc
         internal/generic_request_test.cc
         internal/hash_function_impl_test.cc

google/cloud/storage/internal/feature_tracker.cc

@@ -0,0 +1,58 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "google/cloud/storage/internal/feature_tracker.h"
+#include "google/cloud/storage/internal/base64.h"
+#include "google/cloud/storage/options.h"
+#include "google/cloud/options.h"
+#include <memory>
+#include <vector>
+
+namespace google {
+namespace cloud {
+namespace storage {
+GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
+namespace internal {
+
+std::string EncodeFeatureTrackerBitmask(std::uint32_t mask) {
+  if (mask == 0) return {};
+
+  std::vector<std::uint8_t> bytes;
+  for (int i = 0; i < 4; ++i) {
+    auto b = static_cast<std::uint8_t>((mask >> (24 - i * 8)) & 0xFF);
+    if (!bytes.empty() || b != 0) {
+      bytes.push_back(b);
+    }
+  }
+  return Base64Encode(bytes);
+}
+
+Options SetupFeatureTracker(Options opts) {
+  if (opts.has<EnableFeatureReportsOption>() &&
+      !opts.get<EnableFeatureReportsOption>()) {
+    return std::move(opts);
+  }
+  if (opts.has<FeatureTrackerOption>()) return std::move(opts);
+
+  std::uint32_t mask = 0;
+  // Add checks for configuration-driven options here as they are introduced.
+  opts.set<FeatureTrackerOption>(std::make_shared<FeatureTracker>(mask));
+  return std::move(opts);
+}
+
+}  // namespace internal
+GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END
+}  // namespace storage
+}  // namespace cloud
+}  // namespace google

google/cloud/storage/internal/feature_tracker.h

@@ -0,0 +1,88 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STORAGE_INTERNAL_FEATURE_TRACKER_H
+#define GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STORAGE_INTERNAL_FEATURE_TRACKER_H
+
+#include "google/cloud/storage/version.h"
+#include "google/cloud/options.h"
+#include <atomic>
+#include <cstdint>
+#include <memory>
+#include <string>
+
+namespace google {
+namespace cloud {
+namespace storage {
+GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
+namespace internal {
+
+inline constexpr auto kFeatureTrackerHeaderName = "x-goog-storage-cpp-features";
+
+// Tracked features represented as bit positions in a bitmask.
+enum class TrackedFeature : std::uint32_t {
+  // Operation-Driven Optimizations
+  kMultiStreamInMRD = 0,
+  kPCU = 1,
+
+  // Configuration-Driven Options
+  kGrpcDirectPathEnforced = 2,
+  kJsonReads = 3,
+};
+
+// Converts a feature bitmask to a Base64-encoded string, stripping leading
+// zero bytes to maintain compact representation.
+std::string EncodeFeatureTrackerBitmask(std::uint32_t mask);
+
+// A thread-safe state object that can be shared between operation connections
+// (like ObjectDescriptorImpl) and RPC factories (like OpenStreamFactory)
+// to track which features are adopted during an operation.
+class FeatureTracker {
+ public:
+  FeatureTracker() = default;
+  explicit FeatureTracker(std::uint32_t initial) : mask_(initial) {}
+
+  void RegisterFeature(TrackedFeature feature) {
+    mask_.fetch_or(1U << static_cast<std::uint32_t>(feature),
+                   std::memory_order_relaxed);
+  }
+
+  std::uint32_t GetMask() const {
+    return mask_.load(std::memory_order_relaxed);
+  }
+
+  std::string HeaderValue() const {
+    return EncodeFeatureTrackerBitmask(GetMask());
+  }
+
+ private:
+  std::atomic<std::uint32_t> mask_{0};
+};
+
+struct FeatureTrackerOption {
+  using Type = std::shared_ptr<FeatureTracker>;
+};
+
+// Evaluates client configuration options, creates a shared FeatureTracker
+// initialized with configuration-driven feature flags (if any), and stores it
+// into the Options list under FeatureTrackerOption.
+Options SetupFeatureTracker(Options opts);
+
+}  // namespace internal
+GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END
+}  // namespace storage
+}  // namespace cloud
+}  // namespace google
+
+#endif  // GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STORAGE_INTERNAL_FEATURE_TRACKER_H

google/cloud/storage/options.h

@@ -317,14 +317,27 @@ struct IdempotencyPolicyOption {
   using Type = std::shared_ptr<IdempotencyPolicy>;
 };
 
+/**
+ * Enable adoption reporting of client-side optimizations and configuration
+ * choices.
+ *
+ * When this option is enabled (the default), the GCS client sends a bitmask in
+ * the `x-goog-storage-cpp-features` header.
+ *
+ * @ingroup storage-options
+ */
+struct EnableFeatureReportsOption {
+  using Type = bool;
+};
+
 /// The complete list of options accepted by `storage::Client`.
 using ClientOptionList = ::google::cloud::OptionList<
     RestEndpointOption, IamEndpointOption, ProjectIdOption, ProjectIdOption,
     ConnectionPoolSizeOption, DownloadBufferSizeOption, UploadBufferSizeOption,
     EnableCurlSslLockingOption, EnableCurlSigpipeHandlerOption,
     MaximumCurlSocketRecvSizeOption, MaximumCurlSocketSendSizeOption,
     TransferStallTimeoutOption, RetryPolicyOption, BackoffPolicyOption,
-    IdempotencyPolicyOption, CARootsFilePathOption,
+    IdempotencyPolicyOption, CARootsFilePathOption, EnableFeatureReportsOption,
     storage_experimental::HttpVersionOption>;
 
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END

google/cloud/storage/storage_client_unit_tests.bzl

@@ -60,6 +60,7 @@ storage_client_unit_tests = [
     "internal/const_buffer_test.cc",
     "internal/crc32c_test.cc",
     "internal/default_object_acl_requests_test.cc",
+    "internal/feature_tracker_test.cc",
     "internal/generate_message_boundary_test.cc",
     "internal/generic_request_test.cc",
     "internal/hash_function_impl_test.cc",