CXX-890 Spin in instance constructor if we observe concurrent destruction

Author: acmorrow

src/mongocxx/instance.cpp

@@ -61,6 +61,10 @@ void user_log_handler(::mongoc_log_level_t mongoc_log_level, const char *log_dom
                                         stdx::string_view{log_domain}, stdx::string_view{message});
 }
 
+// A region of memory that acts as a sentintel value indicating that an instance object is being
+// destroyed. We only care about the address of this object, never its contents.
+typename std::aligned_storage<sizeof(instance), alignof(instance)>::type sentinel;
+
 std::atomic<instance*> current_instance{nullptr};
 static_assert(std::is_standard_layout<decltype(current_instance)>::value, "Must be standard layout");
 static_assert(std::is_trivially_constructible<decltype(current_instance)>::value, "Must be trivially constructible");
@@ -96,16 +100,23 @@ instance::instance() : instance(nullptr) {
 }
 
 instance::instance(std::unique_ptr<logger> logger) {
-    instance* expected = nullptr;
-    if (!current_instance.compare_exchange_strong(expected, this))
-        throw logic_error{error_code::k_instance_already_exists};
+
+    while (true) {
+        instance* expected = nullptr;
+        if (current_instance.compare_exchange_strong(expected, this))
+            break;
+        if (expected != reinterpret_cast<instance*>(&sentinel))
+            throw logic_error{error_code::k_instance_already_exists};
+    }
+
     _impl = stdx::make_unique<impl>(std::move(logger));
 }
 
 instance::instance(instance &&) noexcept = default;
 instance &instance::operator=(instance &&) noexcept = default;
 
 instance::~instance() {
+    current_instance.store(reinterpret_cast<instance*>(&sentinel));
     _impl.reset();
     current_instance.store(nullptr);
 }