OpenSSL::SSL::SSLSocket#ssl_read

static VALUE
ossl_ssl_read(VALUE self, VALUE len, VALUE str)
{
    SSL *ssl;
    int ilen;
    VALUE cb_state;

    GetSSL(self, ssl);
    if (!ssl_started(ssl))
        rb_raise(eSSLError, "SSL session is not started yet");

    ilen = NUM2INT(len);
    StringValue(str);
    if ((rb_len_t)rb_str_capacity(str) >= ilen)
        rb_str_modify(str);
    else
        rb_str_modify_expand(str, ilen - RSTRING_LEN(str));

    if (ilen == 0) {
        rb_str_set_len(str, 0);
        return str;
    }

    rb_str_locktmp(str);
    int nread = SSL_read(ssl, RSTRING_PTR(str), ilen);
    rb_str_unlocktmp(str);
    int saved_errno = errno_mapped();

    cb_state = rb_attr_get(self, ID_callback_state);
    if (!NIL_P(cb_state)) {
        rb_ivar_set(self, ID_callback_state, Qnil);
        ossl_clear_error();
        rb_jump_tag(NUM2INT(cb_state));
    }

    switch (SSL_get_error(ssl, nread)) {
      case SSL_ERROR_NONE:
        rb_str_set_len(str, nread);
        return str;
      case SSL_ERROR_ZERO_RETURN:
        return Qnil;
      case SSL_ERROR_WANT_WRITE:
        return sym_wait_writable;
      case SSL_ERROR_WANT_READ:
        return sym_wait_readable;
      case SSL_ERROR_SYSCALL:
        if (saved_errno) {
            ossl_clear_error();
            rb_exc_raise(rb_syserr_new(saved_errno, "SSL_read"));
        }
        if (!ERR_peek_error()) {
            /*
             * The underlying BIO returned 0. This is actually a
             * protocol error. But unfortunately, not all
             * implementations cleanly shutdown the TLS connection
             * but just shutdown/close the TCP connection. So report
             * EOF for now...
             */
            return Qnil;
        }
        /* fall through */
      default:
        ossl_raise(eSSLError, "SSL_read");
    }
}