Add signal flags support via sigaction() on POSIX

[sgerbino]

Summary

Add support for POSIX signal flags (e.g., SA_RESTART, SA_NOCLDSTOP) to signal_set::add(), matching the Boost.Asio API.

Background

The current implementation uses the C runtime signal() function on both POSIX and Windows platforms. This works but doesn't support POSIX signal flags that control signal handling behavior.

Boost.Asio provides an add() overload that accepts flags:

void add(int signal_number, signal_set_base::flags_t flags);
void add(int signal_number, signal_set_base::flags_t flags, error_code& ec);

Motivation

Signal flags provide important functionality for robust signal handling:

Flag	Purpose
SA_RESTART	Automatically restart interrupted system calls
SA_NOCLDSTOP	Don't generate SIGCHLD when children stop (only pause)
SA_NOCLDWAIT	Don't create zombie processes on child termination
SA_NODEFER	Don't block the signal while its handler runs
SA_RESETHAND	Reset handler to SIG_DFL after one invocation

SA_RESTART is particularly important - without it, blocking calls like read() can fail with EINTR when a signal arrives.

Proposed API

// New flags enum (matches Boost.Asio)
class signal_set_base {
public:
    enum flags_t {
        none = 0,
        restart = SA_RESTART,         // POSIX only
        no_child_stop = SA_NOCLDSTOP, // POSIX only
        no_child_wait = SA_NOCLDWAIT, // POSIX only
        no_defer = SA_NODEFER,        // POSIX only
        reset_handler = SA_RESETHAND  // POSIX only
    };
};

class signal_set : public signal_set_base {
public:
    // Existing
    system::result<void> add(int signal_number);

    // New overload
    system::result<void> add(int signal_number, flags_t flags);
};

Implementation Notes

POSIX

• Replace signal() with sigaction() in posix/signals.cpp
• Store flags per-signal in signal_registration
• Apply flags when installing handler via sigaction()

struct sigaction sa;
sa.sa_handler = corosio_posix_signal_handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = flags;  // SA_RESTART, etc.
sigaction(signal_number, &sa, nullptr);

Windows

• Flags parameter is accepted but ignored (Windows signal() has no flags)
• Alternatively, return an error if non-zero flags are passed
• Document platform limitation

Tasks

• Add signal_set_base::flags_t enum
• Add add(int, flags_t) overload to signal_set
• Refactor POSIX implementation to use sigaction() instead of signal()
• Store and apply flags per-registration
• Handle Windows (ignore flags or error)
• Add tests for flag behavior
• Update documentation

References

• Boost.Asio signal_set::add()
• POSIX sigaction()
• sigaction(2) Linux manual

[vinniefalco]

no SA_RESTART in public headers