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+Mail Plugins
+============
+
+Typically plugins add hooks in their init() function by calling
+'mail_storage_hooks_add()', and remove the hooks at deinit() with
+'mail_storage_hooks_remove()'. Hooks that are currently supported:
+
+ * mail_user_created: A new mail user was created. It doesn't yet have any
+   namespaces.
+ * mail_storage_created: A new mail storage was created. It's not connected to
+   any namespaces/mailbox lists yet.
+ * mailbox_list_created: A new mailbox list was created. It's not connected to
+   any storages yet. Because of this, some internal virtual methods haven't
+   been overridden by the storage yet, so plugins rarely want to use this hook.
+   Instead they should use:
+ * mail_namespace_storage_added: Storage was connected to its first
+   namespace/mailbox list. This hook should usually be used if plugin wants to
+   override mailbox_list's methods.
+ * mail_namespaces_created: User's all namespaces have been created. This hook
+   is called only per user at startup. More internal namespaces may be created
+   later when using shared mailboxes.
+ * mailbox_allocated: 'mailbox_alloc()' was called.
+ * mailbox_opened: Mailbox (and its index) was actually opened, either
+   explicitly with 'mailbox_open()' or implicitly by some other function.
+
+Overriding methods
+------------------
+
+When the hook gets called, you usually want to override some method of the
+created object. This is the easy part, for example:
+
+---%<-------------------------------------------------------------------------
+static void plugin_mailbox_allocated(struct mailbox *box)
+..
+        box->v.transaction_begin = plugin_transaction_begin;
+---%<-------------------------------------------------------------------------
+
+The problem is that once 'plugin_transaction_begin()' is called, it should call
+the original 'transaction_begin()'. There may also be multiple plugins that
+want to override the same method, so the idea is to just have each plugin call
+the previous 'transaction_begin()'. The next problem is where do you save the
+previous value? Most objects have a 'module_contexts' array for storing
+per-plugin pointers for this purpose. There are several helper functions to
+make setting and accessing them in a quite safe way.
+
+Easiest way to set up the module context is to just copy&paste code from an
+existing plugin that sets the same context. Here's some documentation about it
+anyway:
+
+First you start by creating register for the plugin. There are different
+registers for different types of objects:
+
+ * mail_user_module_register: For mail_user.
+ * mailbox_list_module_register: For mailbox_list.
+ * mail_storage_module_register: For mail_storage, mailbox, mailbox_transaction
+   and mail_search.
+ * mail_module_register: For mail.
+
+We'll assume you want to use mail_storage_module_register:
+
+---%<-------------------------------------------------------------------------
+static MODULE_CONTEXT_DEFINE_INIT(plugin_storage_module,
+&mail_storage_module_register);
+---%<-------------------------------------------------------------------------
+
+If you need to make it external, use:
+
+---%<-------------------------------------------------------------------------
+extern MODULE_CONTEXT_DEFINE(plugin_storage_module,
+&mail_storage_module_register);
+struct plugin_storage_module plugin_storage_module =
+        MODULE_CONTEXT_INIT(&mail_storage_module_register);
+---%<-------------------------------------------------------------------------
+
+Next you'll need to allocate memory for the structure you want to place in the
+context. If you only want to override some methods, you can use:
+
+---%<-------------------------------------------------------------------------
+union mailbox_module_context *mbox;
+struct mailbox_vfuncs *v = box->vlast;
+
+mbox = p_new(box->pool, union mailbox_module_context, 1);
+mbox->super = *v;
+box->vlast = &mbox->super;
+
+v->transaction_begin = plugin_transaction_begin;
+MODULE_CONTEXT_SET_SELF(box, plugin_storage_module, mbox);
+---%<-------------------------------------------------------------------------
+
+If you want to store some more plugin-specific data to the object instead of
+just the super methods, you can do:
+
+---%<-------------------------------------------------------------------------
+struct plugin_mailbox {
+        /* must be called module_ctx */
+        union mailbox_module_context module_ctx;
+};
+/* .. */
+
+struct plugin_mailbox *mbox;
+struct mailbox_vfuncs *v = box->vlast;
+
+mbox = p_new(box->pool, struct plugin_mailbox, 1);
+mbox->module_ctx.super = *v;
+box->vlast = &mbox->super;
+
+v->transaction_begin = plugin_transaction_begin;
+MODULE_CONTEXT_SET(box, plugin_storage_module, mbox);
+---%<-------------------------------------------------------------------------
+
+Note that when using union directly you use 'MODULE_CONTEXT_SET_SELF()', while
+when it's inside a struct you use 'MODULE_CONTEXT_SET()'.
+
+Once all this initialization is done, you can look up the module context with:
+
+---%<-------------------------------------------------------------------------
+#define PLUGIN_CONTEXT(obj) MODULE_CONTEXT(obj, plugin_storage_module)
+/* .. */
+struct plugin_mailbox *mbox = PLUGIN_CONTEXT(box);
+---%<-------------------------------------------------------------------------
+
+(Yes, this API seems a bit too difficult to use and could use a redesign.)
+
+(This file was created from the wiki on 2019-06-19 12:42)