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Filename : Design.Plugins.txt
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Plugins
=======

Plugins in Dovecot are really simple. They basically have two functions:

 * <plugin_name>_init(module) is called when 'module_dir_init()' is called.
 * <plugin_name>_deinit() is called when 'module_dir_deinit()' or
   'module_dir_unload()' is called.

The <plugin_name> is the short version of the plugin name, based on the
filename. For example if the filename is 'lib11_imap_quota_plugin.so', the
<plugin_name> is "imap_quota" and the init function to be called is
imap_quota_plugin_init().

Versioning
----------

Since different Dovecot versions can have different APIs, your plugin should
usually also define<plugin_name>_version, like:

---%<-------------------------------------------------------------------------
const char *imap_quota_plugin_version = DOVECOT_ABI_VERSION;
---%<-------------------------------------------------------------------------

If the version string in plugin doesn't match the version of the running
binary, the plugin loading fails. The DOVECOT_ABI_VERSION is defined in
Dovecot's 'config.h', which you're typically including.

Dependencies
------------

Some plugins depend on another one. In some systems (but not all) it's possible
to handle this by giving a nicer error message than "symbol xyz not found".
There are two steps for this:

First create <plugin_name>_dependencies array listing plugin names that the
plugin depends on, like:

---%<-------------------------------------------------------------------------
const char *imap_quota_plugin_dependencies[] = { "quota", NULL };
---%<-------------------------------------------------------------------------

Then you'll also have to make the plugin .so binary link to the other plugins:

---%<-------------------------------------------------------------------------
if PLUGIN_DEPS
lib11_imap_quota_plugin_la_LIBADD = \
        ../quota/lib10_quota_plugin.la
endif
---%<-------------------------------------------------------------------------

PLUGIN_DEPS is set only if plugin dependencies are actually supported.
Otherwise the build might fail or plugin loading might fail.

Once all this is done, trying to load imap_quota plugin without quota plugin
gives a nice error message:

---%<-------------------------------------------------------------------------
Error: Can't load plugin imap_quota_plugin: Plugin quota must be loaded also
---%<-------------------------------------------------------------------------

(This file was created from the wiki on 2019-06-19 12:42)