[llvm-dev] question about xray tls data initialization

[comic fans via llvm-dev]

I'm learning the xray library and try if it can be built on windows,  in
xray_fdr_logging_impl.h

line 152  , comment written as
// Using pthread_once(...) to initialize the thread-local data structures


but at line 175, 183, code written as

thread_local pthread_key_t key;

  // Ensure that we only actually ever do the pthread initialization once.
  thread_local bool UNUSED Unused = [] {
    new (&TLSBuffer) ThreadLocalData();
    auto result = pthread_key_create(&key, +[](void *) {
      auto &TLD = *reinterpret_cast<ThreadLocalData *>(&TLSBuffer);


I'm confused that pthread_key_t and Unused are both thread_local
variable, doesn't it mean the following lambda will run for each
thread , and create one pthread_key_t for only one tls data(instead of
only one pthread_key_t for all thread) ? also what does the '+' before
lambda expression mean ?  this may be stupid questions, could somebody
kindly  helped ?