ipc: Fix mpgen capnp tool path for vcpkg/Windows builds - #317
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Error seen (MSVC native build): mpgen error: mp\util.cpp:332: failed: CreateProcessA(...): bitcoin-core#2 The system cannot find the file specified. Problem: mpgen constructs capnp tool paths as capnp_PREFIX + "/bin/capnp" and capnp_PREFIX + "/bin/capnpc-c++". capnp_PREFIX is derived from the capnproto include directory parent, so on a vcpkg install it resolves to something like vcpkg_installed/x64-windows-release. But vcpkg installs executables under tools/capnproto/, not bin/, so the constructed path does not exist and CreateProcessA fails with ERROR_FILE_NOT_FOUND. Fix: pass the CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE cmake variables from CapnProtoConfig.cmake as compile definitions to mpgen, and use them in gen.cpp instead of constructing paths from capnp_PREFIX. Using these cmake variables rather than reading the CapnProto::capnp_tool imported target IMPORTED_LOCATION directly is important: on Debian/Ubuntu, IMPORTED_LOCATION is set to a non-existent path due to a multiarch cmake packaging bug in libcapnp-dev (see bitcoin-core#328). The cmake variables avoid this because CapnProtoConfig.cmake hardcodes the correct paths on Debian/Ubuntu, and on other platforms (including vcpkg) the variables hold generator expressions that target_compile_definitions evaluates correctly at generation time. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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I reviewed 049dae1 and played with the code on Windows using the following vcpkg.json file:
{
"$comment": "The builtin-baseline corresponds to 2025.08.27 Release",
"$schema": "https://raw.githubusercontent.com/microsoft/vcpkg-tool/main/docs/vcpkg.schema.json",
"builtin-baseline": "120deac3062162151622ca4860575a33844ba10b",
"dependencies": [
"capnproto"
]
}The code works. However, I believe the build system could do better and always define CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE to verified paths.
See this branch: https://github.com/hebasto/libmultiprocess/commits/pr317/0731.alt/
Additionally, the code is organised into a function, and unnecessary quotes are removed.
Feel free to pick any part of that branch.
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Thanks for the review! I dug into the debian bug a little more and think I found a simpler workaround. The details of the bug are in #328. I will push an update to bitcoin/bitcoin#32387 including this workaround so we can be sure it still works for MSVC. Updated 049dae1 -> a26a084 ( |
3 participants
It seems that in the vcpkg capnproto package, the
capnpcode generator can't be found in<capnp_PREFIX>/bin/capnpbut is in another path that needs to be read from the cmake configuration, so this change adds cmake code to handle that.Unfortunately the previous code using
<capnp_PREFIX>/bincan't be removed because the llbcapnp-dev 1.0.1 package in Ubuntu Noble is buggy and specifies the wrong executable path in its cmake information. (If this problem is fixed, the oldcapnp_PREFIXcode could be dropped.)This change is needed to get MSVC builds working in bitcoin/bitcoin#32387