Dovecots header files not optimized for external plugins

[Jan Engelhardt]

Making third-party plugins for Dovecot is really hard and frustrating.
Using dovecot 2.2.33.2 and the following sources, the compile errors keep
coming. The use of g++ is mandated as the underlying backend this plugin
will access only has a C++ interface.


== Source 1
/* g++-7 -c a.cpp */
#include <byteswap.h>
#include <dovecot/lib.h>
#include <dovecot/auth-request.h>
#include <dovecot/passdb.h>
static enum passdb_result
authdb_mapi_logon(struct auth_request *rq, const char *password)
{
        int x = dev_null_fd + bswap_16(1);
}

== Problems 1
/usr/include/dovecot/compat.h:45:4: error: #error uoff_t size not set
 #  error uoff_t size not set

== Source 2
/* g++-7 -DHAVE_CONFIG_H -c a.cpp */
#include "config.h" /* generated by my configure */
#include <byteswap.h>
#include <dovecot/lib.h>
#include <dovecot/auth-request.h>
#include <dovecot/passdb.h>
static enum passdb_result
authdb_mapi_logon(struct auth_request *rq, const char *password)
{
        int x = dev_null_fd + bswap_16(1);
}

== Problems 2
In file included from /usr/include/bits/byteswap.h:35:0,
                 from /usr/include/byteswap.h:24,
                 from a.cpp:2:
/usr/include/dovecot/byteorder.h:32:24: error: expected unqualified-id before ‘__extension__’
 static inline uint16_t bswap_16(uint16_t in);

== Other problems
dovecot headers files are missing 'extern "C"' lines.
This means the linkage of symbols like dev_null_fd is not what it should be.
I can't use
 extern "C" {
 #include <dovecot/...>
 }
either because then the standardy library headers that are included
by dovecot headers can easily start acting up too.