LUMIERA.clone/doc/technical/code/linkingStructure.txt

Linking and Application Structure
=================================
:Date: Autumn 2014
:Author: Ichthyostega
:toc:

This page focusses on some quite intricate aspects of the code structure,
the build system organisation and the interplay of application parts on
a rather technical level.

Arrangement of code
-------------------
Since ``code'' may denote several different entities, the place ``where''
some piece of code is located differs according to the context in question.

Visibility vs Timing: the translation unit
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To start with, when it comes to building code in C/C++, the fundamental entity
is _a single translation unit_. Assembler code is emitted while the compiler
progresses through a translation unit. Each translation unit is self contained
and represents a path of definition and understanding. Each translation unit
starts anew at a state of complete ignorance, at the end leading to a fully
specified, coherent operational structure.

Within this _definition of a coded structure_, there is an inherent tension
between the _absoluteness_ of a definition (a definition in mathematical sense
can not be changed, once given) and the _order of spelling out_ this definition.
When described in such an abstract way, these observations might be deemed self evident
and trivial, but let's just consider the following complications in practice...

- Headers are included into multiple translation units. Which means, they appear
  in several disjoint contexts, and must be written in a way independent of the
  specific context.
- Macros, from the point of their definition onwards, change the way the compiler
  ``sees'' the actual code.
- Namespaces are ``open'' -- meaning they can be re-opened several times and
  populated with further definitions. The actual contents of any given namespace
  will be slightly different in each and every translation unit.
- a Template is not in itself code, but a constructor function for actual code.
  It needs to be instantiated with concrete type arguments to produce code.
  And when this happens, the template instantiation picks up definitions
  _as visible at that specific point_ in the path through the translation unit.
  A template instantiation might even pick up specific definitions depending
  on the actual parameters, and the current content of the namespace these
  parameter types are defined in. So it very much matters at which point a
  specific template instantiation is first mentioned.
- it is possible to generate globally visible (or statically visible) code
  from a template instantiation. This may even happen several times when
  compiling multiple translation units; the final linking stage will
  silently remove such duplicate instantiations stemming from templates --
  and this resolution step just assumes that these duplicate code entities
  are actually equivalent. Mind me, this is an assumption and can not be
  easily verified by the compiler. With a bit of criminal energy (think
  namespaces), it is very much possible to produce several instantiations
  of, say, a static initialiser within a template class, which are in
  fact different operations. Such a setup puts us at the mercy of the
  random way in which the linker sees these instances.

Now the quest is to make _good use_ of these various ways of defining things.
We want to write code which clearly conveys its meaning, without boring the
reader with tedious details not necessary to understand the main point in
question. And at the same time we want to write code which is easy to
understand, easy to write and can be altered, extended and maintained.
footnote:[Put blatantly, a ``simple clean language'' without any means of expression
would not be of much help. All the complexities of reality would creep into the usage
of our ``ideal'' language, and, even worse, be mixed up there with the entropy of
doing the same things several times in a different way.]

Since it is really hard to reconcile all these conflicting goals, we are bound
to rely on *patterns of construction*, which are known to work out well in
this regard.

[yellow-background]#to be written#
Import order, forward decls, placement of ctors, wrappers, PImpl


