Linking and Application Structure
=================================
:Date: Autumn 2014
:Author: Ichthyostega
:toc:
:toclevels: 3

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

Arrangement of code
-------------------
Since the term ``code'' may denote several different kinds of 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 out 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, this kind of observation 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:[to put
it 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 all the entropy
produced by doing the same things several times 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.

Imports and import order
^^^^^^^^^^^^^^^^^^^^^^^^
When we refer to other definitions by importing headers, these imports should be
spelled out precisely to the point. Every relevant facility used in a piece of code
must be reflected by the corresponding `#import` statement, yet there should not be any
spurious imports. Ideally, just by reading the prologue of a source file, the reader should
gain a clear understanding about the dependencies of this code. The standards are somewhat
different for header files, since every user of this header gets these imports too. Each
import incurs cost for the user -- so the _header_ should mention only those imports

- which are really necessary to spell out our definition
- which are likely to be useful for the _typical standard use_ of our definition

Imports are to be listed in a strict order: *always start with our own references*,
preferably starting with the facility most ``on topic''. Besides, for rather fundamental
library headers, it is a good idea to start with a very fundamental header, like e.g. 'lib/error.hpp'.
Of course, these widely used fundamental headers need to be carefully crafted, since the leverage
of any other include pulled in through these headers is high.

Any imports regarding *external or system libraries are given in a second block*, after our
own headers. This discipline opens the possibility for our own headers to configure or modify
some system facilities, in case the need arises. It is desirable for headers to be written
in a way independent of the include order. But in some, rare cases we need to rely on a
specific order of include. In such cases, it is a good idea to encode this specific order
right into some very fundamental header, so it gets fixed and settled early in the include
processing chain. Our 'gui/gtk-base.hpp', as used by 'gui/gtk-lumiera.hpp' is a good example.

Forward declarations
^^^^^^^^^^^^^^^^^^^^
We need the full definition of an entity whenever we need to know its precise memory layout,
be it to allocate space, to pass an argument by-value, or to point into some filed within
a struct, array or object. The full definition may be preceded by an arbitrary number of
redundant, equivalent declarations. We _do not actually need_ a full definition for any
use _not dealing with the space or memory layout_ of an entity. Especially, handling
some element by pointer or reference, or spelling out a function signature to take
this entity other than by-value, does _not require a full definition_.

Exploiting this fact allows us largely to reduce the load of dependencies, especially
when it comes to ``subsystem'' or ``package'' headers, which define the access
point to some central facility. Such headers should start with a list of the relevant
core entities of this subsystem, but only in the form of ``lightweight'' forward declarations.
Because, anyone actually to use _one of these_ participants, is bound to include the specific
header of this element anyway; all other users may safely skip the efforts and transitive
dependencies necessary to spell out the full definition of stuff not actually used and needed.

In a similar vein, a façade interface does not actually need to pull in definitions for all
the entities it is able to orchestrate. In most cases, it is sufficient to supply suitable
and compatible `typedef`s in the public part of the interface, just to the point that we're
able to spell out the bare API function signatures without compilation error.

Placement of constructors
^^^^^^^^^^^^^^^^^^^^^^^^^
At the point, where a ctor is actually invoked, we require the full definition of the element
about to be created. Consequently, at the place, where the ctor itself is _defined_ (not just
declared), the full definition of _all the members_ of a class plus the full definition of
all base classes is required. The impact of moving this point down into a single implementation
translation unit can be huge, compared to incurring the same cost in each and every other
translation unit just _using_ an entity.

Yet there is a flip side of the coin: Whenever the compiler sees the full definition of an
entity, it is able to inline operations. And the C\++ compiler uses elaborate metrics
to judge the feasibility of inlining. Especially when almost all ctor implementations are
trivial (which is the case when writing good C++ style code), the runtime impact can be
huge, basically boiling down a whole pile of calls and recursive invocations into precisely
zero assembler code to be generated. This way, abstraction barriers can evaporate
to nothingness. So we're really dealing with a run time vs. development time
and code size tradeoff here.

