Re: support for dynamically loadable modules (ITS#196)

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

Forwarded to devel for review....

Bastiaan.Bakker@lifeline.nl wrote:
>I've built a patch for loadable module support against OpenLDAP-devel of 19990608.
>You can enable it by adding --enable-modules to the options for configure. To
>compile you'll need GLib/GModule v1.2 (see  ftp://ftp.gtk.org/pub/gtk/v1.2/ ). The
>patch can be found here:
> ftp://ftp.openldap.org/incoming/bb_dynmods_for_HEAD19990608.v0.1.gz
>
>This patch is not complete: it will build a functional slapd server that can
>dynamically load modules. However slapd will not contain any backends, expecting
>them to be loaded dynamically. At the moment backend modules still have to be built
>be hand. I do the following:
>1) Run configure with every backend you want a module for enabled.
>2) Make everything.
>3) Go to the particular backend directories for example back-perl and do:
>% gcc -g -O2 -shared -o libback-perl.so *.o
>This works on my development platform (x86 linux with glibc2.0) at least. If you
>use another compiler you may very well have to give it other options.
>4) copy all modules to a suitable directory, say %installroot%/lib/modules
>5) add loadmodule statements to your slapd.conf before any database configurations:
>
>loadmodule /opt/openldap/lib/modules/libback-perl.so
>etc.
>
>The patch allows any shared object to be loaded, but only the slapd backends have
>been modularized so far.
>
>Brief description of implementation:
>
>If the config parser finds a line starting with loadmodule it will call
>load_module(const char* file_name, int argc, char *argv[]) in module.c. This
>function will try to load the shared object / DLL in file 'file_name'. Then it
>looks whether the loaded module contains the function init_module(int argc, char
>*argv[]). If so, this function is called and the module does it's initialization
>stuff like adding the backend it implements to the backend info list. If not, a
>warning message is issued and slapd continues.
>To facilitate modularizing of backends the function backend_add(BackendInfo
>*aBackendInfo) has been added to backend.c. The changes to the backends are
>minimal: just add an init_module function that call backend_add.
>
>Issues & remarks:
>
>As mentioned above the makefiles do not automatically build any modules. At the
>moment I lack the knowledge  to add this in a portable way. Any help will be
>appreciated much!
>
>A choice has to be made about where modules will be located within the install dir
>subtree. I propose lib/modules. Let me know if you think different. The
>installation of modules is not in the makefile yet. Again, help will be
>appreciated.
>
>The current policy of --enable-modules is not to statically link in any backends.
>Maybe it is useful to specify which backends should be linked statically even if
>module support is enabled? Let me know if you think so, and how you think it should
>be told to configure.
>
>It's not illegal to load the same module twice or more! The module will be loaded
>only once but the init_module function will be called each time (with the arguments
>given in the slapd.conf).
>
>Backend_add does not check whether the backend name you are trying to add already
>is in the list. If so the new backend never will be used.
>
>So fare I've tested the dynamic module support only on a Linux RedHat 5.2 x86
>machine. I'd very much like to hear the results for other architectures.
>
>
>If you have any remarks, questions or other feedback, please let me know!
>
>Regards,
>
>Bastiaan Bakker