Refactor client-connect / add support for deferred handling

Hi,

changes from v1:
- rebased to the current master,
- reports errors during file delete

Introduction to the patch-set:

A few years ago I wrote a patch-set concerning OpenVPN's client-connect code. The first part of the patch-set (patches 1 to 6) refactors and unifies the client-connect code.

The second part of the patch-set allows client-connect handling to proceed asynchronously, similar to how OpenVPN supports asynchronous (or deferred) authentication. Basically, the scripts or v1-plugins handling the client-connect event can now write back an additional status code that indicates deferred handling of the client-connect. This causes the OpenVPN server to continue with other things and to regularly re-read the status file. As soon as the status changes from "deferred" to "failed" or "succeeded", the client-connect processing for the connection is continued. (As mentioned before, the auth code does something very similar.)

The motivation for the deferred handling approach were relatively long running client-connect scripts (> 2s) intended for high-traffic servers. As the OpenVPN server completely blocks while synchronously waiting for client-connect scripts to complete, the asynchronous / deferred approach was needed.

The deferred script code path has been in production use for a few years now, although this freshly rebased patch-set has only seen light testing so far.

Feed-back would be very welcome.

The patches are also availabe on a Github branch:
https://github.com/fknittel/openvpn/tree/feat_deferred_client-connect

Cheers
Fabian

PS: See
https://github.com/fknittel/openvpn/wiki/Patch-set-%22deferred-client-connect%22
in case you're interested in ready-made Debian packages.

Fabian Knittel (9):
client-connect: Split multi_connection_established into separate
functions
client-connect: Properly indent all functions
client-connect: Refactor multi_client_connect_source_ccd
client-connect: Move multi_client_connect_setenv into early_setup
client-connect: Refactor to use return values instead of modifying a
passed-in flag
client-connect: Refactor client-connect handling to calling a bunch of
hooks in a loop
client-connect: Add CC_RET_DEFERRED and cope with deferred
client-connect
client-connect: Add deferred support to the client-connect script
handler
client-connect: Add deferred support to the client-connect plugin v1
handler

 src/openvpn/multi.c   | 839 +++++++++++++++++++++++++++++++++++---------------
 src/openvpn/multi.h   |  32 +-
 src/openvpn/options.h |   6 +
 3 files changed, 636 insertions(+), 241 deletions(-)

2.1.4