How to read the gem file specifications from extconf.rb? - ruby

Question

I'm building a Ruby gem file that has various 3rd party executable requirements (some non-ruby tools that the gem will then use through system calls).
The requirement list is specified using the gemspec informative requirements list, and I was tasked with creating a set up where that requirement list is verified during installation - essentially converting the requirements list from being informative to being authoritative.
My initial approach is to use extensions, like in the extconf.rb pattern, to run some ruby code during installation that will verify the list of requirements and fail the installation if the requirements are not met.
So my current implementation is using a ext/Rakefile, that reads gemspec file using a hard-coded path, parses the requirements list using a custom (but straight-forward and simple) syntax and run some simple tests on the required executables.
I would really want to factor out the dependency on knowing the path to the gemspec file, so I can reuse the code in a modular way on other projects.
For completion (and in the hopes that it will be useful for someone), here's my code - which uses the Gem::Ext::Builder "Rakefile mode" to just run some arbitrary code.
require 'rubygems'
require 'mkmf'
def verify_requirement(reqspec)
exe, version_spec, version_regex = reqspec.split(/\s*,\s*/,3)
exe = $1 if exe =~ /\[([^\]]+)\]/ # support human readable alias for executable name
abort "Missing required executable #{exe}!" unless find_executable(exe)
return unless version_spec # no version is specified, so executable existing is good enough
version = %x|#{exe} --version 2>&1|.chomp
version = %x|#{exe} -version 2>&1|.chomp unless $?.success?
abort "Error checking for version of #{exe}: (#{$?.exitstatus}) #{version}" unless $?.success?
if version_regex
abort "Version test '#{version_regex}' failed to match '#{version}'!" unless version =~ /#{version_regex}/m
abort "Version test '#{version_regex}' failed to generate a version number!" unless $1
version = $1
end
abort "Failed to locate a valid version number for #{exe}, found '#{version}'" unless version =~ /^[0-9\.]+/
abort "Insufficient version '#{version}' for #{exe} - requires #{version_spec}" unless Gem::Dependency.new(exe, version_spec).match? exe, version
end
task :default do
gemspec = File.absolute_path('../my.gemspec')
Gem::Specification::load(gemspec).requirements.each do |req|
verify_requirement(req)
end
end
This code requires that the requirements uses either of these syntaxes:
exe-file[, <semantic-version-spec>[, <version-garabbing-regex>]]
alias '[' exe-file ']'[, <semantic-version-spec>[, <version-garabbing-regex>]]
For example:
spec.requirements << 'sox'
spec.requirements << 'ffmpeg, >= 2.5, ffmpeg version ([\d\.]+)'
spec.requirements << 'ImageMagick [convert], >= 6.7.5, ImageMagick ([\d\.]+)'

Answer 1

The Gem::Specification class can be used to load and parse the gemspec. I'm using the following code in ext/Rakefile to perform tasks based on the gemspec file:
task :default do
gemspec = File.absolute_path('../gemname.gemspec')
Gem::Specification::load(gemspec).requirements.each do |req|
verify_requirement(req)
end
end