8.1 External I/O event loops in the client.

The reference implementation for using external I/O event loops in the client is the GPGME library, which exports its own external I/O event loop mechanism and utilizes the Assuan library transparently for the user. The following steps document how GPGME achieves this.

1. Before connecting, set up pipes for bulk data transfer (using the INPUT/OUTPUT commands, for example). These are passed to the server either by inheritance (using a pipe server) or by FD passing (using a socket server).
2. Then you need to connect to the server. GPGME uses a pipe server, so it just spawns a server process, which is a non-blocking operation. FIXME: Currently, using a client with external event loop over a socket connection is not supported. It is easy to support (we just need a variation of assuan_socket_connect which takes an already connected socket FD and turns it into an Assuan context), so if you need this let us know.
3. After connecting, get the inbound status FD with assuan_get_active_fds (the first one returned is the status FD). This FD can be duplicated if it is convenient (GPGME does this to be able to close this FD and associated callback handlers without disrupting Assuan's internals).
4. Then register the Assuan inbound status FD and all bulk data FDs with the I/O event mechanism. In general, this requires setting up callback handlers for these FDs and registering them with the main event loop.
5. When bulk data FDs become ready, you can simply perform the corresponding read or write operations. When the inbound status FD becomes ready, you can receive the next server line with assuan_read_line().
6. You should close and unregister the bulk data FDs when you wrote all data (for outbound FDs) or receive an EOF (for inbound FDs). When you receive an ERR from the server, or an OK for the final operation, you can unregister the inbound status FD and call assuan_disconnect to close it.
7. As noted above, all send operations on the outbound status FD are done immediate with blocking. In GPGME, this has never caused any problems.
8. The INQUIRE function can be handled in two ways: If the requested data is immediately available, the client can just send the data blockingly. If the requested data needs to be fetched from a blocking source, a callback handler can be registered for the FD with the main event loop. GPGME does not support the INQUIRE function, so we do not have any practical experience with this.

Currently, the client can not cancel a pending operation gracefully. It can, however, disconnect from the server at any time. It is the responsibility of the server to periodically send status messages to the client to probe if the connection remains alive.