User:Peter/Erlang Packaging Guidelines

THIS DOCUMENT IS WIP

This document seeks to document the conventions and customs surrounding the proper packaging of Erlang modules and applications in Fedora and EPEL. It does not intend to cover all situations, but to codify those practices which have served the Fedora Erlang community well.

Naming

Erlang packages should be named as erlang-something, except for applications which can be named without the erlang- prefix (e.g. couchdb, rabbitmq-server, wings).

Building

Rebar is usually used to build Erlang packages, as it is the de-facto standard Erlang build tool. It is often bundled along with the sources, so the packager must ensure that the system installed rebar is used instead of the one included with the package. We recommend using our build macros for building (%{erlang_compile} or less generic %{rebar_compile}) to ensure that the correct rebar is used. However some old software cannot be built with rebar - this is an outstanding issue to be addressed.

Debug symbols / source installation / dialyzer

Erlang packages should not install their original sources. Instead packages should ensure that they are built with +debug_info flag. It does NOT impact the runtime performance of the application at all (beam loader disregards debug symbols before loading the build images into the VM). Dialyzer requires either this debug info or original sources. If the package was built with our recommended macros (%{erlang_compile} or less generic %{rebar_compile}), then the needed debug_info is generated automatically.

If the Erlang package doesn't contain any NIF-libraries, port-applications, or driver libraries, it must contain the "%global debug_package %{nil}" directive to suppress building an empty debuginfo sub-package. Unfortunately, this adds two additional rpmlint messages:

<code>Auriga ~: rpmlint ~/rpmbuild/RPMS/x86_64/erlang-cowboy-2.0.0-0.1.pre.3.fc24.x86_64.rpm 
erlang-cowboy.x86_64: E: no-binary
erlang-cowboy.x86_64: W: only-non-binary-in-usr-lib
1 packages and 0 specfiles checked; 1 errors, 1 warnings.
Auriga ~:</code>

What rpmlint it trying to say here is that we're installing arch-independent data into arch-dependent library space. This is expected, and these messages can be ignored for now.

File Locations

Erlang packages should be installed to %{_erllibdir}/%{realname}-%{version}. The handy macro %{erlang_appdir} has been provided as a shorthand expression of this path for spec files that define %{realname}. Large applications such as rabbitmq-server or riak install their content somewhere else for historical reasons.

Header files

Header files for erlang modules stored in the source ./include directory must be bundled with the main package (not in a *-devel subpackage). They are used by system administrators sometimes right from the REPL console. Headers from ./src directory normally shouldn't be packaged, however sometimes it's required. In this case you have to package them, but consider reporting an issue upstream about their include file placements.

Dependencies

Erlang packagers need to build the list of BuildRequires by hand, but RPM builds a list of Requires for the packager automatically (except for few rare cases). Please take a look at the Koji build log. If you see messages like the following one, then the packager either missed one of the BuildRequires or the package contains an error:

error: invalid dependency (bad format): ERROR: Cant find lager:error/2 while processing '/builddir/build/BUILDROOT/erlang-ibrowse-4.2.4-1.fc25.x86_64/usr/lib64/erlang/lib/ibrowse-4.2.4/ebin/ibrowse.beam'

In the case cited above, the package contained an error which was addressed upstream already. Errors like this could also mean that the packager forgot to add "BuildRequires: erlang-lager". We can't give packagers a hint on what's missing, but in the case of this message the packager should try running "sudo dnf provides "*/lager.beam".

An example of spec-file

Here is an example of a typical Erlang package's spec-file. The RPM macro script use %{realname} and  %{upstream}, so please be sure to include these definitions in your spec files.

<code>%global realname foo

Name:		erlang-%{realname}
Version:	1.2.3
Release:	1%{?dist}
Summary:	Erlang library for doing cool things
License:	Apache-2.0
URL:		https://github.com/awesomeperson/%{realname}
Source0:	%{url}/archive/%{version}/%{realname}-%{version}.tar.gz
#Source0:	%{url}/archive/<long git hash>/%{realname}-%{version}.tar.gz
# Normally erlang-rebar3 installs almost everything required for building. But sometimes some extra packages are required.
# Please try building in Koji - you'll likely see some hints.
BuildRequires:	erlang-rebar3

# Normally RPM detects Erlang dependencies automatically (if built with erlang-rebar3). However in some rare cases it can't be detected,
# and you have to list those dependencies manually.
Requires: erlang-bar

%description
Erlang library for doing cool things.

%prep
%autosetup -p1 -n %{realname}-%{version}

%build
%{erlang3_compile}

%install
%{erlang3_install}

# Additionally install some cool stuff required to run application properly
cp -arv priv/ %{buildroot}%{erlang_appdir}/

%check
%{erlang3_test}
#
# Sometimes upstream uses different config-file for testing
#%%{erlang3_test -C rebar-test.config}

%files
%license LICENSE.txt
%doc doc/ examples/ README.md
%{erlang_appdir}/

* Wed Nov  6 2024 Peter Lemenkov <lemenkov@gmail.com> - 1.2.3-1
- Initial build</code>