doc: multiprocess documentation improvements

Most improvements suggested by stickies-v <stickies-v@protonmail.com>
https://github.com/bitcoin/bitcoin/pull/28978#pullrequestreview-1800375604

Omission of CustomReadMessage and CustomBuildMessage was noticed by
TheCharlatan in
https://github.com/bitcoin/bitcoin/pull/30510#discussion_r1708996040 and is
fixed here as well.

Co-authored-by: stickies-v <stickies-v@protonmail.com>

doc/design/multiprocess.md

@@ -81,7 +81,7 @@ This section describes the major components of the Inter-Process Communication (
 - In the generated code, we have C++ client subclasses that inherit from the abstract classes in [`src/interfaces/`](../../src/interfaces/). These subclasses are the workhorses of the IPC mechanism.
 - They implement all the methods of the interface, marshalling arguments into a structured format, sending them as requests to the IPC server via a UNIX socket, and handling the responses.
 - These subclasses effectively mask the complexity of IPC, presenting a familiar C++ interface to developers.
-- Internally, the client subclasses generated by the `mpgen` tool wrap [client classes generated by Cap'n Proto](https://capnproto.org/cxxrpc.html#clients), and use them to send IPC requests.
+- Internally, the client subclasses generated by the `mpgen` tool wrap [client classes generated by Cap'n Proto](https://capnproto.org/cxxrpc.html#clients), and use them to send IPC requests. The Cap'n Proto client classes are low-level, with non-blocking methods that use asynchronous I/O and pass request and response objects, while mpgen client subclasses provide normal C++ methods that block while executing and convert between request/response objects and arguments/return values.
 
 ### C++ Server Classes in Generated Code
 - On the server side, corresponding generated C++ classes receive IPC requests. These server classes are responsible for unmarshalling method arguments, invoking the corresponding methods in the local [`src/interfaces/`](../../src/interfaces/) objects, and creating the IPC response.
@@ -94,7 +94,7 @@ This section describes the major components of the Inter-Process Communication (
 - **Asynchronous I/O and Thread Management**: The library is also responsible for managing I/O and threading. Particularly, it ensures that IPC requests never block each other and that new threads on either side of a connection can always make client calls. It also manages worker threads on the server side of calls, ensuring that calls from the same client thread always execute on the same server thread (to avoid locking issues and support nested callbacks).
 
 ### Type Hooks in [`src/ipc/capnp/*-types.h`](../../src/ipc/capnp/)
-- **Custom Type Conversions**: In [`src/ipc/capnp/*-types.h`](../../src/ipc/capnp/), function overloads of two `libmultiprocess` C++ functions, `mp::CustomReadField` and `mp::CustomBuildFields`, are defined. These overloads are used for customizing the conversion of specific C++ types to and from Cap’n Proto types.
+- **Custom Type Conversions**: In [`src/ipc/capnp/*-types.h`](../../src/ipc/capnp/), function overloads of `libmultiprocess` C++ functions, `mp::CustomReadField`, `mp::CustomBuildField`, `mp::CustomReadMessage` and `mp::CustomBuildMessage`, are defined. These overloads are used for customizing the conversion of specific C++ types to and from Cap’n Proto types.
 - **Handling Special Cases**: The `mpgen` tool and `libmultiprocess` library can convert most C++ types to and from Cap’n Proto types automatically, including interface types, primitive C++ types, standard C++ types like `std::vector`, `std::set`, `std::map`, `std::tuple`, and `std::function`, as well as simple C++ structs that consist of aforementioned types and whose fields correspond 1:1 with Cap’n Proto struct fields. For other types, `*-types.h` files provide custom code to convert between C++ and Cap’n Proto data representations.
 
 ### Protocol-Agnostic IPC Code in [`src/ipc/`](../../src/ipc/)
@@ -197,7 +197,7 @@ sequenceDiagram
    - Upon receiving the request, the Cap'n Proto dispatching code in the `bitcoin-node` process calls the `getBlockHash` method of the `Chain` [server class](#c-server-classes-in-generated-code).
    - The server class is automatically generated by the `mpgen` tool from the [`chain.capnp`](https://github.com/ryanofsky/bitcoin/blob/pr/ipc/src/ipc/capnp/chain.capnp) file in [`src/ipc/capnp/`](../../src/ipc/capnp/).
    - The `getBlockHash` method of the generated `Chain` server subclass in `bitcoin-wallet` receives a Cap’n Proto request object with the `height` parameter, and calls the `getBlockHash` method on its local `Chain` object with the provided `height`.
-   - When the call returns, it encapsulates the return value in a Cap’n Proto response, which it sends back to the `bitcoin-wallet` process,
+   - When the call returns, it encapsulates the return value in a Cap’n Proto response, which it sends back to the `bitcoin-wallet` process.
 
 5. **Response and Return**
    - The `getBlockHash` method of the generated `Chain` client subclass in `bitcoin-wallet` which sent the request now receives the response.
@@ -232,7 +232,7 @@ This modularization represents an advancement in Bitcoin Core's architecture, of
 
 - **Cap’n Proto struct**: A structured data format used in Cap’n Proto, similar to structs in C++, for organizing and transporting data across different processes.
 
-- **client class (in generated code)**: A C++ class generated from a Cap’n Proto interface which inherits from a Bitcoin core abstract class, and implements each virtual method to send IPC requests to another process. (see also [components section](#c-client-subclasses-in-generated-code))
+- **client class (in generated code)**: A C++ class generated from a Cap’n Proto interface which inherits from a Bitcoin Core abstract class, and implements each virtual method to send IPC requests to another process. (see also [components section](#c-client-subclasses-in-generated-code))
 
 - **IPC (inter-process communication)**: Mechanisms that enable processes to exchange requests and data.
 

doc/multiprocess.md

@@ -17,7 +17,9 @@ The multiprocess feature requires [Cap'n Proto](https://capnproto.org/) and [lib
 ```
 cd <BITCOIN_SOURCE_DIRECTORY>
 make -C depends NO_QT=1 MULTIPROCESS=1
-cmake -B build --toolchain=depends/x86_64-pc-linux-gnu/toolchain.cmake
+# Set host platform to output of gcc -dumpmachine or clang -dumpmachine or check the depends/ directory for the generated subdirectory name
+HOST_PLATFORM="x86_64-pc-linux-gnu"
+cmake -B build --toolchain=depends/$HOST_PLATFORM/toolchain.cmake
 cmake --build build
 build/src/bitcoin-node -regtest -printtoconsole -debug=ipc
 BITCOIND=$(pwd)/build/src/bitcoin-node build/test/functional/test_runner.py