Extract changelog template parser to new module

Template parser that was added with changelog generator can be used
outside of changelogs, for example for merge commit templates.
This commit simply refactors the generic parts of template parser and
moves them out to module Gitlab::TemplateParser.

lib/gitlab/changelog/config.rb

@@ -52,7 +52,12 @@ module Gitlab
         end
 
         if (template = hash['template'])
-          config.template = Parser.new.parse_and_transform(template)
+          config.template =
+            begin
+              TemplateParser::Parser.new.parse_and_transform(template)
+            rescue TemplateParser::Error => e
+              raise Error, e.message
+            end
         end
 
         if (categories = hash['categories'])
@@ -73,7 +78,12 @@ module Gitlab
       def initialize(project)
         @project = project
         @date_format = DEFAULT_DATE_FORMAT
-        @template = Parser.new.parse_and_transform(DEFAULT_TEMPLATE)
+        @template =
+          begin
+            TemplateParser::Parser.new.parse_and_transform(DEFAULT_TEMPLATE)
+          rescue TemplateParser::Error => e
+            raise Error, e.message
+          end
         @categories = {}
         @tag_regex = DEFAULT_TAG_REGEX
       end

lib/gitlab/changelog/release.rb

@@ -54,7 +54,7 @@ module Gitlab
       end
 
       def to_markdown
-        state = EvalState.new
+        state = TemplateParser::EvalState.new
         data = { 'categories' => entries_for_template }
 
         # While not critical, we would like release sections to be separated by
@@ -63,7 +63,12 @@ module Gitlab
         #
         # Since it can be a bit tricky to get this right in a template, we
         # enforce an empty line separator ourselves.
-        markdown = @config.template.evaluate(state, data).strip
+        markdown =
+          begin
+            @config.template.evaluate(state, data).strip
+          rescue TemplateParser::ParseError => e
+            raise Error, e.message
+          end
 
         # The release header can't be changed using the Liquid template, as we
         # need this to be in a known format. Without this restriction, we won't

lib/gitlab/changelog/ast.rb → lib/gitlab/template_parser/ast.rb

 # frozen_string_literal: true
 
 module Gitlab
-  module Changelog
+  module TemplateParser
     # AST nodes to evaluate when rendering a template.
     #
     # Evaluating an AST is done by walking over the nodes and calling

lib/gitlab/template_parser/error.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module TemplateParser
+    # An error raised when a template couldn't be rendered.
+    Error = