`syscall(SYS_pidfd_open)` should produce clear error when `ENOSYS`

[krwalker]

While building on an older Linux kernel for compatibility, all V2 process spawns failed with Bad file descriptor. We traced this back to syscall(SYS_pidfd_open) returning -1 and errno == ENOSYS.

In config.hpp, even though SYS_pidfd_open is #defined, it doesn't appear to mean that it will work.

We now detect and define BOOST_PROCESS_V2_DISABLE_PIDFD_OPEN and things work as expected, but it took some time to figure out that we needed to do this.

It would be nice if an exception was thrown when this failed syscall produced ENOSYS.

[klemens-morgenstern]

Which boost version are you using? I think that is already fixed.

[krwalker]

I am currently using Boost v1.87.0 with Boost.Process V2 (develop branch) from early January, 2026. Although 1bfe21b / #513 checks for -1 in the basic_process_handle_fd constructor body, the exception is coming from the descriptor_ constructor:

What: assign: Bad file descriptor [system:9 at /3rdparty/Boost/libs/asio/include/boost/asio/detail/impl/reactive_descriptor_service.ipp:120:5 in function 'boost::system::error_code boost::asio::detail::reactive_descriptor_service::assign(implementation_type&, const native_handle_type&, boost::system::error_code&)']

/3rdparty/Boost/libs/asio/include/boost/asio/posix/basic_descriptor.hpp(139): Throw in function boost::asio::posix::basic_descriptor<Executor>::basic_descriptor(const executor_type&, const native_handle_type&) [with Executor = boost::asio::any_io_executor; executor_type = boost::asio::any_io_executor; native_handle_type = int]