CurlEasyClient is not thread-safe #124
Description
The CurlEasyClient class calls curl_global_init/curl_global_cleanup in its constructor/destructor. These functions increase/decrease an initialized count, but when called from multiple threads this breaks since it's not using an atomic increment/decrement (the initialized count gets read and increased by two threads and then written with the new value which is only increased by one, or similar in the destructor). This lead to crashes when running one of our application for several hours because curl would suddenly be deinitialized when it shouldn't be.
We've had to patch azure-storage-cpplite to remove the calls to curl_global_init/curl_global_cleanup from the CurlEasyClient class, but this change could possibly break other applications that depend on automatic cleanup when using azure-storage-cpplite in a single-threaded application so I'm not sure how you would want to solve this. curl_easy_init will automatically call curl_global_init if it hasn't already been called, but in that case it won't clean up on exit (see https://github.com/curl/curl/blob/master/lib/easy.c).