Many applications that use OpenSSL with ENGINE support might face a

situation where they've initialised the ENGINE, loaded keys (which are then
linked to that ENGINE), and performed other checks (such as verifying
certificate chains etc). At that point, if the application goes
multi-threaded or multi-process it creates problems for any ENGINE
implementations that are either not thread/process safe or that perform
optimally when they do not have to perform locking and other contention
management tasks at "run-time".

This defines a new ENGINE_ctrl() command that can be supported by engines
at their discretion. If ENGINE_ctrl(..., ENGINE_CTRL_HUP,...) returns an
error then the caller should check if the *_R_COMMAND_NOT_IMPLEMENTED error
reason was set - it may just be that the engine doesn't support or need the
HUP command, or it could be that the attempted reinitialisation failed. A
crude alternative is to ignore the return value from ENGINE_ctrl() (and
clear any errors with ERR_clear_error()) and perform a test operation
immediately after the "HUP". Very crude indeed.

ENGINEs can support this command to close and reopen connections, files,
handles, or whatever as an alternative to run-time locking when such things
would otherwise be needed. In such a case, it's advisable for the engine
implementations to support locking by default but disable it after the
arrival of a HUP command, or any other indication by the application that
locking is not required. NB: This command exists to allow an ENGINE to
reinitialise without the ENGINE's functional reference count having to sink
down to zero and back up - which is what is normally required for the
finish() and init() handlers to get invoked. It would also be a bad idea
for engine_lib to catch this command itself and interpret it by calling the
engine's init() and finish() handlers directly, because reinitialisation
may need special handling on a case-by-case basis that is distinct from a
finish/init pair - eg. calling a finish() handler may invalidate the state
stored inside individual keys that have already loaded for this engine.