cmake: check for Cap'n Proto / Clang / C++20 incompatibility #205

willcl-ark · 2025-09-03T10:13:16Z

Cap'n Proto 0.9.x and 0.10.x are incompatible with Clang 16+ when using C++20 due to P2468R2 implementation. This was fixed in Cap'n Proto 1.0+.

Generate a helpful error when these combinations are detected.

See: #199 for more context

DrahtBot · 2025-09-03T10:14:04Z

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willcl-ark · 2025-09-03T10:14:38Z

I've left direct reference to Bitcoin Core in the error message as, although this library is pretty generic, it seems that Bitcoin Core is the primary (or perhaps only) user, and therefore worth including for the tip on disabling IPC to bypass.

CMakeLists.txt

ryanofsky

Code review ACK ee5e284. This seems like it should be a helpful change.

The approach suggested bitcoin/bitcoin#33290 (comment) could potentially be used to avoid needing to mention the bitcoin core option here, and could be a followup

ryanofsky · 2025-09-03T22:07:10Z

LLM linter also suggested some reasonable edits #205 (comment)

CMakeLists.txt

willcl-ark · 2025-09-04T15:12:52Z

OK, I am a bit of a cmake language noob, so apologies in advance if I've butchered this, but I've tried to share the common error message in 197aef2 to along with @fanquake's suggestion of not recommending downgrading (which seems sensible).

To avoid also the situation where we have two seperate checks which a user could fail sequentially (but only after installing a second version of capnproto!), I have tried to merge them into a single check, with logic for each check type.

This way a user should only ever fail on this version check once, and should be shown appropriate options.

ryanofsky · 2025-09-04T15:16:54Z

@willcl-ark I started to look at 197aef2 which seems like a nice approach.

Any interested in extending it to replace bitcoin/bitcoin#33290? It would be great to show a better error message when find_package(CapnProto) fails here and it looks like the new code you've written could be easily extended to do it.

I think you would just need to replace

find_package(CapnProto 0.7 REQUIRED NO_MODULE)

with

find_package(CapnProto 0.7 QUIET NO_MODULE)
if (NOT CapnProto_FOUND)
   # ... custom handling here ...
endif()

ryanofsky

Code review ACK ee5e284. New code is clean and pretty straightforward and I really like the approach because it clearly spells out the solutions instead of requiring users and developers to look for them.

Cap'n Proto 0.9.x and 0.10.x are incompatible with Clang 16+ when using C++20 due to P2468R2 implementation. This was fixed in Cap'n Proto 1.0+. Generate a helpful error when these combinations are detected. See: bitcoin-core#199

Add a helpful error when we fail to find the package at all.

willcl-ark · 2025-09-04T18:34:51Z

@ryanofsky I added the suggested check here.

I did look at having a single giant "check package, then version compatibility" section (i.e. extending what is there currently), but the code is much cleaner using a specific

find_package(CapnProto 0.7 QUIET NO_MODULE)
 if(NOT CapnProto_FOUND)
   message(FATAL_ERROR

...even if that means we repeat a string or two.

Sjors · 2025-09-05T07:36:37Z

I tested 657d806 by updating the subtree in Bitcoin Core, and uninstalling capnp:

CMake Error at src/ipc/libmultiprocess/CMakeLists.txt:18 (message):
  Cap'n Proto is required but was not found.

  To resolve, choose one of the following:

    - Install Cap'n Proto (version 1.0+ recommended)
    - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support

That's quite nice.

ryanofsky · 2025-09-05T13:16:29Z

Might make sense to update PR title and description to reflect broader scope of this change now

ryanofsky · 2025-09-05T13:36:03Z

Testing 657d806 with different settings in olddeps.bash and it seems to work well.

setting 0.9.0 in olddeps.bash warns about CVE and suggests upgrading to major or minor version or disabling

$ rm -rvf build-olddeps; CI_CONFIG=ci/configs/olddeps.bash ci/scripts/run.sh
[...]
CMake Error at CMakeLists.txt:76 (message):
  The version of Cap'n Proto detected: 0.9.0 has known compatibility issues:

