As of r758185:

Design issues

1. MPM arg rewriting, signal server support
   see old main.c, look for AP_MPM_WANT_SIGNAL_SERVER and AP_MPM_WANT_SET_GRACEFUL_SHUTDOWN
   also, see
      struct module_struct {
        ...
        /** Function to allow MPMs to re-write command line arguments.  This
         *  hook is only available to MPMs.
         *  @param The process that the server is running in.
         */
        void (*rewrite_args) (process_rec *process);
        ...
      };

   and its use in many MPMs

   Proposal: Confirm common rewrite args use in loadable MPMs, and provide a
             module for static linking which specifies the arg rewriting hook
             in mpm_common.c

2. retain data

   SOLVED with ap_set_retained_data()/ap_get_retained_data()

3. core.c generation of PidFile, ScoreBoardFile, LockFile, MaxRequestsPerChild,

   CoreDumpDirectory, AcceptMutex, MaxMemFree, ThreadStackSize directives

   Proposal: punt for the moment

4. don't allow multiple MPMs to be loaded

   Current feedback: open-logs is run-all, so you get EADDRINUSE errors when the second
                     MPM tries to open sockets

   Proposal: punt for the moment

Bugs

1. Windows still installs mpm's *.h
2. Check setting of return codes from new MPM hooks; appropriate for the hook defs?
3. main.c needs a command-line option to call show_mpm_settings();
   like -M, this needs to load the modules first

   Proposal 1: Move show_mpm_settings() to mpm_common.c and convert to a test-config hook
               that checks for a command-line define.
               MPMs use that for a test-config hook.
               user does ./httpd -t -D SHOW_MPM (or some synonym)

   Proposal 2: leave show_mpm_settings() as-is; proceed through loading modules
               and then call show_mpm_settings() and exit, when some new command-line
               switch is enabled

4. ap_show_mpm() returns hard-coded string; make it call a hook implemented by
   the MPMs
5. CHANGES and ap_mmn.h not updated for MPM churn
6. need to hide ap_pglobal

Stuff to test (other than "everything")

1. make sure arg rewriting and signalling still work for built-in MPMs
2. test the module which uses register-timed-callback
3. test idle process termination

Other issues

1. a shared MPM must be built with apxs or similar instead of like other shared
   modules

   (punt until there are fewer problems with them ;) )

2. MPMs not updated (unclear which of these were previously working)
    beos
    mpmt_os2
    netware
    leader
    perchild
    threadpool