Category Archives: Stuff

This is Alue

I have made a couple of references in my blog to the new software suite I am writing, which I am calling Alue. It is time to explain what it is all about.

Alue will be a discussion forum system providing a web-based forum interface, a NNTP (Netnews) interface and an email interface, all with equal status. What will be unusual compared to most of the competition is that all these interfaces will be coequal views to the same abstract discussion, instead of being primarily one of these things and providing the others as bolted-on gateways. (I am aware of at least one other such system, but it is proprietary and thus not useful to my needs. Besides, I get to learn all kinds of fun things while doing this.)

I have, over several years, come across many times the need for such systems and never found a good, free implementation. I am now building this software for the use of one new discussion site that is being formed (which is graciously willing to serve as my guinea pig), but I hope it will eventually be of use to many other places as well.

I now have the first increment ready for beta testing. Note that this is not even close to being what I described above; it is merely a start. It currently provides a fully functional NNTP interface to a rudimentary (unreliable and unscalable) discussion database.

The NNTP server implements most of RFC 3977 (the base NNTP spec – IHAVE, MODE-READER, NEWNEWS and HDR are missing), all of RFC 4642 (STARTTLS) and a part of RFC 4643 (AUTHINFO USER – the SASL part is missing). The article database is intended to support – with certain deliberate omissions – the upcoming Netnews standards (USEFOR and USEPRO), but currently omits most of the mandatory checks.

There is a test installation at verbosify.org (port 119), which allows anonymous reading but requires identification and authentication for posting. I am currently handing out accounts only by invitation.

Code can be browsed in a Gitweb; git clone requests should be directed to git://git.verbosify.org/git/alue.git/.

There are some tweaks to be done to the NNTP frontend, but after that I expect to be rewriting the message filing system to be at least reliable if not scalable. After that, it is time for a web interface.

Asynchronous transput and gnutls

CC0
To the extent possible under law,
Antti-Juhani Kaijanaho has waived all copyright and related or neighboring rights to
Asynchronous transput and gnutls. This work is published from Finland.

GnuTLS is a wonderful thing. It even has a thick manual – but nevertheless its documentation is severely lacking from the programmer’s point of view (and there doesn’t even seem to be independent howtos floating on the net). My hope is to remedy with this post, in small part, that problem.

I spent the weekend adding STARTTLS support to the NNTP (reading) server component of Alue. Since Alue is written in C++ and uses the Boost ASIO library as its primary concurrency framework, it seemed prudent to use ASIO’s SSL sublibrary. However, the result wasn’t stable and debugging it looked unappetizing. So, I wrote my own TLS layer on top of ASIO, based on gnutls.

Now, the gnutls API looks like it works only with synchronous transput: all TLS network operations are of the form “do this and return when done”; for example gnutls_handshake returns once the handshake is finished. So how does one adapt this to asynchronous transput? Fortunately, there are (badly documented) hooks for this purpose.

An application can tell gnutls to call application-supplied functions instead of the read(2) and write(2) system calls. Thus, when setting up a TLS session but before the handshake, I do the following:

                gnutls_transport_set_ptr(gs, this);
                gnutls_transport_set_push_function(gs, push_static);
                gnutls_transport_set_pull_function(gs, pull_static);
                gnutls_transport_set_lowat(gs, 0);

Here, gs is my private copy of the gnutls session structure, and the push_static and pull_static are static member functions in my sesssion wrapper class. The first line tells gnutls to give the current this pointer (a pointer to the current session wrapper) as the first argument to them. The last line tells gnutls not to try treating the this pointer as a Berkeley socket.

