mumble/docs/dev/ExtendingTheIceInterface.md

Extending the Ice interface

The server component supports RPC via ZeroC Ice. This document describes how the Ice interface of the server can be extended.

Note: If not stated otherwise all referenced files live in src/murmur/.

The files involved in extending the Ice interface are

File	Description
Murmur.ice	Contains the formal definition of the interface
Murmur.h	Contains the C++ interface definition (abstract base classes). This file is automatically generated based on Murmur.ice when invoking cmake (via slice2cpp). It lives in <build directory>/src/murmur. This file is needed to generate MurmurIceWrapper.cpp
Murmur.cpp	Contains some boilerplate and Ice-internal implementation code. This file is generated and lives alongside Murmur.h
MurmurI.h	Contains the definition of the actually implemented API classes (ServerI and MetaI). These extend the abstract base classes from Murmur.h
MurmurIceWrapper.cpp	Contains wrapper implementation of the *I API classes. This file is auto-generated by the scripts/generateIceWrapper.py script
MurmurIce.h	Contains the definition of a statically used helper class
MurmurIce.cpp	Contains the implementation of that helper class and static functions used to actually implement the server-side functionality of the Ice API functions
RPC.cpp	Contains the implementations of the Server (the Mumble server, not the Ice API type) class's member functions that are required to make certain functionality accessible to the static functions in MurmurIce.cpp

Overview

The steps are

1. Let cmake invoke slice2cpp to generate Murmur.h and Murmur.cpp
2. Invoke generateIceWrapper.py to generate MurmurIceWrapper.cpp
3. Add new function declarations to MurmurU.h
4. Write impl function in MurmurIce.cpp
5. Potentially write new public API functions (declare in Server.h and define in RPC.cpp

Detailed instructions

Before proceeding any further you should run cmake once in order for these files to get generated as they are needed for the next step. Assuming your build directory lives directly under the repository's root, you can do this by invoking

cmake ..

The next step is the generation of the MurmurIceWrapper.cpp file by executing the following command (assuming the call is performed from the repository's root and the build directory is called build):

$ python3 scripts/generateIceWrapper.py --ice-file src/murmur/Murmur.ice --generated-ice-header build/src/murmur/Murmur.h --out-file src/murmur/MurmurIceWrapper.cpp 
Using ICE-file at                   "src/murmur/Murmur.ice"
Using ICE-generated header file at  "build/src/murmur/Murmur.h"

The paths that are given in the example may have to be adapted in order for it to work on your machine.

The MurmurIceWrapper.cpp file generates the *_async versions of the Ice callbacks that handle the async nature of these callbacks and also contain the boilerplate for e.g. verification of the caller and things like this. Most importantly though these functions call the impl_* functions defined in MurmurIce.cpp. For instance a function called updateCertificates inside the Server class will call impl_Server_updateCertificate which has to be defined as a static function inside MurmurIce.cpp.

The declaration of the async functions generated this way are contained inside MurmurI.h. You have to manually add the function's declaration into there. The easiest way to do this is to let the script generate the implementation as described above and then copy the function signature from there into the Murmur.h file (make the declaration virtual though).

The impl function's signature is always

static void impl_<className>_<functionName>(const ::Murmur::AMD_<className>_<functionName>Ptr cb [, int server_id] [, <function arguments>]) {
    // Implementation goes here

    cb->ice_response([<function return value>]);
}

• <className>: Name of the class the function is declared in (e.g. Server or Meta)
• <functionName>: Name of the function as declared in the Murmur.ice file
• [, int server_id]: Only needed when extending the Server API class (the brackets are not part of what needs to be written in code)
• [, <function arguments>]: To be replaced by the list of arguments the function takes
• [<function return value>]: To be replaced with the value this function returns or to be removed if the function does not return anything.

If you have used non-default types that are declared in Murmur.ice (e.g. IdList), you can reference them here as ::Murmur::<typeName> (e.g. ::Murmur::IdList).

Error reporting works via the cb->ice_exception function and if everything went well, the function must end by calling cb->ice_response (potentially passing a value to that function that shall be returned to the caller of the function).

In general it is a good idea to have a look at the existing implementation inside MurmurIce.cpp and take inspiration from those.

Note that the implementations make heavy use of macros (e.g. NEED_SERVER, NEED_CHANNEL, etc.). These will initialize the corresponding variables (server, channel, etc.) based in the parameters fed into the function (In order to obtain the channel, user, etc. you always have to initialize the server variable first). For this to work it is essential that you are using the same parameter names as the existing implementations (e.g. server_id for the server's ID). (For historical reasons the macro to obtain the user variable is called NEED_PLAYER)

If the function requires action on the server's side (beyond its public API), you have to declare a new public function in the Server class (this time the Mumble server though; not the Ice server class) defined in Server.h (the definitions belong to the group of other RPC functions in there - section marked by a comment). The implementation of this new function should then be written in RPC.cpp.

An example of when this is needed is for instance if you have to access the list of connected clients (e.g. because you want to send them a message). While in the current state of the code it would be possible to access this list from the outside (public visibility), you should prefer creating a public API function in the Mumble Server class that has the implementation in RPC.cpp.