doc: Multiprocess misc doc and comment updates

doc/build-osx.md

@@ -48,7 +48,7 @@ See [dependencies.md](dependencies.md) for a complete overview.
 To install, run the following from your terminal:
 
 ``` bash
-brew install cmake boost pkg-config libevent
+brew install cmake boost pkg-config libevent capnp
 ```
 
 For macOS 11 (Big Sur) and 12 (Monterey) you need to install a more recent version of llvm.

doc/build-unix.md

@@ -77,7 +77,7 @@ GUI dependencies:
 Bitcoin Core includes a GUI built with the cross-platform Qt Framework. To compile the GUI, we need to install
 the necessary parts of Qt, the libqrencode and pass `-DBUILD_GUI=ON`. Skip if you don't intend to use the GUI.
 
-    sudo apt-get install qtbase5-dev qttools5-dev qttools5-dev-tools
+    sudo apt-get install qtbase5-dev qttools5-dev qttools5-dev-tools libcapnp-dev capnproto
 
 Additionally, to support Wayland protocol for modern desktop environments:
 

doc/dependencies.md

@@ -47,3 +47,7 @@ You can find installation instructions in the `build-*.md` file for your platfor
 | --- | --- | --- | --- | --- |
 | [Berkeley DB](../depends/packages/bdb.mk) (legacy wallet) | [link](https://www.oracle.com/technetwork/database/database-technologies/berkeleydb/downloads/index.html) | 4.8.30 | 4.8.x | No |
 | [SQLite](../depends/packages/sqlite.mk) | [link](https://sqlite.org) | [3.38.5](https://github.com/bitcoin/bitcoin/pull/25378) | [3.7.17](https://github.com/bitcoin/bitcoin/pull/19077) | No |
+
+### Multiprocess support
+| Dependency | Releases | Version used | Minimum required | Runtime |
+| [Cap'n Proto](../depends/packages/capnp.mk) | [link](https://capnproto.org) | [0.6.1](https://capnproto.org/install.html) | 0.5.3 | Yes |

doc/multiprocess.md

@@ -34,3 +34,9 @@ Alternately, you can install [Cap'n Proto](https://capnproto.org/) and [libmulti
 `bitcoin-node` is a drop-in replacement for `bitcoind`, and `bitcoin-gui` is a drop-in replacement for `bitcoin-qt`, and there are no differences in use or external behavior between the new and old executables. But internally after [#10102](https://github.com/bitcoin/bitcoin/pull/10102), `bitcoin-gui` will spawn a `bitcoin-node` process to run P2P and RPC code, communicating with it across a socket pair, and `bitcoin-node` will spawn `bitcoin-wallet` to run wallet code, also communicating over a socket pair. This will let node, wallet, and GUI code run in separate address spaces for better isolation, and allow future improvements like being able to start and stop components independently on different machines and environments.
 [#19460](https://github.com/bitcoin/bitcoin/pull/19460) also adds a new `bitcoin-node` `-ipcbind` option and an `bitcoind-wallet` `-ipcconnect` option to allow new wallet processes to connect to an existing node process.
 And [#19461](https://github.com/bitcoin/bitcoin/pull/19461) adds a new `bitcoin-gui` `-ipcconnect` option to allow new GUI processes to connect to an existing node process.
+
+## Known issues
+
+- Unexpected socket disconnects aren't handled cleanly many places. Interface calls that used to never throw can now throw exceptions if a socket is disconnected (typically because a process on the other side of the connection has crashed or been killed), leading to errors.
+
+- Internally spawned bitcoin-node and bitcoin-wallet processes don't currently install signal handlers and so won't shut down cleanly if terminated with [CTRL-C](https://github.com/bitcoin/bitcoin/pull/10102#issuecomment-595353238). Shutting down with `bitcoin-cli stop` should still shut down cleanly, and is a suggested alternative.