Fix CFI icall violation in Future callbacks

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Eliminate unsafe reinterpret_cast of TypedCompletionCallback and std::function completion callbacks to FutureBase::CompletionCallback in Future::OnCompletion and Future::AddOnCompletion.

Introduce type-safe trampoline functions and wrapper data structures (TypedCompletionCallbackTrampoline and TypedCompletionCallbackData) in firebase::detail to bridge the typed callback to the base callback signature expected by ReferenceCountedFutureImpl::RunCallback, avoiding undefined behavior and runtime CFI type check aborts.

---

Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @a-maurice on commit 126a8dd
Last updated: Thu Aug 27 16:08 PDT 2026
View integration test log & download artifacts

Failures	Configs
app_check	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirebaseAppCheckTest.TestSignIn
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: windows] [1/4 android_device: android_latest] (1 failed tests)   FilterTest.QueryEmptyWhereComposite [TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   CRASH/TIMEOUT
messaging	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/4 android_device: android_target] (1 failed tests)   FirebaseMessagingTest.TestSendMessageToToken
storage	[TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/4 android_device: emulator_ftl_latest] (1 failed tests)   CRASH/TIMEOUT

Add flaky tests to go/fpl-cpp-flake-tracker

[gemini-code-assist]

Code Review

This pull request refactors the Future<T>::OnCompletion and Future<T>::AddOnCompletion implementations to avoid unsafe reinterpret_cast on callbacks, introducing a typed callback data structure and trampoline functions instead. The review feedback highlights critical safety issues regarding null or empty callbacks: passing a null callback can cause a crash if the future is already completed, and wrapping an empty std::function without validation can trigger a std::bad_function_call exception or crash. Suggestions are provided to handle these cases safely by using dummy callbacks or adding validity checks.

[gemini-code-assist]

If callback is nullptr and the future is already completed, passing nullptr to api_->AddCompletionCallback will cause a crash (null pointer dereference) in ReferenceCountedFutureImpl::RunCallback when it attempts to execute the callback. To prevent this, pass a dummy callback that does nothing instead of nullptr.

Suggested change

	if (callback == nullptr) {
	api_->AddCompletionCallback(handle_, nullptr, nullptr, nullptr,
	/*clear_existing_callbacks=*/true);
	if (callback == nullptr) {
	api_->AddCompletionCallback(handle_, [](const FutureBase&, void*){}, nullptr, nullptr,
	/*clear_existing_callbacks=*/true);

[gemini-code-assist]

If callback is an empty std::function (i.e., !callback), executing it inside the lambda will throw std::bad_function_call or cause an immediate crash if exceptions are disabled. To prevent this, check if the callback is valid. If it is empty, safely clear the existing callback using a dummy callback.

  if (!callback) {
    FutureBase::OnCompletion([](const FutureBase&, void*){}, nullptr);
  } else {
    FutureBase::OnCompletion([callback](const FutureBase& future) {
      callback(static_cast<const Future<ResultType>&>(future));
    });
  }

[gemini-code-assist]

If callback is an empty std::function (i.e., !callback), executing it inside the lambda will throw std::bad_function_call or cause an immediate crash if exceptions are disabled. To prevent this, check if the callback is valid and return an empty CompletionCallbackHandle if it is empty.

  if (!callback) {
    return CompletionCallbackHandle();
  }
  return FutureBase::AddOnCompletion([callback](const FutureBase& future) {
    callback(static_cast<const Future<ResultType>&>(future));
  });