On a related note: care has to be taken whenever a templated class defines virtual methods.
Each instantiation of the template will cause the compiler to emit a function which generates
the VTable, together with code for each of the virtual functions. This effect is known as
``template code bloat''.

The PImpl pattern
^^^^^^^^^^^^^^^^^
It is is the very nature of a good design pattern, the reason why it is remembered and applied
over and over again: to allow otherwise destructive forces to move past each other in a
seemingly ``friction-less'' way. In our case, there is a design pattern known to resolve
the high tension and potential conflict inherent to the situations and issues described above.
And, in addition, it circumvents the lack of a real interface definition construct in C++ elegantly:

Whenever a facility has to offer an outward façade for the client, while at the same time engaging
into heavy weight implementation activities, then you may split this entity into an interface shell
and a private implementation delegate.footnote:[the common name for this pattern, »PImpl« means
``point-to-implementation''] The interface part is defined in the header, fully eligible
for inlining. It might even be generic -- templated to adapt to a wide array of parameter types.
The implementation of the API functions is also given inline, and just performs the necessary
administrative steps to accept the given parameters, before passing on the calls to the
private implementation delegate. This implementation object is managed by (smart) pointer,
so all of the dependencies and complexities of the implementation is moved into a single
dedicated translation unit, which may even be reshaped and reworked without the need to
recompile the usage site.

Wrappers and opaque holders
^^^^^^^^^^^^^^^^^^^^^^^^^^^
These constructs serve a similar purpose: To segregate concerns, together with the related
dependencies and overhead. They, too, represent some trade-off: a typically very intricate
library construct is traded for a lean and flexible construction at usage site.

A wrapper (smart-pointer or smart handle), based on the ability of C++ to invoke ctors and
dtors of stack-allocated values and object members automatically, can be used so push some
cross-cutting concern into a separate code location, together with all the accompanying
management facilities and dependencies, so the actual ``business code'' remains untainted.

In a related, but somewhat different style, an opaque holder allows to ``piggyback'' a value
without revealing the actual implementation type. When hooked this way behind a strategy interface,
extended compounds of implementation facilities can be secluded into a dedicated facility, without
incurring dependency overhead or tight coupling or even in-depth knowledge onto the client, yet
typesafe and with automatic tracking for clean-up and failure management.


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, need to be. 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] This way, the client 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.


Locating dependencies at start-up
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We hope for Lumiera to be not only installed on desktop systems, but also used in a studio
or production context, where you'll use a given system just for the duration of one project.
This might even be specific hardware booted with a Live-System, or it might be a ``headless''
render-farm node. For this reason, we impose the explicit requirement that Lumiera must be
fully *usable without installation*. Unzip the application into some folder, launch,
start working. There might be some problems with required media handling libraries,
but the basic idea is to use a self-contained bundle or sub-tree, and the application
needs to locate all the further required resources actively on start-up.

On the other hand, we want Lumiera to be a good citizen, packaged in the usual way,
compliant to the Unix **F**ile**S**ystem **H**ierarchy standard. It turns out these two
rather conflicting goals can be reconciled by leveraging some of the advanced features
of the GNU dynamic linker: The application will figure out the whereabouts relatively,
starting from the location of the executable, and with the help of some search paths
and symlinks, the same mechanism can be made to work in the usual FSH compliant
installation into `/usr/lib/lumiera` and `/usr/share/lumiera`

This way, we end up with a rather elaborate start-up sequence, where the application
works out it's own installation location and establishes all the further resources
actively step by step

. the first challenge is posed by the parts of the application built as dynamic libraries;
  effectively most of the application code resides in some shared modules. Since we
  most definitively want one global link step in the build process, where unresolved
  symbols will be spotted, and we do want a coherent application core, so we use
  dynamic linking right at start-up and thus need a way to make the linker locate
  our further modules and components relative to the executable. Fortunately, the
  GNU linker supports some extended attributes in the `.dynamic` section of ELF
  executables, known as the ``new style d-tags'' -- see the extensive description
  in the next section below