Code size and Code Bloat
~~~~~~~~~~~~~~~~~~~~~~~~
Each piece of code incurs cost of various kinds

- it needs to be understood by the reader. Otherwise it will die
  sooner or later and from then on haunt the code base as a zombie.
- writing code produces bugs and defects at a largely constant rate.
  The best code, the perfect code is code you _do not write_.
- actual implementation produces machine code, which occupies
  space, needs to be loaded into memory (think caching) and performed.
- to the contrary, mere definitions are for free, _but_ --
- even definitions consume compiler time (read: development cycle turnaround)
- and since we're developing with debug builds, each and every definition
  produces debug information in each and every translation unit referring it.

Thus, for every piece of code we must ask ourselves how much _visible_ this
code is. And we must consider the dependencies the code incurs. It pays off to
turn something into a detail and ``push it into the backyard''. This explains
why we're using the frontend - backend split so frequently.


Source and binary dependencies
------------------------------
To _use_ stuff while writing code, a definition or at least a declaration needs to
be brought into scope. This is fine as long as definitions are rather cheap,
omitting and hiding the details of implementation. The user does not need to understand
these details, and the compiler does not need to parse them.

The situation is somewhat different when it comes to _binary dependencies_ though.
At execution time, all we get is pieces of data, and functions able to process specific
data. Thus, whenever some piece of data is to be used, the corresponding functions need
to be loaded and made available. Most of the time we're linking dynamically,
and thus the above means that a _dynamic library_ providing those functions needs to be loaded.
This other dynamic library becomes a dependency of our executable or library; it is recorded
in the 'dynamic' section of the headers of our ELF binary (executable or library). Such a
'needed' dependency is recorded there in the form of a ``SONAME'': this is an unique, symbolic
ID denoting the library we're depending on. At runtime, it is the responsibility of the system's
dynamic linker to translate these SONAMEs into actual libraries installed somewhere on the system,
to load those libraries and to map the respective memory pages into our current process' address
space, and finally to _relocate_ the references in our assembly code to point properly to the
functions of this library we're depending on.

Application Layer structure and dependency structure
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Lumiera application uses a layered architecture, where upper layers may depend on the services
of lower layers, but not vice versa. The top layer, the GUI is defined to be _strictly optional_.
As long as all we had to deal with was code in upper layers using and invoking services in lower
layers, there would not be much to worry. Yet to produce any tangible value, software has to
collaborate on shared data. So the naive ``natural'' form of architecture would be to build
everything around shared knowledge about the layout of this data. Unfortunately such an approach
endangers the most central property of software, namely to be ``soft'', to adapt to change.
Inevitably, data centric architectures either grow into a rigid immobile structure,
or they breed an intangible insider culture with esoteric knowledge and obscure conventions
and incantations. The only known solution to this problem (incidentally a solution known
since millennia), is to rely on subsidiarity. ``Tell, don't ask''

This gets us into a tricky situation regarding binary dependencies. Subsidiarity leads to an
interaction pattern based on handshakes and exchanges, which leads to mutual dependency. One
side places a contract for offering some service, the other side reshapes its internal entities
to comply to that contract superficially. Dealing with the entities involved in such a handshake
effectively involves the internal functions of both sides. Which is in contradiction to a
``clean'' layer hierarchy.

For a more tangible example, lets assume our backend has to do some work on behalf of the GUI;
so the backend offers a contract to outline the properties of stuff it can work on. In compliance
with this contract, the GUI hands over some data entities to the backend to work on -- but by their
very nature, these data entities are and always remain GUI entities. When the backend invokes compliant
operations on these entities, it effectively invokes functionality implemented in the GUI. Which
makes the backend _binary dependent on the GUI_.

While this problem can not be resolved in principle, there are ways to work around it, to the degree
necessary to get hierarchically ordered binary dependencies -- which is what we need to make a lower
layer operative, standalone, without the upper layer(s). The key is to introduce an _abstraction_,
and then to _segregate_ along the realm of this abstraction, which needs to be chosen large enough
in scope to cast the service and its contract entirely in terms of this abstraction, but at the same
time it needs to be kept tight enough to prevent details of the client to leak into the abstraction.
When this is achieved (which is the hard part), then any operations dealing with the abstraction solely
can be migrated into the entity offering the service, while the client hides the extended knowledge about
the nature of the manipulated data behind a builder function footnote:[frequently this leads to the
``type erasure'' pattern, where specific knowledge about the nature of the fabricated entities -- thus
a specific type -- is relinquished and dropped once fabrication is complete], but retains ownership
on these entities, passing just a reference to the service implementation. This move ties the binary
dependency on the client implementation to this factory function -- as long as _this factory_ remains
within the client, the decoupling works and eliminates binary cross dependencies.

This solution pattern can be found at various places within the code base; in support we link with
strict dependency checking (Link flag `--no-undefined`), so every violation of the predefined
hierarchical dependency order of our shared modules is spotted immediately during build.


Finding dependencies at start-up
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[yellow-background]#to be written#


Transitive binary dependencies
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Binary dependencies can be recursive:
When our code depends on some library, this library might in turn depend on other libraries.
At runtime, the dynamic linker/loader will detect all these transitive dependencies and try to load
all the required shared libraries; thus our binary is unable to start, unless all these dependencies
are already present on the target system. It is the job of the packager to declare all necessary dependencies
in the software package definition, so users can install them through the package manager of the distribution.

There is a natural tendency to define those installation requirements too wide. For one, it is better
to be on the safe side, otherwise users won't be able to run the executable at all. And on top of that,
there is the general tendency towards frameworks, toolkit sets and library collections -- basically
a setup which is known to work under a wide range of conditions. Using any of these typically means
to add a _standard set of dependencies_, which is often way more than actually required to load and
execute our code. One way to fight this kind of ``distribution dependency bloat'' is to link `--as-needed`.
In this mode, the linker silently drops any binary dependency not necessary for _this concrete piece
of code_ to work. This is just awesome, and indeed we set this toggle by default in our build process.
But there are some issues to be aware of.

Static registration magic
^^^^^^^^^^^^^^^^^^^^^^^^^
[yellow-background]#to be written#

Relative dependency location
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Locating binary dependencies relative to the executable (as described above) is complicated when several
of _our own dynamically linked modules_ depend on each other transitively. For example, a plug-in might
depend on `liblumierabackend.so`, which in turn depends on `liblumierasupport.so`. Now, when we link
`--as-needed`, the linker will add the direct dependency, but omit the transitive dependency on the
support library. Which means, at runtime, that we'd need to find the support library _when we are
about to load the backend library_. With the typical, external libraries already installed to the
system this works, since the linker has built-in ``magic'' knowledge about the standard installation
locations of system libraries. Not so for our own loadable components -- recall, the idea was to provide
a self-contained directory tree, which can be relocated in the file system as appropriate, without the
need to ``install'' the package officially. The GNU dynamic linker can handle this requirement, though,
if we supply an additional, relative search information _with the library pulling in the transitive
dependency_. In our example, `liblumierabackend.so` needs an additional search path to locate
`liblumierasupport.so` _relative_ to the backend lib (and not relative to the executable). For this
reason, our build system by default supplies such a search hint with every Lumiera lib or dynamic
module -- assuming that our own shared libraries are installed into a subdirectory `modules` below
the location of the executable; other dynamic modules (plug-ins) may be placed in sibling directories.
So, to summarise, the build defines the following `RPATH` and `RUNPATH` specs:

for executables::      `$ORIGIN/modules`
for libs and modules:: `$ORIGIN/../modules`