* "ex_data" - a CRYPTO_EX_DATA structure in the ENGINE structure itself
that allows an ENGINE to store its own information there rather than in
global variables. It follows the declarations and implementations used
in RSA code, for better or worse. However there's a problem when storing
state with ENGINEs because, unlike related structure types in OpenSSL,
there is no ENGINE-vs-ENGINE_METHOD separation. Because of what ENGINE
is, it has method pointers as its structure elements ... which leads
to;
* ENGINE_FLAGS_BY_ID_COPY - if an ENGINE should not be used just as a
reference to an "implementation" (eg. to get to a hardware device), but
should also be able to maintain state, then this flag can be set by the
ENGINE implementation. The result is that any call to ENGINE_by_id()
will not result in the existing ENGINE being returned (with its
structural reference count incremented) but instead a new copy of the
ENGINE will be returned that can maintain its own state independantly of
any other copies returned in the past or future. Eg. key-generation
might involve a series of ENGINE-specific control commands to set
algorithms, sizes, module-keys, ids, ACLs, etc. A final command could
generate the key. An ENGINE doing this would *have* to declare
ENGINE_FLAGS_BY_ID_COPY so that the state of that process can be
maintained "per-handle" and unaffected by other code having a reference
to the same ENGINE structure.
0ce5f3e4f5