UNIXSocket#recv_io

static VALUE
unix_recv_io(int argc, VALUE *argv, VALUE sock)
{
    VALUE klass, mode;
    rb_io_t *fptr;
    struct iomsg_arg arg;
    struct iovec vec[2];
    char buf[1];
    unsigned int gc_reason = 0;
    enum {
        GC_REASON_EMSGSIZE = 0x1,
        GC_REASON_TRUNCATE = 0x2,
        GC_REASON_ENOMEM = 0x4
    };

    int fd;
#if FD_PASSING_BY_MSG_CONTROL
    union {
        struct cmsghdr hdr;
        char pad[sizeof(struct cmsghdr)+8+sizeof(int)+8];
    } cmsg;
#endif

    rb_scan_args(argc, argv, "02", &klass, &mode);
    if (argc == 0)
        klass = rb_cIO;
    if (argc <= 1)
        mode = Qnil;

retry:
    GetOpenFile(sock, fptr);

    arg.msg.msg_name = NULL;
    arg.msg.msg_namelen = 0;

    vec[0].iov_base = buf;
    vec[0].iov_len = sizeof(buf);
    arg.msg.msg_iov = vec;
    arg.msg.msg_iovlen = 1;

#if FD_PASSING_BY_MSG_CONTROL
    arg.msg.msg_control = (caddr_t)&cmsg;
    arg.msg.msg_controllen = (socklen_t)CMSG_SPACE(sizeof(int));
    arg.msg.msg_flags = 0;
    cmsg.hdr.cmsg_len = (socklen_t)CMSG_LEN(sizeof(int));
    cmsg.hdr.cmsg_level = SOL_SOCKET;
    cmsg.hdr.cmsg_type = SCM_RIGHTS;
    fd = -1;
    memcpy(CMSG_DATA(&cmsg.hdr), &fd, sizeof(int));
#else
    arg.msg.msg_accrights = (caddr_t)&fd;
    arg.msg.msg_accrightslen = sizeof(fd);
    fd = -1;
#endif

    arg.fd = fptr->fd;
    while ((int)BLOCKING_REGION_FD(recvmsg_blocking, &arg) == -1) {
        int e = errno;
        if (e == EMSGSIZE && !(gc_reason & GC_REASON_EMSGSIZE)) {
            /* FreeBSD gets here when we're out of FDs */
            gc_reason |= GC_REASON_EMSGSIZE;
            rb_gc_for_fd(EMFILE);
            goto retry;
        }
        else if (e == ENOMEM && !(gc_reason & GC_REASON_ENOMEM)) {
            /* ENOMEM is documented in recvmsg manpages */
            gc_reason |= GC_REASON_ENOMEM;
            rb_gc_for_fd(e);
            goto retry;
        }
        if (!rb_io_wait_readable(arg.fd))
            rsock_syserr_fail_path(e, "recvmsg(2)", fptr->pathv);
    }

#if FD_PASSING_BY_MSG_CONTROL
    if (arg.msg.msg_controllen < (socklen_t)sizeof(struct cmsghdr)) {
        /* FreeBSD and Linux both get here when we're out of FDs */
        if (!(gc_reason & GC_REASON_TRUNCATE)) {
            gc_reason |= GC_REASON_TRUNCATE;
            rb_gc_for_fd(EMFILE);
            goto retry;
        }
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (msg_controllen=%d smaller than sizeof(struct cmsghdr)=%d)",
                 (int)arg.msg.msg_controllen, (int)sizeof(struct cmsghdr));
    }
    if (cmsg.hdr.cmsg_level != SOL_SOCKET) {
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (cmsg_level=%d, %d expected)",
                 cmsg.hdr.cmsg_level, SOL_SOCKET);
    }
    if (cmsg.hdr.cmsg_type != SCM_RIGHTS) {
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (cmsg_type=%d, %d expected)",
                 cmsg.hdr.cmsg_type, SCM_RIGHTS);
    }
    if (arg.msg.msg_controllen < (socklen_t)CMSG_LEN(sizeof(int))) {
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (msg_controllen=%d smaller than CMSG_LEN(sizeof(int))=%d)",
                 (int)arg.msg.msg_controllen, (int)CMSG_LEN(sizeof(int)));
    }
    if ((socklen_t)CMSG_SPACE(sizeof(int)) < arg.msg.msg_controllen) {
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (msg_controllen=%d bigger than CMSG_SPACE(sizeof(int))=%d)",
                 (int)arg.msg.msg_controllen, (int)CMSG_SPACE(sizeof(int)));
    }
    if (cmsg.hdr.cmsg_len != CMSG_LEN(sizeof(int))) {
        rsock_discard_cmsg_resource(&arg.msg, 0);
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (cmsg_len=%d, %d expected)",
                 (int)cmsg.hdr.cmsg_len, (int)CMSG_LEN(sizeof(int)));
    }
#else
    if (arg.msg.msg_accrightslen != sizeof(fd)) {
        rb_raise(rb_eSocket,
                 "file descriptor was not passed (accrightslen=%d, %d expected)",
                 arg.msg.msg_accrightslen, (int)sizeof(fd));
    }
#endif

#if FD_PASSING_BY_MSG_CONTROL
    memcpy(&fd, CMSG_DATA(&cmsg.hdr), sizeof(int));
#endif

    rb_update_max_fd(fd);
    rb_maygvl_fd_fix_cloexec(fd);

    if (klass == Qnil)
        return INT2FIX(fd);
    else {
        ID for_fd;
        int ff_argc;
        VALUE ff_argv[2];
        CONST_ID(for_fd, "for_fd");
        ff_argc = mode == Qnil ? 1 : 2;
        ff_argv[0] = INT2FIX(fd);
        ff_argv[1] = mode;
        return rb_funcallv(klass, for_fd, ff_argc, ff_argv);
    }
}