Parallel Proxy Requests Need Separate libcurl Handle Ownership
Keep libcurl handles under one thread’s ownership at a time when requests use different proxies.
A proxy option belongs to a libcurl easy handle. Changing that option from one thread while another thread performs a transfer on the same handle violates the handle's ownership rules.
libcurl is thread-safe when its documented rules are followed. It does not internally synchronize arbitrary simultaneous access to one handle. Give concurrent workers their own handles, or transfer ownership only when the previous user has finished.
The share interface has separate locking requirements and limits. It is not permission to mutate every setting of a shared easy handle from every worker.
Old proxy-thread examples often include locking callbacks for old TLS-library versions. Check the current thread-safety documentation for the actual TLS backend and build. Copying historical OpenSSL callbacks into a current application is not a substitute for correct handle ownership.
Initialize global library state according to the documented startup requirements before using it concurrently. For public requests permitted through public intermediaries, the free proxy list offers candidates for each worker's proxy option; match the protocol to the libcurl build. For each failed operation, retain the handle's configured proxy and its own error result. A global “current proxy” variable is a poor identifier when several transfers are in flight.