  * any executable may define an extended search path through the `RUNPATH` tag,
    which is searched to locate dynamic libraries and modules before looking
    into the standard directories
    footnote:[the linker also supports the old-style `RPATH` tag, which is deprecated.
    In the age of multi-arch installation and virtual machines, the practice of baking
    in the installation location of each library into the `RPATH` is no longer adequate.
    For the time being, our build system sets both `RPATH` and the new-style, more
    secure `RUNPATH` to the same value relative to the executable. We will cease
    using `RPATH` at some point in the future]
  * and this search patch may contain _relative_ entries, using the special
    magic token `$ORIGIN` to point at the directory holding the executable
    +
    By convention, the Lumiera buildsystem bakes in the search location `$ORIGIN/modules`
    -- so this subdirectory below the location of the executable is where all the dynamic
    modules of the application will be placed by default

. after the core application has been loaded and all direct dependencies are resolved,
  but still before entering `main()`, the class `lumiera::AppState` will be initialised,
  which in turn holds a member of type `lumiera::BasicSetup`. The latter will figure out
  the location of the executable footnote:[this is a Linux-only trick, using `/proc/self/exe`]
  and require a 'setup.ini' file in the same directory. This setup file is mandatory.

. from there, the _search paths_ are retrieved to locate the extended resources of the
  application. All these search paths are a colon separated list; the entries may
  optionally also use the token `$ORIGIN` to refer to the location of the main executable.
  The default version of 'setup.ini' is outfitted with search paths to cover both the
  situation of a self-contained bundle, but also the situation of a FSH compliant
  installation.

  Lumiera.modulepath:: this is where the plugin loader looks for additional extensions and
    plug-ins, most notably the *GUI plugin*

  Lumiera.gui:: defines the name of this GUI plugin, which is loaded and activated from
    `main()` -- unless Lumiera starts in ``headless'' mode

  Lumiera.configpath:: all the extended application configuration will be picked up from
    these directories (_not yet implemented as of 2015_)

  Gui.iconpath:: root of the folder structure used to load icons and similar graphical
    elements for the GTK-UI. Below, several subdirectories for various icon sizes are
    recognised. Actually, most of our icons are defined as SVG and rendered using
    libCairo during the build process.

  Gui.resourcepath:: the place where the GTK-UI looks for further resources, most notably...

  Gui.stylesheet:: the name of the CSS-stylesheet for GTK-3, which defines the
    application specific look, link:{ldoc}/technical/gui/guiTheme.html[skinning and theme].

While the first two steps, the relative locations `$ORIGIN/modules` and `$ORIGIN/setup.ini`
are hard-wired, the further resolution steps rely on the contents of 'setup.ini' and are
open for adjustments and reconfiguration, both for the packager or the advanced user.
Any failure or mismatch during this start-up sequence will be considered fatal and abort
the application execution.

About RPATH, RUNPATH
^^^^^^^^^^^^^^^^^^^^
Management of library dependencies can be a tricky subject, both obscure and contentious.
There is a natural tension between the application developers (»upstream«), the packagers
and distributors. Developers have a natural tendency to cut short on ``secondary'' concerns
in order to keep matters simple. As a developer, you always try to stretch to the limit,
and this very tendency _limits your ability_ to care for intricate issues faced by just a
few users, or to care for compatibility or even for extended tutorial documentation. So,
as an upstream developer, if you know that the stuff you've built works just fine with
a specific library version -- be it bleeding edge or be it outdated and obsoleted since
years -- the most pragmatic and adequate answer is to demand from your users just to
``come along with you'' -- frankly, the active developer has zero inclination to look
back to past issues already overcome in the course of development, nor has he much interest
to engage in other fields of ongoing debate not conductive to his own concerns right now.
So the best solution for the developer would be just to wrap-up his own system and ship
it as a huge bundle to the users. Every other solution seems inferior, just adding weight,
efforts and pain without tangible benefit.

