cmake: Support building with libmultiprocess subtree

When ENABLE_IPC option is on, build with libmultiprocess subtree and
`add_subdirectory(src/ipc/libmultiprocess)` instead of external package
and `find_package(Libmultiprocess)` by default.

Behavior can be toggled with `WITH_EXTERNAL_LIBMULTIPROCESS` option. Using a
subtree should be more convenient for most bitcoin developers, but using an
external package is more convenient for developing in the libmultiprocess
repository.

The `WITH_EXTERNAL_LIBMULTIPROCESS` option is also used to avoid needing to
changing the depends build here. But in later commits, the depends build is
switched to use the add_subdirectory build as well.

Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>
Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

CMakeLists.txt

@@ -154,7 +154,8 @@ endif()
 cmake_dependent_option(WITH_DBUS "Enable DBus support." ON "CMAKE_SYSTEM_NAME STREQUAL \"Linux\" AND BUILD_GUI" OFF)
 
 option(ENABLE_IPC "Build multiprocess bitcoin-node and bitcoin-gui executables in addition to monolithic bitcoind and bitcoin-qt executables. Requires libmultiprocess library. Experimental." OFF)
-if(ENABLE_IPC)
+cmake_dependent_option(WITH_EXTERNAL_LIBMULTIPROCESS "Build with external libmultiprocess library instead of with local git subtree when ENABLE_IPC is enabled. This is not normally recommended, but can be useful when cross-compiling or making changes to the upstream project." OFF "ENABLE_IPC" OFF)
+if(ENABLE_IPC AND WITH_EXTERNAL_LIBMULTIPROCESS)
   find_package(Libmultiprocess REQUIRED COMPONENTS Lib)
   find_package(LibmultiprocessNative REQUIRED COMPONENTS Bin
     NAMES Libmultiprocess
@@ -674,6 +675,16 @@ if(ENABLE_WALLET)
 endif()
 message("  external signer ..................... ${ENABLE_EXTERNAL_SIGNER}")
 message("  ZeroMQ .............................. ${WITH_ZMQ}")
+if(ENABLE_IPC)
+  if (WITH_EXTERNAL_LIBMULTIPROCESS)
+    set(ipc_status "ON (with external libmultiprocess)")
+  else()
+    set(ipc_status ON)
+  endif()
+else()
+  set(ipc_status OFF)
+endif()
+message("  IPC ................................. ${ipc_status}")
 message("  USDT tracing ........................ ${WITH_USDT}")
 message("  QR code (GUI) ....................... ${WITH_QRENCODE}")
 message("  DBus (GUI, Linux only) .............. ${WITH_DBUS}")

cmake/libmultiprocess.cmake

@@ -0,0 +1,28 @@
+# Copyright (c) 2025 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
+function(add_libmultiprocess subdir)
+  # Set BUILD_TESTING to match BUILD_TESTS. BUILD_TESTING is a standard cmake
+  # option that controls whether enable_testing() is called, but in the bitcoin
+  # build a BUILD_TESTS option is used instead.
+  set(BUILD_TESTING "${BUILD_TESTS}")
+  add_subdirectory(${subdir} EXCLUDE_FROM_ALL)
+  # Apply core_interface compile options to libmultiprocess runtime library.
+  target_link_libraries(multiprocess PUBLIC $<BUILD_INTERFACE:core_interface>)
+  target_link_libraries(mputil PUBLIC $<BUILD_INTERFACE:core_interface>)
+  target_link_libraries(mpgen PUBLIC $<BUILD_INTERFACE:core_interface>)
+  # Mark capproto options as advanced to hide by default from cmake UI
+  mark_as_advanced(CapnProto_DIR)
+  mark_as_advanced(CapnProto_capnpc_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_capnp_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_capnp-json_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_capnp-rpc_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_capnp-websocket_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_kj-async_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_kj-gzip_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_kj-http_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_kj_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_kj-test_IMPORTED_LOCATION)
+  mark_as_advanced(CapnProto_kj-tls_IMPORTED_LOCATION)
+endfunction()

depends/toolchain.cmake.in

@@ -160,8 +160,10 @@ endif()
 
 if("@multiprocess@" STREQUAL "1")
   set(ENABLE_IPC ON CACHE BOOL "")
+  set(WITH_EXTERNAL_LIBMULTIPROCESS ON CACHE BOOL "")
   set(Libmultiprocess_ROOT "${CMAKE_CURRENT_LIST_DIR}" CACHE PATH "")
   set(LibmultiprocessNative_ROOT "${CMAKE_CURRENT_LIST_DIR}/native" CACHE PATH "")
 else()
   set(ENABLE_IPC OFF CACHE BOOL "")
+  set(WITH_EXTERNAL_LIBMULTIPROCESS OFF CACHE BOOL "")
 endif()

src/CMakeLists.txt

@@ -19,6 +19,10 @@ include(../cmake/crc32c.cmake)
 include(../cmake/leveldb.cmake)
 include(../cmake/minisketch.cmake)
 add_subdirectory(univalue)
+if (ENABLE_IPC AND NOT WITH_EXTERNAL_LIBMULTIPROCESS)
+  include(../cmake/libmultiprocess.cmake)
+  add_libmultiprocess(ipc/libmultiprocess)
+endif()
 #=============================
 # secp256k1 subtree
 #=============================