  - CVE-2022-46149 security vulnerability (details:
  https://github.com/advisories/GHSA-qqff-4vw4-f6hx)

  To resolve, choose one of the following:

    - Upgrade to Cap'n Proto version 1.0 or newer (recommended)
    - Upgrade to a patched minor version (0.7.1, 0.8.1, 0.9.2, 0.10.3, or later)
    - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support

setting 0.9.0 in and export CXX=clang++ in olddeps.bash suggests upgrading to major version or disabling

$ rm -rf build-olddeps; CI_CONFIG=ci/configs/olddeps.bash ci/scripts/run.sh
[...]
CMake Error at CMakeLists.txt:76 (message):
  The version of Cap'n Proto detected: 0.9.0 has known compatibility issues:

  - CVE-2022-46149 security vulnerability (details:
  https://github.com/advisories/GHSA-qqff-4vw4-f6hx)

  - Incompatible with Clang 20.1.5 when using C++20

  To resolve, choose one of the following:

    - Upgrade to Cap'n Proto version 1.0 or newer (recommended)
    - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support

setting 0.9.2 in and export CXX=clang++ in olddeps.bash suggests using gcc or updating to major version or disabling

$ rm -rf build-olddeps; CI_CONFIG=ci/configs/olddeps.bash ci/scripts/run.sh
[...]
CMake Error at CMakeLists.txt:76 (message):
  The version of Cap'n Proto detected: 0.9.2 has known compatibility issues:

  - Incompatible with Clang 20.1.5 when using C++20

  To resolve, choose one of the following:

    - Upgrade to Cap'n Proto version 1.0 or newer (recommended)
    - Use GCC instead of Clang
    - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support

commenting out capnproto input in shell.nix suggests installing cap'n proto or disabling

$ rm -rf build-olddeps; CI_CONFIG=ci/configs/olddeps.bash ci/scripts/run.sh
[...]
CMake Error at CMakeLists.txt:18 (message):
  Cap'n Proto is required but was not found.

  To resolve, choose one of the following:

    - Install Cap'n Proto (version 1.0+ recommended)
    - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support

ryanofsky

Code review ACK 657d806. Thanks for these changes! I think they should very helpful and prevent unnecessary confusion.

ryanofsky · 2025-09-05T13:47:33Z

CMakeLists.txt

+    string(APPEND RESOLUTION_OPTIONS "  - Use GCC instead of Clang\n")
+  endif()
+
+  string(APPEND RESOLUTION_OPTIONS "  - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support\n")


In commit "cmake: check for Cap'n Proto / Clang / C++20 incompatibility" (d314057)

This might be more readable with a comma after "compilation". Also I think the "if you do not need IPC functionality" phrasing from bitcoin/bitcoin#33290 might make the effects of this easier to understand than "to disable multiprocess support". Maybe my suggestion would be to change this to something like "If building Bitcoin Core and you do not need IPC functionality, configure cmake with -DENABLE_IPC=OFF" here in and in the next commit.

This also seems fine for now though, and later we can drop if "If building Bitcoin Core" part by supporting a MP_SUBPROJECT_ERROR variable as suggested in the other issue.

hebasto

Concept ACK.

As a follow-up, all code related to handling the CapnProto package could be encapsulated in a cmake/FindCapnProto.cmake module.

hebasto · 2025-09-05T15:09:00Z

CMakeLists.txt

+ if(NOT CapnProto_FOUND)
+   message(FATAL_ERROR
+     "Cap'n Proto is required but was not found.\n"
+     "To resolve, choose one of the following:\n"
+     "  - Install Cap'n Proto (version 1.0+ recommended)\n"
+     "  - For Bitcoin Core compilation build with -DENABLE_IPC=OFF to disable multiprocess support\n"
+   )
+ endif()


mis-indentation?

