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This folder contains a proof of concept of 389 Directory Server components in
rust.

The architecture of these libraries is:

slapi_r_plugin -- (wraps and links) --> libslapd.la
plugins/* --(links)--> slapi_r_plugin

slapi_r_plugin contains rust definitions and macros that are copied from
slapi_plugin.h. It provides wrappers to functions in slapi_plugin.h ie
slapi_pblock_get().


Plugins are built to *only* need native rust types! When the plugin actually
registers itself, it is installing the slapi_r_plugin_manager into the DS
code, and then finds all the correct rust calls. This allows it to proxy all
the accessses and convert types.

This example is an interaction between libslapd (Directory Server) and the
rust plugin manager and rust plugin.


libslapd                      slapi_r_plugin_mana                  hellorust

+------+                                                         +-------------+
|      |               -- (1) slapi_r_plugin_init_fn -->         | (2) init    |
|      |                                                         |     |       |
|      |                        +-------------+                  |     v       |
|      |                        |             | <- (3) new cb -- | plugin init |
|      |  <- (4) add callback   |             |                  |             |
|      |   -- (5) result -->    |             |                  |             |
|      |                        |             | -- (6) result -> |             |
|      |                        |             |                  |             |
z      z                        z             z                  z             z
|      |                        |             |                  |             |
|      |                        |             | <- (N1) new cb --|             |
|      |  <- (N2) add callback  |             |                  |             |
|      |   -- (N3) result -->   |             |                  |             |
|      |                        |             | - (N4) result -> |             |
|      |                        |             |                  |             |
z      z                        z             z                  z             z
|      |                        |             |                  |             |
|      |                        |             | <- (7) register  |             |
|      |  <- (8) set private -- |             |                  |             |
|      |    -- (9) result -->   |             |                  |             |
|      |                        |             |                  |             |
|      |  < (10) add srpm cbs - |             |                  |             |
|      |   -- (11) result -->   |             |                  |             |
|      |                        |             | - (12) result -> |             |
|      |                        |             |                  | (13) done   |
|      |                        |             |                  |             |
z      z                        z             z                  z             z
|      |                        |             |                  |             |
|      |  -- (13) op call -->   |             |                  |             |
|      |  <- (14) get private - |             |                  |             |
|      |   -- (15) result -->   |             |                  |             |
|      |                        | (16) get cb |                  |             |
|      |                        |             | - (17) op call > |             |
|      |                        |             | <- (18) result - |             |
|      |                        | (19) wrap   |                  |             |
|      |   <-- (20) result --   |             |                  |             |
+------+                        +-------------+                  +-------------+


The key interaction is that hellorust is not registered to Directory Server:
slapi_r_plugin_manager has all it's callbacks registered on Directory Server.
When a request is made to the s_r_p_m, it looks up the current plugins rust
callbacks, wraps the Slapi_PBlock into a Slapi_R_PBlock, and makes the call.

The plugin then returns a Result<(), PluginOperationError>. s_r_p_m then
transforms Ok(()) into constant::LDAP_SUCCESS, or Err(e), is converted to the
correct Directory Server integer results and returned.

This adds more code to the slapi_r_plugin library, but makes the interactions of
the hellorust plugin very simple, and purely in rust types. This gives a safe
abstraction boundary: A important aspect of using Rust! This means we have Safe
Rust Plugins avaliable to us!



In the future if directory server components were to be added to this as part
of the main binary, they should be in:

slapi_r_private/src/{modules}

Calls follows arrows: Linker is opposite direction.

We would then have the modules linked such that:

Hybrid:

    ns-slapd main()
    |
    \--> libslapd.so
         |
         |--> slapi_r_private() // Contains modules from "ground up" and build out.
         |                      // Cannot depend on libslapd.so
         |
         \--> slapi_r_plugin() 
              |
              \--> plugins/<pluginname>()

So while a transition were occuring, we build slapi_r_private out from the core
up to assimilate more functions of libslapd.so.

From the other side, we have slapi_r_plugin() that wraps calls to libslapd.so.

Eventually, the two would meet in the middle, and we would have:

Pure rust:

    main()
    |
    \--> slapi_r_private()
         |
         |--> slapi_r_backend()
         |    |
         |    \--> backend-plugin
         |
         \--> slapi_r_plugin()
              |
              \--> plugins/<pluginname>()


This design may be subject to change!!