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\startcomponent evenmore-libraries

\environment evenmore-style

\startchapter[title={Libraries}]

\startsection[title={Introduction}]

The \LUAMETATEX\ binary comes with a couple of libraries built in. These normally
provide enough functionality to get a \TEX\ job done. But take the case where
need to manipulate (or convert) an image before we can include it? It would be
nice if \CONTEXT\ does that for you so having some features in the binary that
handle it make sense. However, given that such a conversion only happens once it
makes more sense to just call an external program and let that deal with it. It
is for that reason that the \CONTEXT\ code base has hardly any library related
code: most of what one wants to do can be done by calling a program. Some callers
are built in, others can be dealt with using the Adityas filter module. The most
significant runtime exception is probably accessing \SQL\ databases where it
might be more efficient to use a library call instead of calling a client. And
even then the main reason for that interface being present is the simple fact
that I (ab)use the engine to serve requests that need some kind of database
access. Another example of where we need some external program is in generating
barcodes. Here one can argue that it does make sense to do that runtime, for
instance because they change or because one doesn't like to have dozens of cached
barcode images on disk.

In this chapter I will explain how we deal with libraries in \LUAMETATEX. Because
libraries create a dependency an approach is chosen that tries to avoid bloating
the source tree with additional header and source files. This is made easy by the
fact that we don't need full blown interfaces to libraries where all methods are
exposed. We know what we need and most of these tasks somehow relate to
typesetting which is a limited application with known demands in terms of input,
output and performance. We don't need to serve every possible scenario.

\stopsection

\startsection[title={Using \LUA\ libraries}]

One approach is to use a \LUA\ library that sits between the embedded \LUA\ instance
and the external library. Say that one does this:

\starttyping
local mylib = require("mylib")
\stoptyping

This can locate and load the file \type {mylib.lua} which implements a bunch of
(\LUA) functions. But, it can also load a library, for instance \type
{mylib.dll}, a binary that provides functions that themselves can call external
ones. Often such a library is also responsible for some resource management which
is then done via userdata objects. Such a connector library on the one hand
refers to \LUA\ library methods (like \type {const char * str = lua_tostring (L,
1);} for fetching a \LUA\ string variable from the argument list) and on the
other hand to those in the external library (like passing that string \type {str}
to a function and passing the result back to \LUA\ with \type {lua_pushstring (L,
result);}). If we would follow that approach in \LUAMETATEX\ it means that in
addition to the main binary (on \MSWINDOWS\ that is \type {luametatex.exe}) there
is also an extra intermediate binary (on \MSWINDOWS\ that is \type {mylib.dll})
plus the external library (on \MSWINDOWS\ that could be \type {foolib.dll}) which
itself can depend on other libraries.

In this approach we need to compile the extra intermediate libraries alongside
the main \LUAMETATEX\ binary. Quite likely we then need access to the header
files of the external libraries too. We might even decide to put the dependencies
in our source tree. But, this is not what we like to do: it adds extra work, we
need to keep an eye on updates and operating specific patches, we complicate the
compilation, etc. This all contradicts the fact that we want \LUAMETATEX\ to be
simple. There is no need to complicate the setup just because a very few users
want to use some library. Add to this the fact that quite likely we need to
provide a version of \LUAMETATEX\ that exposes its \LUA\ related symbols which
makes for a larger binary. So, this approach is not really an option because
at the same time we like to keep the binary (and memory footprint) as small
as possible (think of running in a container or on a low energy device).

\stopsection

\startsection[title=A variant]

There are a few issues when you use \LUA\ libraries from elsewhere. First of all,
you need to get hold of one that matches the version of \LUA\ that you use. There
are not that many and some only can be set up as part of a larger framework.
Also, you can find plenty of modules that seem not to be maintained (or maybe
they are just very stable and I'm wrong here). Also, not all platforms are
supported equally well. Then there is the question to what extend libraries are
to stay. What is considered to be the standard today might not be tomorrow. Even
in the rather stable \TEX\ ecosystem we see them come and go. These are all
reasons to avoid hard coded dependencies. Ideally we like users to be able to
compile \LUAMETATEX\ in the future without too must hassle.

A couple of years after we started the \LUATEX\ project, a solution for using
libraries was implemented, called \SWIGLIB, because it uses the swig
infrastructure. It was an attempt to come up with a more or less standard
approach, a rather one|-|to|-|one mapping so that basically any library could be
interfaced. But, probably because no one really needs libraries, it never catched
on. In \MKIV\ we still support loading libraries made that way but in \LMTX\ that
code has been removed.

