Improve and document act support

[Sjors]

Facilitate using act with Docker / Podman as an alternative for using nix directly.

I found this very useful for testing the sanitizer job against individual commits in #222. That is, once I finally got it to work...

This disables sandbox and filter-syscalls (but not in the CI environment), which seems fine given that anyone building this code on their own machine has to trust it anyway.

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ACK	ryanofsky

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[Sjors]

I'm still testing whether act works against all jobs, but even if only a few like sanitize work that's useful.

---

It works. Thanks to the cache it runs a lot faster the second time. Output is a bit verbose and there's no summary.

[ryanofsky]

Nice changes. They seem very useful. It seems like they could be especially useful for testing non-nix jobs like openbsd locally, though not sure if that requires KVM support.

Am a little curious why you needed this to run the sanitize job. Was it a macos issue? I regularly use the steps in the readme to run CI scripts locally on my machine without needing any containers (e.g. CI_CONFIG=ci/configs/sanitize.bash ci/scripts/run.sh) and that has seemed like the most fast and efficient way possible to run them.

[Sjors]

It's been a while, but last time I tried I couldn't get the nix based CI jobs to run on my Mac.

---

Still doesn't work, e.g. for master @ a4f9296:

...
-- Configuring done (11.5s)
-- Performing Test CXX_SUPPORTS_EHSC_FLAG - Failed
-- Performing Test C_SUPPORTS_FUNWIND_TABLES_FLAG
-- Performing Test C_SUPPORTS_FUNWIND_TABLES_FLAG - Success
CMake Warning at /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/cmake/Modules/HandleCompilerRT.cmake:8 (message):
  Failed to find compiler-rt builtins library for
Call Stack (most recent call first):
  /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/cmake/Modules/HandleCompilerRT.cmake:102 (cache_compiler_rt_library)
  /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/libcxxabi/src/CMakeLists.txt:127 (find_compiler_rt_library)


CMake Error at /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/libcxxabi/src/CMakeLists.txt:130 (message):
  Cannot find compiler-rt directory on OS X required for LLVM_USE_SANITIZER


-- Configuring incomplete, errors occurred!
error: builder for '/nix/store/m5r6a9hrpgbmqhc8i46m1gvcmlhlxz6i-libcxx-21.1.1.drv' failed with exit code 1;
       last 25 log lines:
       > -- Looking for pthread_once in pthread - found
       > -- Looking for __cxa_thread_atexit_impl in c
       > -- Looking for __cxa_thread_atexit_impl in c - not found
       > -- Performing Test CXX_SUPPORTS_FNO_OMIT_FRAME_POINTER_FLAG
       > -- Performing Test CXX_SUPPORTS_FNO_OMIT_FRAME_POINTER_FLAG - Success
       > -- Performing Test CXX_SUPPORTS_GLINE_TABLES_ONLY_FLAG
       > -- Performing Test CXX_SUPPORTS_GLINE_TABLES_ONLY_FLAG - Success
       > -- Performing Test CXX_SUPPORTS_FSTRICT_ALIASING_FLAG
       > -- Performing Test CXX_SUPPORTS_FSTRICT_ALIASING_FLAG - Success
       > -- Performing Test CXX_SUPPORTS_EHSC_FLAG
       > -- Performing Test CXX_SUPPORTS_EHSC_FLAG - Failed
       > -- Performing Test C_SUPPORTS_FUNWIND_TABLES_FLAG
       > -- Performing Test C_SUPPORTS_FUNWIND_TABLES_FLAG - Success
       > CMake Warning at /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/cmake/Modules/HandleCompilerRT.cmake:8 (message):
       >   Failed to find compiler-rt builtins library for
       > Call Stack (most recent call first):
       >   /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/cmake/Modules/HandleCompilerRT.cmake:102 (cache_compiler_rt_library)
       >   /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/libcxxabi/src/CMakeLists.txt:127 (find_compiler_rt_library)
       >
       >
       > CMake Error at /tmp/nix-build-libcxx-21.1.1.drv-0/libcxx-src-21.1.1/libcxxabi/src/CMakeLists.txt:130 (message):
       >   Cannot find compiler-rt directory on OS X required for LLVM_USE_SANITIZER
       >
       >
       > -- Configuring incomplete, errors occurred!
       For full logs, run:
         nix log /nix/store/m5r6a9hrpgbmqhc8i46m1gvcmlhlxz6i-libcxx-21.1.1.drv
error: build of '/nix/store/m5r6a9hrpgbmqhc8i46m1gvcmlhlxz6i-libcxx-21.1.1.drv', '/nix/store/n8c59j8znsgi8bzw349g7af5x1hzs79p-capnproto-1.2.0.drv' failed

https://gist.github.com/Sjors/bda49b5576a668493575081b3abb11a9

M4 Apple Silicon macOS 26.0.1

[maflcko]

It should be possible to do so, see https://github.com/maflcko/libmultiprocess/actions/runs/17243581016/job/48927248082 / e8dc12d.

[ryanofsky]

Code review ACK 4e3f8fa. Nice changes, and sorry for not reviewing this earlier. This seems like a useful feature and I plan to use it myself to test githuba ctions changes. Setting up the /nix subvolume in the instructions is also really nice since it should enable builds to run really quickly.