Avoid setting an empty (null) TraceProvider in trace::Provider::SetTracerProvider

[grrtrr]

Problem Description

By default, the global TracerProvider returns shared pointer to a NoopTracerProvider from trace::Provider::GetTracerProvider(), which has this comment:

// api/include/opentelemetry/trace/provider.h
  /**
   * Returns the singleton TracerProvider.
   *
   * By default, a no-op TracerProvider is returned. This will never return a
   * nullptr TracerProvider.
   */
  static nostd::shared_ptr<TracerProvider> GetTracerProvider() noexcept

The SetTracerProvider allows to set the global singleton instance to a null/empty TracerProvider:

// Similar for empty shared_ptr / unique_ptr:
opentelemetry::trace::Provider::SetTracerProvider(nullptr);

Some of the examples do that to clean up.

While this is fine at the end of a program, when one is sure that there are no more calls to trace::Provider::GetTracerProvider(), this can easily break.

The following segfaults:

opentelemetry::trace::Provider::SetTracerProvider(nullptr);

// ... Many lines below (or in some other file):
auto provider = opentelemetry::trace::Provider::GetTracerProvider();
auto tracer = provider->GetTracer("main");  // <== SEGMENTATION FAULT

Suggestion

Instead of accepting an empty shared_ptr in SetTracerProvider, replace with NoopTracerProvider:

// api/include/opentelemetry/trace/provider.h
  static void SetTracerProvider(const nostd::shared_ptr<TracerProvider> &tp) noexcept
  {
    std::lock_guard<common::SpinLockMutex> guard(GetLock());
    if (!tp) {
        // Ensure that GetTracerProvider never returns a null/empty TracerProvider:
        GetProvider() = std::make_shared<NoopTracerProvider>();
    } else {
        GetProvider() = tp;
    }
  }