The pull_static static member function just passes control on to a non-static member, for convenience:

ssize_t session::pull_static(void * th, void *b, size_t n)
{
        return static_cast<session *>(th)->pull(b, n);
}

The basic idea of the pull function is to try to return immediately with data from a buffer, and if the buffer is empty, to fail with an error code signalling the absence of data with the possibility that data may become available later (the POSIX EAGAIN code):

class session
{
        [...]
        std::vector<unsigned char> ins;
        size_t ins_low, ins_high;
        [...]
};
ssize_t session::pull(void *b, size_t n_wanted)
{
        unsigned char *cs = static_cast<unsigned char *>(b);
        if (ins_high - ins_low > 0)
        {
                errno = EAGAIN;
                return -1;
        }
        size_t n = ins_high - ins_low < n_wanted 
                ?  ins_high - ins_low 
                :  n_wanted;
        for (size_t i = 0; i < n; i++)
        {
                cs[i] = ins[ins_low+i];
        }
        ins_low += n;
        return n;
}

Here, ins_low is an index to the ins vector specifying the first byte which has not already been passed on to gnutls, while ins_high is an index to the ins vector specifying the first byte that does not contain data read from the network. The assertions 0 <= ins_low, ins_low <= ins_high and ins_high <= ins.size() are obvious invariants in this buffering scheme.

The push case is simpler: all one needs to do is buffer the data that gnutls wants to send, for later transmission:

class session
{
        [...]
        std::vector<unsigned char> outs;
        size_t outs_low;
        [...]
};
ssize_t session::push(const void *b, size_t n)
{
        const unsigned char *cs = static_cast<const unsigned char *>(b);
        for (size_t i = 0; i < n; i++)
        {
                outs.push_back(cs[i]);
        }
        return n;
}

The low water mark outs_low (indicating the first byte that has not yet been sent to the network) is not needed in the push function. It would be possible for the push callback to signal EAGAIN, but it is not necessary in this scheme (assuming that one does not need to establish hard buffer limits).

Once gnutls receives an EAGAIN condition from the pull callback, it suspends the current operation and returns to its caller with the gnutls condition GNUTLS_E_AGAIN. The caller must arrange for more data to become available to the pull callback (in this case by scheduling an asynchronous write of the data in the outs buffer scheme and scheduling an asynchronous read to the ins buffer scheme) and then call the operation again, allowing the operation to resume.

The code so far does not actually perform any network transput. For this, I have written two auxiliary methods:

class session
{
        [...]
        bool read_active, write_active;
        [...]
};
void session::post_write()
{
        if (write_active) return;
        if (outs_low > 0 && outs_low == outs.size())
        {
                outs.clear();
                outs_low = 0;
        }
        else if (outs_low > 4096)
        {
                outs.erase(outs.begin(), outs.begin() + outs_low);
                outs_low = 0;
        }
        if (outs_low < outs.size())
        {
                stream.async_write_some
                        (boost::asio::buffer(outs.data()+outs_low,
                                             outs.size()-outs_low),
                         boost::bind(&session::sent_some,
                                     this, _1, _2));
                write_active = true;
        }
}

void session::post_read()
{
        if (read_active) return;
        if (ins_low > 0 && ins_low == ins.size())
        {
                ins.clear();
                ins_low = 0;
                ins_high = 0;
        }
        else if (ins_low > 4096)
        {
                ins.erase(ins.begin(), ins.begin() + ins_low);
                ins_high -= ins_low;
                ins_low = 0;
        }
        
        if (ins_high + 4096 >= ins.size()) ins.resize(ins_high + 4096);

        stream.async_read_some(boost::asio::buffer(ins.data()+ins_high,
                                                   ins.size()-ins_high),
                               boost::bind(&session::received_some,
                                           this, _1, _2));
        read_active = true;
}

Both helpers prune the buffers when necessary. (I should really remove those magic 4096s and make them a symbolic constant.)

The data members read_active and write_active ensure that at most one asynchronous read and at most one asynchronous write is pending at any given time. My first version did not have this safeguard (instead trying to rely on the ASIO stream reset method to cancel any outstanding asynchronous transput at need), and the code sent some TLS records twice – which is not good: sending the ServerHello twice is guaranteed to confuse the client.

Once ASIO completes an asynchronous transput request, it calls the corresponding handler:

void session::received_some(boost::system::error_code ec, size_t n)
{
        read_active = false;
        if (ec) { pending_error = ec; return; }
        ins_high += n;
        post_pending_actions();
}
void session::sent_some(boost::system::error_code ec, size_t n)
{
        write_active = false;
        if (ec) { pending_error = ec; return; }
        outs_low += n;
        post_pending_actions();
}

Their job is to update the bookkeeping and to trigger the resumption of suspended gnutls operations (which is done by post_pending_actions).