Obviously, a distributor can not agree to that stance. To create a coherent and reliable whole
out of the thousand individual variations the upstream developers breed, is in itself a
herculean task and would simply be impossible without forcing some degree of standardisation
onto the developers. In fact, the distributor's task becomes feasible _only_ by offloading some
efforts for compatibility in small portions onto the shoulders of the individual upstream
projects. A preference for using a common set of shared libraries is even built into
the toolchain, and there is a distinct tendency to discourage and even hide away the
mechanisms otherwise available to deal with a self contained bundle of libraries.

Speaking in terms of history, explicit mechanisms to support packaging and distribution management
are a rather new phenomenon and tend to conflict with the _glorious »Unix Way«_ of doing things,
they are cross-cutting, global and invasive. On Unix systems, traditionally there used to be two
_overriding mechanisms_ for library resolution, one for the user, one for the developer:

- the `LD_LIBRARY_PATH` environment variable allows the _user_ on invocation to manipulate
  the search location for loading dynamic libraries; it takes precedence over all default
  configured system wide installation locations and over the contents of the linker cache.
  In the past, astute users frequently configured an elaborate `LD_LIBRARY_PATH` into their
  login profile and managed an extended set of private installation locations with custom
  compiled libraries. Obviously this caused an avalanche of problems at the point where
  significant functionality of a system started to be just ``provided'' and no single
  person was able to manage all of the everyday functionality alone on their own.
  Basically, the advent of graphical desktop systems marked this breaking point.

- the `RPATH` tag, which is the other override mechanism available for _the builder of software_,
  was made to defeat and overrule the effect of `LD_LIBRARY_PATH`. Search locations baked
  in as `DT_RPATH` take absolute precedence and can not be altered with any other means besides
  recompiling the executable (or at least rewriting the `.dynamic` section in the ELF binary).
  Over time, it became »best practice« to bake in the installation location into each and
  every binary, which kind-of helped the upstream developers to re-gain control over the
  libraries actually being used to execute their code. But unfortunately, the distributors
  were left with zero options to manage large-scale library dependency transitions, beyond
  patching the build system of each and every package.

Based on this situation, the _new-style d-tags_ were designed to implement a different
precedence hierarchy. Whenever the new d-tags are enabled,footnote:[the `--enable-new-dtags`
linker flag is default in many current distributions, and especially with the »gold« linker.]
the presence of a `DT_RUNPATH` tag in the `.dynamic` section of an ELF binary completely disables
the effect of any `DT_RPATH`. Moreover, the `LD_LIBRARY_PATH` is automatically disabled, whenever
a binary is installed as _set-user-ID_ or _set-group-ID_ -- which closes a blatant security loophole.

The use of both `RPATH` and `LD_LIBRARY_PATH` is strongly discouraged. Debian policy for example
does not explicitly rule out the use of `RPATH` / `RUNPATH`, but it is considered bad practice
footnote:[a good summary of the situation can be found on
https://wiki.debian.org/RpathIssue[this Debian page] ] -- with the exception of using a _relative_
`RUNPATH` with `$ORIGIN` to add non-standard library search locations to libraries that are only
intended for usage by the given executable or other libraries within the same source package.

In the new system, the precedence order is as follows footnote:[see
http://linux.die.net/man/8/ld.so[ld.so manpage on die.net] or
http://manpages.ubuntu.com/manpages/lucid/man8/ld.so.8.html[ld.so manpage ubuntu.com (more recent)] ]

. `LD_LIBRARY_PATH` entries, unless the executable is `setuid`/`setgid`
. `DT_RUNPATH` from the `.dynamic` section of that ELF binary, library or executable
  _causing_ the actual library lookup.
. '/etc/ld.so.cache' entries, unless the `-z nodeflib` linker flaw was given at link time
. '/lib', '/usr/lib' (and the platform-decorated variants) unless `-z nodeflib` was given at link time
. otherwise linking fails (``not found'').

