Merge bitcoin/bitcoin#30509: multiprocess: Add -ipcbind option to bitcoin-node

30073e6b3a multiprocess: Add -ipcbind option to bitcoin-node (Russell Yanofsky)
73fe7d7230 multiprocess: Add unit tests for connect, serve, and listen functions (Ryan Ofsky)
955d4077aa multiprocess: Add IPC connectAddress and listenAddress methods (Russell Yanofsky)
4da20434d4 depends: Update libmultiprocess library for CustomMessage function and ThreadContext bugfix (Ryan Ofsky)

Pull request description:

  Add `-ipcbind` option to `bitcoin-node` to make it listen on a unix socket and accept connections from other processes. The default socket path is `<datadir>/node.sock`, but this can be customized.

  This option lets potential wallet, gui, index, and mining processes connect to the node and control it. See examples in #19460, #19461, and #30437.

  Motivation for this PR, in combination with #30510, is be able to release a bitcoin core node binary that can generate block templates for a separate Stratum v2 mining service, like the one being implemented in https://github.com/Sjors/bitcoin/pull/48, that connects over IPC.

  Other things to know about this PR:

  - While the `-ipcbind` option lets other processes to connect to the `bitcoin-node` process, the only thing they can actually do after connecting is call methods on the [`Init`](https://github.com/bitcoin/bitcoin/blob/master/src/ipc/capnp/init.capnp#L17-L20) interface which is currently very limited and doesn't do much. But PRs [#30510](https://github.com/bitcoin/bitcoin/pull/30510), [#29409](https://github.com/bitcoin/bitcoin/pull/29409), and [#10102](https://github.com/bitcoin/bitcoin/pull/10102) expand the `Init` interface to expose mining, wallet, and gui functionality respectively.

  - This PR is not needed for [#10102](https://github.com/bitcoin/bitcoin/pull/10102), which runs GUI, node, and wallet code in different processes, because [#10102](https://github.com/bitcoin/bitcoin/pull/10102) does not use unix sockets or allow outside processes to connect to existing processes. [#10102](https://github.com/bitcoin/bitcoin/pull/10102) lets parent and child processes communicate over internal socketpairs, not externally accessible sockets.

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/issues/28722).

ACKs for top commit:
  achow101:
    ACK 30073e6b3a
  TheCharlatan:
    Re-ACK 30073e6b3a
  itornaza:
    Code review ACK 30073e6b3a

Tree-SHA512: 2b766e60535f57352e8afda9c3748a32acb5a57b2827371b48ba865fa9aa1df00f340732654f2e300c6823dbc6f3e14377fca87e4e959e613fe85a6d2312d9c8

src/qt/bitcoin.cpp

@@ -525,7 +525,7 @@ int GuiMain(int argc, char* argv[])
 
     /// 2. Parse command-line options. We do this after qt in order to show an error if there are problems parsing these
     // Command-line options take precedence:
-    SetupServerArgs(gArgs);
+    SetupServerArgs(gArgs, init->canListenIpc());
     SetupUIArgs(gArgs);
     std::string error;
     if (!gArgs.ParseParameters(argc, argv, error)) {