Note for adding sinks on the fly

We had a need to add a callback sink to our logs so we can reprocess log messages to different formats (json and MQTT) while keeping the original logging. I discovered a small trap based on not knowing the C++ standard inside out. To add a sink, be careful how you access sinks() in the logger. This doesn't work:

auto sinks = log_->sinks();
sinks.push_back(std::make_shared<spdlog::sinks::callback_sink<std::mutex> >([this](const spdlog::details::log_msg& msg) {this->LogCallback(msg);}));

"auto sinks = ..." gives you a copy of the sinks vector so you're not modifying the sinks vector in the logger. The C++ compiler is actually doing what it should with the C++ standard, "auto var = xyz" is never supposed to give you a reference, it calls the copy constructor of the RHS object.

You need to use auto& or the original sink_ptr type defined in common.h, but it needs to be a reference to the vector.

auto& sinks = log_->sinks();
sinks.push_back( .....etc

Hopefully this saves someone else an hour of head-scratching.

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See details #3014