Now we have all the main pieces of the puzzle. The remaining pieces are obvious but rather messy, and I’d rather not repeat them here (not even in a cleaned-up form). But their essential idea goes as follows:

When called by the application code or when resumed by post_pending_actions, an asynchronous wrapper of a gnutls operation first examines the session state for a saved error code. If one is found, it is propagated to the application using the usual ASIO techniques, and the operation is cancelled. Otherwise, the wrapper calls the actual gnutls operation. When it returns, the wrapper examines the return value. If successful completion is indicated, the handler given by the application is posted in the ASIO io_service for later execution. If GNUTLS_E_AGAIN is indicated, post_read and post_write are called to schedule actual network transput, and the wrapper is suspended (by pushing it into a queue of pending actions). If any other kind of failure is indicated, it is propagated to the application using the usual ASIO techniques.

The post_pending_actions merely empties the queue of pending actions and schedules the actions that it found in the queue for resumption.

The code snippets above are not my actual working code. I have mainly removed from them some irrelevant details (mostly certain template parameters, debug logging and mutex handling). I don’t expect the snippets to compile. I expect I will be able to post my actual git repository to the web in a couple of days.

Please note that my (actual) code has received only rudimentary testing. I believe it is correct, but I won’t be surprised to find it contains bugs in the edge cases. I hope this is, still, of some use to somebody :)

The NaNoWriMo site TOS is onerous

From the NaNoWriMo TOS:

The Office of Letters and Light reserves the right to change the terms and conditions hereof without prior notice at any time and any such change shall be effective immediately upon posting the changed Agreement on NaNoWriMo.org. The effective date indicated at the top of this page will be updated to indicate that changes have been made. You should, as a condition of continuing to access NaNoWriMo.org and use its features, periodically review the terms of this Agreement and such continued access and/or use shall be deemed to be conclusive evidence of your agreement to be bound by any such changed Agreement.

No, I will not write the Office of Letter and Light a blank check.

Analogiset antenniradiokanavat Jyväskylässä

Kun en löytänyt nopeasti netistä tällaista listaa, kasasin sen itse (päälähteinä liikenne- ja viestintäministeriön radiotoiminnan toimilupaluettelo ja kanavien nettisivut).

[Lisäys 22.8.: Tämä lista ei ole hyödyllinen jos radiosi on kaapeliverkossa kiinni. Kaapelitaajuudet löytyvät Elisalta.]

87,6 MHz
YLE Radio Peili (Yleisradio)
89,9 MHz
YLE Radio 1 (Yleisradio)
91,3 MHz
Järviradio (Järviradio Oy)
92,5 MHz
YLEX (Yleisradio)
94,1 MHz
Radio Dei (Kristillinen Media Oy)
95,1 MHz
SuomiPOP (Metroradio Finland Oy)
96,2 MHz
Classic Radio (Metroradio Finland Oy)
97,3 MHz
Energy (NRJ Finland Oy)
97,7 MHz
Radio Rock (Swelcom Oy)
99,3 MHz
Radio Keski-Suomi (Yleisradio)
101,0 MHz
Radio Aalto (Swelcom Oy)
101,6 MHz
The Voice (Pro Radio Oy)
102,5 MHz
Radio Jyväskylä (Pro Radio Oy)
103,5 MHz
Radio Vega (Yleisradio)
104,5 MHz
GrooveFM (Metroradio Finland Oy)
105,8 MHz
Radio Nova (Oy Suomen Uutisradio Ab)
107,1 MHz
Iskelmä (Pro Radio Oy)

Making it official

I just wrote this in my user page in the English wikipedia:

Antti-Juhani Kaijanaho is no longer actively working on Wikipedia. While I approve, in principle, all the new and stricted editor policies, it is emotionally very demanding to be challenged under new policies for edits that were made in good faith under older policies. There just aren’t enough positive aspects to doing Wikipedia to offset that very big negative aspect.

Some of the challenges toward my edits have happened in the English Wikipedia, and some in the Finnish Wikipedia. I have felt like this for about a year now – it’s probably time to make it official.

I may make minor edits, or write in talk pages occasionally, but any substantial edits I won’t bother with any more.