As a side note: the code that deals with this in \MKIV\ also deals with version
specific loading. When we were playing with for instance \MYSQL\ libs we found
out that it made sense to be able to support different \API s, but in the end,
given the rare usage of libraries, that made no sense either. Therefore in \LMTX\
locating libraries has version support removes and as a consequence is much
simpler (code|-|wise).

\stopsection

\startsection[title=Foreign function interfaces]

Then there is a \FFI\ interface, first introduced in \LUAJITTEX\ as it is part of
\LUAJIT, and later a similar library was built|-|in \LUATEX. But \LUAJIT\ doesn't
conceptually follow \LUA\ upgrades and its future is unsure so in \LUAMETATEX\
there is no \JIT\ variant (the \JIT\ part was never used anyway as it only slowed
down a run; we just used the \FFI\ part plus the fact that the restricted virtual
machine performs better). The \FFI\ library used in \LUATEX\ also comes from
elsewhere and it doesn't seem to be maintained any longer, so that code is to be
kept working in the perspective of \LUATEX. Both technologies hook into the
processor architecture and are somewhat complex so when their maintenance becomes
unsure we have to reconsider using them. Not all hardware platforms are supported
\footnote {As I write this only Intel works while ARM doesn't and only on
\MSWINDOWS, \LINUX\ and \OSX\ I can compile without alignment warnings} and the
functionality can differ in details per platform. To some extend we can keep
using \FFI\ in \LUATEX\ because Luigi takes care of it, but who knows when it
becomes too problematic. Does it make sense to adopt a library that needs tweaks
depending on architectures? For now we're good for \LUATEX, so for a while we're
also okay (in \MKIV).

The nice thing about \FFI\ is that one can define the interface at runtime. Of
course this interface has to fit the current version of the library \API, but
that is doable. It is up to a user of a library to determine where it comes from.
It can be put in the \TEX\ tree but also being taken from wherever the operating
system put it in the path. Of course that can then be a bit of an issue when
there are different versions because programs can ship their own variants, but
when you use a library you probably are aware of that and know what you're doing.
A drawback of \FFI\ is that it opens up the whole machinery pretty low level,
which can be considered a risk. Some can consider that to be a security threat.
It for these reasons that \LUAMETATEX\ doesn't provide the \FFI\ feature; users
who depend on it can of course use \MKIV\ with \LUATEX.

\stopsection

\startsection[title=So how to proceed?]

When a library and its \LUA\ interface are kept external the main binary has to
be compiled in a way that permits loading libraries (read: symbols need to be
known). When we use \FFI\ that is not needed. And when a library is internal we
have the disadvantage that we mentioned at the start of this chapter.

So, how do we combine the advantages of \FFI\ (runtime binding), external
libraries (no need to have all that code in the code base) and internal libraries
(no loading issues)? At some point it stroke me that we actually can do that with
not that much effort. The solution was probably subconsciously implanted by
noticing the fact that the \LUAMETATEX\ machinery uses function pointers in some
places and the fact that when a \LUA\ library is loaded by \LUA\ itself, a
specific initialization function is called to initialize it: by combining these
concepts we can delay the binding till when a library is needed.

In \LUAMETATEX\ we can therefore have some optional libraries that offer a
minimal interface because after all we can do a lot at the \LUA\ end. Optional
libraries register themselves in the global \type {optional} table. We're talking
of a couple of hundred lines of \CCODE\ for a simple binding. The functions in an
optional library table can be used (accessed) without loading the library and
then just do nothing useful. So, before using them you need to load the third
party library but we can safely assume that the \LUA\ wrapper code calls an
initializer when it needs some feature. That initializer, which by the way is
located at the \LUA\ end, loads the external library, and when that is successful
the needed helpers are bound by resolving function pointers. There is no
dependency when nothing is used: the main binary stays lean and mean because the
binding normally only adds a few \KB. Users can compile without dependencies and
when used performance is quite okay (no \FFI\ overhead).

The \LUAMETATEX\ distribution only ships a few such bindings but these can serve
as example. What is shipped has a proper \LUA\ companion file and these are then
the standard one used in the \CONTEXT\ distribution. Think of \MYSQL\ and
\SQLITE\ (for databases), \ZINT\ (for barcodes), simple \CURL\ (for fetching
stuff), \GHOSTSCRIPT\ and \GRAPHICSMAGICK\ (for some conversions) bindings . When
compiled into \LUAMETATEX\ these will add some interfacing code to the main
binary but that gets compensated by the removal of the \FFI\ library. The \LUA\
interfaces provide just enough to get us going. At some point we can consider
providing libraries as optional part of an installation because we can generate
them using the buildbot infrastructure managed by Mojca, but the core
distribution (source code) is kept clean.

\stopsection

\stopchapter

\stopcomponent

