Follow depended gem version by fluent-plugin-generate

In the previous versions, generated .gemspec doesn't follow
depended gem version which is used in fluentd.gemspec at all.

Instead, it is changed to retrieve depended gem version from
Gemfile.lock to sync with it.

Before:

  spec.add_development_dependency "bundler", "~> 1.14"
  spec.add_development_dependency "rake", "~> 12.0"
  spec.add_development_dependency "test-unit", "~> 3.0"

It is required to update manually.

After:

  spec.add_development_dependency "bundler", "~> 2.2.15"
  spec.add_development_dependency "rake", "~> 13.0"
  spec.add_development_dependency "test-unit", "~> 3.3"

Above depended version is replaced by <%= %> in template.
No need to update manually.

Signed-off-by: Kentaro Hayashi <kenhys@gmail.com>

lib/fluent/command/plugin_generator.rb

@@ -151,6 +151,36 @@ def plugin_name
     underscore_name
   end
 
+  def gem_file_path
+    File.expand_path(File.join(File.dirname(__FILE__),
+                               "../../../",
+                               "Gemfile"))
+  end
+
+  def lock_file_path
+    File.expand_path(File.join(File.dirname(__FILE__),
+                               "../../../",
+                               "Gemfile.lock"))
+  end
+
+  def locked_gem_version(gem_name)
+    d = Bundler::Definition.build(gem_file_path, lock_file_path, false)
+    d.locked_gems.dependencies[gem_name].requirement.requirements.first.last.version
+  end
+
+  def rake_version
+    locked_gem_version("rake")
+  end
+
+  def test_unit_version
+    locked_gem_version("test-unit")
+  end
+
+  def bundler_version
+    d = Bundler::Definition.build(gem_file_path, lock_file_path, false)
+    d.locked_gems.bundler_version.version
+  end
+
   def class_name
     "#{capitalized_name}#{type.capitalize}"
   end

templates/new_gem/fluent-plugin.gemspec.erb

@@ -20,8 +20,8 @@ Gem::Specification.new do |spec|
   spec.test_files    = test_files
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 1.14"
-  spec.add_development_dependency "rake", "~> 12.0"
-  spec.add_development_dependency "test-unit", "~> 3.0"
+  spec.add_development_dependency "bundler", "~> <%= bundler_version %>"
+  spec.add_development_dependency "rake", "~> <%= rake_version %>"
+  spec.add_development_dependency "test-unit", "~> <%= test_unit_version %>"
   spec.add_runtime_dependency "fluentd", [">= 0.14.10", "< 2"]
 end