ryanofsky · 2025-09-05T19:42:34Z

I think I'll go ahead and merge this now, and we can iterate and improve in future PRs. There are good ideas above for fixing up an indent, changing wording, and moving code to a module. There is also the MP_SUBPROJECT_ERROR idea from bitcoin/bitcoin#33290 (comment) and possibility of extending this to detect an incompatible netbsd package from #196 (comment)

…f2ecc1 13424cf2ecc1 Merge bitcoin-core/libmultiprocess#205: cmake: check for Cap'n Proto / Clang / C++20 incompatibility 72dce118649b Merge bitcoin-core/libmultiprocess#200: event loop: add LogOptions struct and reduce the log size 85003409f964 eventloop: add `LogOptions` struct 657d80622f81 cmake: capnproto pkg missing helpful error d314057775a5 cmake: check for Cap'n Proto / Clang / C++20 incompatibility 878e84dc3030 Merge bitcoin-core/libmultiprocess#203: cmake: search capnproto in package mode only 1a85da5873c2 Merge bitcoin-core/libmultiprocess#202: doc: correct the build instructions for the example df01873e1ecb Merge bitcoin-core/libmultiprocess#197: ci: Add freebsd and macos build 3bee07ab3367 cmake: search capnproto in package mode only b6d3dc44194c doc: correct the build instructions for example fa1ac3000055 ci: Add macos and freebsd task git-subtree-dir: src/ipc/libmultiprocess git-subtree-split: 13424cf2ecc1e5eadc85556cf1f4c65e915f702a

a334bbe Squashed 'src/ipc/libmultiprocess/' changes from 1b8d4a6f1e54..13424cf2ecc1 (Ryan Ofsky) Pull request description: Includes: - bitcoin-core/libmultiprocess#197 - bitcoin-core/libmultiprocess#202 - bitcoin-core/libmultiprocess#203 - bitcoin-core/libmultiprocess#200 - bitcoin-core/libmultiprocess#205 These changes should give better feedback when there are build errors, and also make IPC logs more readable. The 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) ACKs for top commit: Sjors: ACK a4ee70e Tree-SHA512: ddddd0ed44522ade98a5b94f44b57210794d64f8c378a00438082b8c377f41e9b86c0c5ed29add45472549758f7478ab220af8e268b90b30f57a236c639497d3

Sjors reviewed Sep 3, 2025

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willcl-ark force-pushed the clang-capnp-incompat branch from ee5e284 to 197aef2 Compare September 4, 2025 15:07

ryanofsky approved these changes Sep 4, 2025

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ryanofsky mentioned this pull request Sep 4, 2025

build: suggest -DENABLE_IPC=OFF when missing capnp bitcoin/bitcoin#33290

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willcl-ark added 2 commits September 4, 2025 19:32

cmake: capnproto pkg missing helpful error

657d806

Add a helpful error when we fail to find the package at all.

willcl-ark force-pushed the clang-capnp-incompat branch from e1c15e0 to 657d806 Compare September 4, 2025 18:33

Sjors mentioned this pull request Sep 5, 2025

build: suggest -DENABLE_IPC=OFF when missing capnp bitcoin/bitcoin#33315

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ryanofsky approved these changes Sep 5, 2025

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ci: Add NetBSD job #196

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hebasto reviewed Sep 5, 2025

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ryanofsky merged commit 13424cf into bitcoin-core:master Sep 5, 2025
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ryanofsky mentioned this pull request Sep 5, 2025

Update libmultiprocess subtree to improve build and logs bitcoin/bitcoin#33322

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This was referenced Sep 8, 2025

Cap'n Proto 0.9 & 0.10 capnp/blob.h error: invalid operands to binary expression ('const char *' and 'kj::StringPtr') #199

Closed

build: clang build broken on Debian Bookworm bitcoin/bitcoin#33279

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maflcko mentioned this pull request Sep 22, 2025

Set cmake_minimum_required(VERSION 3.22) #175

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cmake: check for Cap'n Proto / Clang / C++20 incompatibility #205

cmake: check for Cap'n Proto / Clang / C++20 incompatibility #205

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