// file      : doc/packaging.cli
// license   : MIT; see accompanying LICENSE file

"\name=build2-packaging-guide"
"\subject=toolchain"
"\title=Packaging Guide"

// NOTES
//
// - Maximum <pre> line is 70 characters.
//

"
\h0#preface|Preface|

This document provides guidelines for converting third-party projects to the
\c{build2} build system and making them available as packages from
\l{https://cppget.org cppget.org}, the \c{build2} community's central package
repository. For additional information, including documentation for individual
\c{build2} toolchain components, man pages, HOWTOs, etc., refer to the project
\l{https://build2.org/doc.xhtml Documentation} page.

\N|This document is a work in progress and is incomplete.|


\h1#dont-do|What Not to Do|


\h#dont-from-scratch|Don't write \c{buildfile}s from scratch, use \c{bdep-new}|

Unless you have good reasons not to, create the initial project layout
automatically using \l{bdep-new(1)}, then tweak it if necessary and fill with
upstream source code.

The main rationale here is that there are many nuances in getting the build
right and auto-generated \c{buildfile}s had years of refinement and
fine-tuning. The familiar structure also makes it easier for others to
understand your build, for example while reviewing your package submission.

The \l{bdep-new(1)} command supports a wide variety of
\l{bdep-new.xhtml#src-layout source layouts}. While it may take a bit of time
to understand the customization points necessary to achieve the desired layout
for your first package, this will pay off in spades when you work on
converting subsequent packages. The recommended sequence of steps is
as follows:

\ol|

\li|Study the upstream source layout. We want to stay as close to upstream as
possible since this has the best chance of producing an issues-free result
(see \l{#dont-change-upstream Don't change upstream source code layout} for
details).|

\li|Craft and execute the \l{bdep-new(1)} command line necessary to achieve
the upstream layout.|

\li|Study the auto-generated \c{buildfile}s for things that don't fit and need
to change. But don't rush to start manually editing the result. First get an
overview of the required changes and then check if it's possible to achieve
these changes automatically using one of \l{bdep-new(1)} sub-options.

For example, if you see that the generated project assumes the wrong C++ file
extensions, these can be changed with \c{--lang|-l} sub-options.|

\li|Once you have squeezed as much as possible out of \l{bdep-new(1)}, it's
time for manual customizations. These would normally include:

\ul|

\li|Replace generated source code with upstream, normally as symlinks from the
    \c{upstream/} \c{git} submodule.|

\li|Tweak source subdirectory \c{buildfile} that builds the main target
    (library, executable).|

\li|Add tests and, if necessary, examples.|

\li|Tweak \c{manifest} (in particular the \c{version}, \c{summary}, and
    \c{license} values).|

\li|Fill in \c{README.md}.|||

|


\h#dont-change-upstream|Don't change upstream source code layout|

It's a good idea to stay as close to the upstream's source code layout as
possible. This has the best chance of giving us a build without any compile
errors since the header inclusion in the project can be sensitive to this
layout. This also makes it easier for upstream to adopt the \c{build2}
build.

Sometimes, however, there are good reasons for deviating from upstream,
especially in cases where upstream is clearly following bad practices, for
example installing generically-named headers without a library prefix. If you
do decide to change the layout, it's usually less disruptive (to the build) to
rearrange things at the outer levels than at the inner. For example, it should
normally be possible to move/rename the top-level \c{tests/} directory or to
place the library source directory into a subdirectory.


\h#dont-header-only|Don't make a library header-only if it can be compiled|

Some libraries offer two alternative modes: header-only and compiled. Unless
there are good reasons not to, a \c{build2} build of such a library should use
the compiled mode.

\N|Some libraries use the \i{precompiled} term to describe the non-header-only
mode. We don't recommend using this term in the \c{build2} build since it has
a strong association with precompiled headers and can therefore be
confusing. Instead, use the \i{compiled} term.|

The main rationale here is that a library would not be offering a compiled
mode if there were no benefits (usually faster compile times of library
consumers) and there is no reason not to take advantage of it in the
\c{build2} build.

There are, however, reasons why a compiled mode cannot be used, the most
common of which are:

\ul|

\li|The compiled mode is not well maintained/tested by upstream and therefore
offers inferior user experience.|

\li|The compiled mode does not work on some platforms, usually Windows due to
the lack of symbol export support (but see \l{b##cc-auto-symexport Automatic
DLL Symbol Exporting}).|

\li|Uses of the compiled version of the library requires changes to the
library consumers, for example, inclusion of different headers.|

|

If a compiled mode cannot be always used, then it may be tempting to support
both modes potentially making the mode user-configurable. Unless there are
strong reasons to, you should resist this temptation and, if the compiled
mode is not universally usable, then use the header-only mode everywhere.

The main rationale here is that variability adds complexity which makes the
result more prone to bugs, more difficult to use, and harder to review and
maintain. If you really want to have the compiled mode, then the right
way to do it is to work with upstream to fix any issues that prevent its
use in \c{build2}.

There are, however, reasons why supporting both mode may be needed, the most
common of which are:

\ul|

\li|The library is widely used in both modes but switching from one mode to
the other requires changes to the library consumers (for example, inclusion of
different headers). In this case only supporting one mode would mean not
supporting a large number of library consumers.|

\li|The library consists of a large number of independent components and its
common for applications to only use a small subset of them. On the other hand,
compiling all of them in the compiled mode takes a substantial amount of time.
(Note that this can also be addressed by making the presence of optional
components user-configurable.)|

|


\h#dont-main-target-root-buildfile|Don't build your main targets in root \c{buldfile}|

It may be tempting to have your main targets (libraries, executables) in the
root \c{buildfile}, especially if it allows you to symlink entire directories
from \c{upstream/} (which is not possible if you have to have a \c{buildfile}
inside). However, this is a bad idea except for the simplest projects.

Firstly, this quickly gets messy since you have to combine managing
\c{README}s, \c{LICENSE}s, and subdirectories with you main target builds.
But, more importantly, this means that when you main target is imported (and
thus the \c{buildfile} that defines this target must be loaded), your entire
project will be loaded, including any \c{tests/} and \c{examples/} subproject,
which is wasteful.

Note also that it's easy to continue symlinking entire directories from
\c{upstream/} without moving everything to the root \c{buildfile} by simply
creating another subdirectory. Let's look at a concrete example. Here is
the directory structure where everything is in the root \c{buildfile}:

\
libigl-core/
├── igl/ -> upstream/igl/
├── tests/
└── buildfile                      # Defines lib{igl-core}.
\

And here is the alternative structure where we have added the \c{libigl-core}
subdirectory with its own \c{buildfile}:

\
libigl-core/
├── libigl-core/
│   ├── igl/ -> ../upstream/igl/
│   └── buildfile                 # Defines lib{igl-core}.
├── tests/
└── buildfile
\

Below is the \l{bdep-new(1)} invocation that can be used to automatically
create this alternative structure (see \l{bdep-new.xhtml#src-layout SOURCE
LAYOUT} for details):

\
$ bdep new -t lib,prefix=libigl-core,no-subdir,no-version libigl-core
\

"