support rubygemsrecipe recipe

The approach is to look in the parts directory and report everything
that has been installed in lib/ruby/gems/specifications/*.gemspec
To find the name and version, instead of trying to deduct from the
filename, we read the stub metadata from the gemspec file, using the
same login as what rubygems does in
https://github.com/rubygems/rubygems/blob/6b360fdcd2cbd72a902f4107a1deb264a6e7d3ca/lib/rubygems/stub_specification.rb#L4-L10

nxdbom/__init__.py

@@ -81,6 +81,8 @@ def bom_software(installed_software_path): # -> {} (name,kind) -> PkgInfo
                     url = '%s/%s-%s.tar.gz' % (nxdbase, name, v)
                 else:
                     url = 'https://pypi.org/project/%s/%s/' % (name, v)
+            elif kind == 'gem':
+                url = 'https://rubygems.org/gems/%s/versions/%s' % (name, ver)
             else:
                 raise NotImplementedError('TODO url for kind %r  (urlpath: %r)' % (kind, urlpath))
 
@@ -257,6 +259,11 @@ def bom_software(installed_software_path): # -> {} (name,kind) -> PkgInfo
                 ver = 'HEAD'
             addbom(repo, 'git', ver)
 
+        elif recipe in ('rubygemsrecipe',):
+            location = part.get('location', raw=True)
+            for gemspec in glob('%s/lib/ruby/gems/specifications/*.gemspec' % location):
+                addbom(gemspec, 'gem')
+
         elif recipe in ('plone.recipe.command',):
             # not downloading anything
             pass
@@ -416,6 +423,14 @@ def _namever(url, failonerr):
     if m is not None:
         url = unquote(m.group('f'))
 
+    if url.endswith('.gemspec'):
+        with open(url) as f:
+            _ = f.readline()
+            stub_line = f.readline()
+        dash, stub, name, ver, _, _ = stub_line.split()
+        assert (dash, stub) == ('#', 'stub:')
+        return name, ver
+
     filename = basename(url)
 
     name   = filename

nxdbom/nxdbom_test.py

@@ -453,6 +453,49 @@ case1("""\
 recipe = plone.recipe.command==1.1
 """, '')
 
+# ruby gems
+case1("""\
+[gems]
+recipe = rubygemsrecipe
+location = /ROOT/parts/gems
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/gitlab-puma-4.3.3.gitlab.2.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: gitlab-puma 4.3.3.gitlab.2 ruby lib
+# stub: ext/puma_http11/extconf.rb
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/pyu-ruby-sasl-0.0.3.3.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: pyu-ruby-sasl 0.0.3.3 ruby lib
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/toml-rb-1.0.0.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: toml-rb 1.0.0 ruby lib
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/tzinfo-1.2.6.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: tzinfo 1.2.6 ruby lib
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/google-protobuf-3.8.0-x86_64-linux.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: google-protobuf 3.8.0 x86_64-linux lib
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/apollo_upload_server-2.0.0.beta.3.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: apollo_upload_server 2.0.0.beta.3 ruby lib
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/omniauth-gitlab-1.0.3.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: omniauth-gitlab 1.0.3 ruby lib
+-- /ROOT/parts/gems/lib/ruby/gems/specifications/diff_match_patch-0.1.0.gemspec --
+# -*- encoding: utf-8 -*-
+# stub: diff_match_patch 0.1.0 ruby lib
+
+""", """
+>>> gems:
+apollo_upload_server         2.0.0.beta.3 https://rubygems.org/gems/apollo_upload_server/versions/2.0.0.beta.3
+diff_match_patch             0.1.0      https://rubygems.org/gems/diff_match_patch/versions/0.1.0
+gitlab-puma                  4.3.3.gitlab.2 https://rubygems.org/gems/gitlab-puma/versions/4.3.3.gitlab.2
+google-protobuf              3.8.0      https://rubygems.org/gems/google-protobuf/versions/3.8.0
+omniauth-gitlab              1.0.3      https://rubygems.org/gems/omniauth-gitlab/versions/1.0.3
+pyu-ruby-sasl                0.0.3.3    https://rubygems.org/gems/pyu-ruby-sasl/versions/0.0.3.3
+toml-rb                      1.0.0      https://rubygems.org/gems/toml-rb/versions/1.0.0
+tzinfo                       1.2.6      https://rubygems.org/gems/tzinfo/versions/1.2.6
+""")
+
 # readline 8 and 5 used simultaneously    ->  older one is presented as readline5
 # libpng   12 and 16 used simultaneously  ->  ----//----                libpng12
 # ----/---- Python2 and Python3