childWaitPosix

fn childWaitPosix(child: *process.Child) process.Child.WaitError!process.Child.Term {
    defer childCleanupPosix(child);

    const pid = child.id.?;

    var ru: posix.rusage = undefined;
    const ru_ptr = if (child.request_resource_usage_statistics) &ru else null;

    if (have_wait4) {
        var status: if (builtin.link_libc) c_int else i32 = undefined;
        const syscall: Syscall = try .start();
        while (true) switch (posix.errno(posix.system.wait4(pid, &status, 0, ru_ptr))) {
            .SUCCESS => {
                syscall.finish();
                if (ru_ptr) |p| child.resource_usage_statistics.rusage = p.*;
                return statusToTerm(@bitCast(status));
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            .CHILD => |err| return syscall.errnoBug(err), // Double-free.
            else => |err| return syscall.unexpectedErrno(err),
        };
    }

    if (have_waitid) {
        const linux = std.os.linux; // Bypass libc which has the wrong signature.
        var info: linux.siginfo_t = undefined;
        const syscall: Syscall = try .start();
        while (true) switch (linux.errno(linux.waitid(.PID, pid, &info, linux.W.EXITED, ru_ptr))) {
            .SUCCESS => {
                syscall.finish();
                if (ru_ptr) |p| child.resource_usage_statistics.rusage = p.*;
                const status: u32 = @bitCast(info.fields.common.second.sigchld.status);
                const code: linux.CLD = @fromBackingInt(@intCast(info.code));
                return switch (code) {
                    .EXITED => .{ .exited = @truncate(status) },
                    .KILLED, .DUMPED => .{ .signal = @fromBackingInt(@intCast(status)) },
                    .TRAPPED, .STOPPED => .{ .stopped = @fromBackingInt(@intCast(status)) },
                    _, .CONTINUED => .{ .unknown = status },
                };
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            .CHILD => |err| return syscall.errnoBug(err), // Double-free.
            else => |err| return syscall.unexpectedErrno(err),
        };
    }

    var status: if (builtin.link_libc) c_int else i32 = undefined;
    const syscall: Syscall = try .start();
    while (true) switch (posix.errno(posix.system.waitpid(pid, &status, 0))) {
        .SUCCESS => {
            syscall.finish();
            return statusToTerm(@bitCast(status));
        },
        .INTR => {
            try syscall.checkCancel();
            continue;
        },
        .CHILD => |err| return syscall.errnoBug(err), // Double-free.
        else => |err| return syscall.unexpectedErrno(err),
    };
}