All the ``search paths'' mentioned here are colon separated lists of directories.

NOTE: The new-style `DT_RUNPATH` is not extended recursively when resolving transitive dependencies,
      as was the case with `RPATH`. That is, if we `dlopen()` 'libA.so', which in turn marks 'libB.so'
      as `DT_NEEDED`, which in turn requires a 'libC.so' -- then the resolution for 'libC.so' will
      visit _only_ the `RUNPATH` given in 'libB.so', but _not_ the `RUNPATH` given in 'libA.so'
      (to the contrary, the old `RPATH` used to visit all these locations). +
      This behaviour was chosen deliberately, in compliance with the ELF spec, as can be seen in this
      link:https://sourceware.org/bugzilla/show_bug.cgi?id=13945[glibc bug #13945] and the
      developer comment by
      link:https://sourceware.org/ml/libc-hacker/2002-11/msg00011.html[Roland McGrath from 2002]
      mentioned therein.


the $ORIGIN token
^^^^^^^^^^^^^^^^^
To support flexible `RUNPATH` (and `RPATH`) settings, the GNU `ld.so` (also the SUN and Irix linkers)
allow the usage of some ``magic'' tokens in the `.dynamic` section of ELF binaries (both libraries
and executables):

$ORIGIN:: the directory containing the executable or library _actually triggering_
          the current (innermost) resolution step. Not to be confused with the entity
          causing the whole linking procedure (an executable to be executed or a `dlopen()` call)
$PLATFORM:: expands to the architecture/platform tag as provided by the OS kernel
$LIB:: the system libraries directory, which is /lib for the native architecture
       on FHS compliant GNU/Linux systems.

Since the new-style `RUNPATH` is not extended for resolving transitive dependencies, each library,
when relying on relative locations, must provide _its own_ `RUNPATH` to point at the direct
dependencies at a relative location. This solution is a bit more tricky to set up, but in fact
more logical and scalable on the long run. Incidentally, it can be quite a challenge to escape
the `$ORIGIN` properly from any shell script or even build system -- it is crucial that the
linker itself, not the compiler driver, actually gets to ``see'' the dollar sign, plain,
without spurious escapes.


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 frameworks 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
^^^^^^^^^^^^^^^^^^^^^^^^^
The linker _actually needs to see the dependency._ Indirect, conceptual dependencies, where the client
takes initiative and enrols itself actively with the server, will slip through unnoticed. Under some
additional conditions, especially with self-configuring systems, this omission might even cause a whole
dependency and subsystem to be disabled.

As a practical example, our C\++ unit-tests are organised into test-suites, where the individual test uses
a registration mechanism, providing the name of the suite and category tags. This way, the test runner
may be started to execute just some category of tests. Now, switching tests to dynamic linking causes
an insidious side effect: The registration mechanism uses static initialisation -- which is the commonly
used mechanism for this kind of tasks in C++. Place a static variable into an anonymous namespace -- it
will be initialised by the runtime system _on start-up_, causing any constructor code to run right when
it is needed to enrol itself with the global (test) service. Unfortunately, static initialisation of
shared objects is performed at load time of the library -- which never happens unless the linker has
figured out the dependency and added the library to the required set. But the linker won't be able
to see this dependency when building the test-runner, since the client, the individual test-case in
the shared library is the one to call into the test-runner, not the other way round. The registration
function resides in a common support library, picked as dependency both by the test-runner and the
individual test case, but this won't help us either. So, in this case (and similar cases), we need
either to fabricate a ``dummy'' call into the library holding the clients (tests), or we need to
link the test-runner with `--no-as-needed` -- which is the preferred solution, and in fact is
what we do.footnote:[see 'tests/SConscript']

Relative dependency location
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Locating binary dependencies relative to the executable (as described above) gets 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, our build defines the following `RPATH` and `RUNPATH` specs:

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

