cpp-static-thread-safety

name	cpp-static-thread-safety
description	Enforce C++ static thread safety annotations and correct synchronization primitives. Use this skill when designing multi-threaded classes or editing guarded member fields.

C++ Static Thread Safety & Synchronization Guidelines

Filament leverages Clang's static thread safety analysis to verify lock holding requirements at compile-time. All multi-threaded classes must use explicit capability annotations to guarantee race-free state access.

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1. Selecting the Correct Lock Primitive

• Filament Primitives (utils::Mutex & utils::Condition) — MANDATORY:
  • Rule: All engine, backend, and utility code under filament/ must use utils::Mutex and utils::Condition exclusively. Do not use std::mutex or std::condition_variable.
  • Deadlock & Order Debugging: utils::Mutex transparently integrates with Filament's compile-time lock debugging facility (FILAMENT_DEBUG_MUTEX / -u). When enabled, it maintains a global cycle dependency graph via BFS during lock() and try_lock(), immediately trapping lock-order inversions and self-deadlocks with exact CallStack traces. Any locks defined via std::mutex bypass this tracker entirely and remain invisible to deadlock diagnostics.
  • Memory & Cache Hygiene: On Android and Linux (linuxutil::Mutex), utils::Mutex is only 4 bytes (a single atomic futex word) versus 40 bytes for std::mutex. For structures allocated in large volumes or embedded in handles/fences, this 10x size reduction prevents struct bloat and maintains cache-line density.
  • Condition Variable Support: utils::Condition (Condition::wait / wait_until) is explicitly templated (template <typename M>) to work seamlessly with UniqueLock<utils::Mutex> for producer-consumer queues and blocking wait loops.
  • Priority Inversion Reality: C++ standard std::mutex (pthread_mutex_t) does not provide priority inheritance out of the box (PTHREAD_PRIO_NONE). Therefore, std::mutex offers zero priority inversion protection over utils::Mutex.

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2. Lock Guard & RAII Lifetime Conventions

• Immutability Invariant: Use LockGuard const as the default for all standard synchronized blocks to guarantee scope-bound read-only lock scopes.

  // Correct
  utils::LockGuard const lock(mLock);
  

• Condition Variables Overloads: Use UniqueLock (non-const) strictly when passing locks to condition variables (wait(lock)) or when explicit .unlock() / .lock() boundaries are required for performance or deadlock prevention.
• Lock Dependency: Any source file (.cpp) that instantiates LockGuard or UniqueLock must explicitly include the matching utility header:

  #include <utils/Mutex.h>
  

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3. Resolving Clang's Lambda Closure Limitations

Clang evaluates C++ anonymous closures (lambdas) as separate context boundaries. Because lambdas lack capability attributes, standard condition variable waits passing local predicates (e.g., using std::ranges::all_of) will trigger false-positive thread safety errors.

To resolve this, use one of the following approved patterns:

Pattern A: Inner Lambda Bypass (Recommended)

Decorate only the CV wait predicate lambda operator with UTILS_NO_THREAD_SAFETY_ANALYSIS to ignore nested boundaries while preserving outer compile-time checks:

UniqueLock lock(mQueueLock);
mQueueCondition.wait(lock, [this]() UTILS_NO_THREAD_SAFETY_ANALYSIS {
    return mExitRequested || 
           (!std::ranges::all_of(mQueues, [](auto&& q) { return q.empty(); }));
});

Pattern B: Manual Loop Inlining

If the check is flat, completely inline the CV predicate as a standard while loop to bring the member variables directly into the parent function's locked scope:

UniqueLock lock(mLock);
while (mFreeSpace < requiredSize) {
    mCondition.wait(lock);
}

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4. Dynamic Threading & Preprocessor Safety

• Single-Threaded Parity: All thread safety annotations (UTILS_GUARDED_BY) are conditionally compiled out on single-threaded configurations. To prevent compile crashes when FILAMENT_SINGLE_THREADED is defined, standard annotations are gated by UTILS_HAS_THREADING in compiler.h.