Merge bitcoin/bitcoin#32345: ipc: Handle unclean shutdowns better

2581258ec2 ipc: Handle bitcoin-wallet disconnections (Ryan Ofsky)
2160995916 ipc: Add Ctrl-C handler for spawned subprocesses (Ryan Ofsky)
0c28068ceb doc: Improve IPC interface comments (Ryan Ofsky)
7f65aac78b ipc: Avoid waiting for clients to disconnect when shutting down (Ryan Ofsky)
6eb09fd614 test: Add unit test coverage for Init and Shutdown code (Ryan Ofsky)
9a9fb19536 ipc: Use EventLoopRef instead of addClient/removeClient (Ryan Ofsky)
e886c65b6b Squashed 'src/ipc/libmultiprocess/' changes from 27c7e8e5a581..b4120d34bad2 (Ryan Ofsky)

Pull request description:

  This PR fixes various problems when IPC connections are broken or hang which were reported in https://github.com/bitcoin-core/libmultiprocess/issues/123, https://github.com/bitcoin-core/libmultiprocess/issues/176, and https://github.com/bitcoin-core/libmultiprocess/pull/182. The different fixes are described in commit messages.

  ---

  The first two commits of this PR update the libmultiprocess subtree including the following PRs:

  - https://github.com/bitcoin-core/libmultiprocess/pull/181
  - https://github.com/bitcoin-core/libmultiprocess/pull/179
  - https://github.com/bitcoin-core/libmultiprocess/pull/160
  - https://github.com/bitcoin-core/libmultiprocess/pull/184
  - https://github.com/bitcoin-core/libmultiprocess/pull/187
  - https://github.com/bitcoin-core/libmultiprocess/pull/186
  - https://github.com/bitcoin-core/libmultiprocess/pull/192

  The subtree changes can be verified by running `test/lint/git-subtree-check.sh src/ipc/libmultiprocess` as described in [developer notes](https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#subtrees) and [lint instructions](https://github.com/bitcoin/bitcoin/tree/master/test/lint#git-subtree-checksh).

  The remaining commits are:

  - [`9a9fb19536fa` ipc: Use EventLoopRef instead of addClient/removeClient](9a9fb19536)
  - [`6eb09fd6141f` test: Add unit test coverage for Init and Shutdown code](6eb09fd614)
  - [`7f65aac78b95` ipc: Avoid waiting for clients to disconnect when shutting down](7f65aac78b)
  - [`0c28068ceb7b` doc: Improve IPC interface comments](0c28068ceb)
  - [`216099591632` ipc: Add Ctrl-C handler for spawned subprocesses](2160995916)
  - [`2581258ec200` ipc: Handle bitcoin-wallet disconnections](2581258ec2)

  The new commits depend on the subtree update, and because the subtree update includes an incompatible API change, the "Use EventLoopRef" commit needs to be part of the same PR to avoid breaking the build. The other commits also make sense to merge at the same time because the bitcoin & libmultiprocess changes were written and tested together.

  ---

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

ACKs for top commit:
  Sjors:
    re-utACK 2581258ec2
  josibake:
    code review ACK 2581258ec2
  pinheadmz:
    re-ACK 2581258ec2

Tree-SHA512: 0095aa22d507803e2a2d46eff51fb6caf965cc0c97ccfa615bd97805d5d51e66a5b4b040640deb92896438b1fb9f6879847124c9d0e120283287bfce37b8d748

src/interfaces/ipc.h

@@ -59,17 +59,20 @@ public:
     //! true. If this is not a spawned child process, return false.
     virtual bool startSpawnedProcess(int argc, char* argv[], int& exit_status) = 0;
 
-    //! Connect to a socket address and make a client interface proxy object
-    //! using provided callback. connectAddress returns an interface pointer if
-    //! the connection was established, returns null if address is empty ("") or
-    //! disabled ("0") or if a connection was refused but not required ("auto"),
-    //! and throws an exception if there was an unexpected error.
+    //! Connect to a socket address and return a pointer to its Init interface.
+    //! Returns a non-null pointer if the connection was established, returns
+    //! null if address is empty ("") or disabled ("0") or if a connection was
+    //! refused but not required ("auto"), and throws an exception if there was
+    //! an unexpected error.
     virtual std::unique_ptr<Init> connectAddress(std::string& address) = 0;
 
-    //! Connect to a socket address and make a client interface proxy object
-    //! using provided callback. Throws an exception if there was an error.
+    //! Listen on a socket address exposing this process's init interface to
+    //! clients. Throws an exception if there was an error.
     virtual void listenAddress(std::string& address) = 0;
 
+    //! Disconnect any incoming connections that are still connected.
+    virtual void disconnectIncoming() = 0;
+
     //! Add cleanup callback to remote interface that will run when the
     //! interface is deleted.
     template<typename Interface>