Support building from git-archive tarballs (vagrant-libvirt#1187)

Adds support for exposing the correct version via export-subst for
git-archive constructed tarballs to make it easier to consume directly
from source.

Will check in the following order of preference:
 - version file
 - format string containing "Tag:"
 - clone remote to describe commit

.gitattributes

@@ -0,0 +1 @@
+lib/vagrant-libvirt/version.rb export-subst

lib/vagrant-libvirt/version.rb

@@ -1,21 +1,49 @@
+require 'open3'
+require 'tmpdir'
+
 module VagrantPlugins
   module ProviderLibvirt
     VERSION_FILE = File.dirname(__FILE__) + "/version"
 
+    GIT_ARCHIVE_VERSION = "$Format:%H %D$"
+
+    HOMEPAGE = 'https://github.com/vagrant-libvirt/vagrant-libvirt'
+
     def self.get_version
       if File.exist?(VERSION_FILE)
+        # built gem
         version = File.read(VERSION_FILE)
-      else
+      elsif self.inside_git_repository
+        # local repo
         git_version = `git describe --tags`
-        version_parts = git_version.split('-').first(2) # drop the git sha if it exists
-        if version_parts.length > 1
-          # increment the patch number so that this is marked as a pre-release of the
-          # next possible release
-          main_version_parts = Gem::Version.new(version_parts[0]).segments
-          main_version_parts[-1] = main_version_parts.last + 1
-          version_parts = main_version_parts + ["pre", version_parts[1]]
+        version = self.version_from_describe(git_version)
+      elsif !GIT_ARCHIVE_VERSION.start_with?('$Format')
+        # archive - format string replaced during export
+        hash, refs = GIT_ARCHIVE_VERSION.split(' ', 2)
+
+        tag = refs.split(',').select { |ref| ref.strip.start_with?("tag:") }.first
+        if tag != nil
+          # tagged
+          version = tag.strip.split(' ').last
+        else
+          version = ""
+          # arbitrary branch/commit
+          Dir.mktmpdir do |dir|
+            stdout_and_stderr, status = Open3.capture2e("git -C #{dir} clone --bare #{HOMEPAGE}")
+            raise "failed to clone original to resolve version: #{stdout_and_stderr}" unless status.success?
+
+            stdout_and_stderr, status = Open3.capture2e("git --git-dir=#{dir}/vagrant-libvirt.git describe --tags #{hash}")
+            raise "failed to determine version for #{hash}: #{stdout_and_stderr}" unless status.success?
+
+            version = version_from_describe(stdout_and_stderr)
+          end
+
+          # in this case write the version file to avoid cloning a second time
+          File.write(VERSION_FILE, version)
         end
-        version = version_parts.join(".")
+      else
+        # no idea
+        version = "9999"
       end
 
       return version.freeze
@@ -24,5 +52,25 @@ def self.get_version
     def self.write_version
       File.write(VERSION_FILE, self.get_version)
     end
+
+    private
+
+    def self.inside_git_repository
+      _, status = Open3.capture2e("git rev-parse --git-dir")
+
+      status.success?
+    end
+
+    def self.version_from_describe(describe)
+      version_parts = describe.split('-').first(2) # drop the git sha if it exists
+      if version_parts.length > 1
+        # increment the patch number so that this is marked as a pre-release of the
+        # next possible release
+        main_version_parts = Gem::Version.new(version_parts[0]).segments
+        main_version_parts[-1] = main_version_parts.last + 1
+        version_parts = main_version_parts + ["pre", version_parts[1]]
+      end
+      version = version_parts.join(".")
+    end
   end
 end

vagrant-libvirt.gemspec

@@ -7,14 +7,14 @@ Gem::Specification.new do |s|
   s.license       = 'MIT'
   s.description   = %q{libvirt provider for Vagrant.}
   s.summary       = %q{libvirt provider for Vagrant.}
-  s.homepage      = 'https://github.com/vagrant-libvirt/vagrant-libvirt'
+  s.homepage      = VagrantPlugins::ProviderLibvirt::HOMEPAGE
 
   s.files         = Dir.glob("{lib,locales}/**/*") + %w(LICENSE README.md)
   s.executables   = Dir.glob("bin/*.*").map{ |f| File.basename(f) }
   s.test_files    = Dir.glob("{test,spec,features}/**/*.*")
   s.name          = 'vagrant-libvirt'
   s.require_paths = ['lib']
-  s.version       = VagrantPlugins::ProviderLibvirt.get_version()
+  s.version       = VagrantPlugins::ProviderLibvirt.get_version
 
   s.add_development_dependency "rspec-core", "~> 3.5.0"
   s.add_development_dependency "rspec-expectations", "~> 3.5.0"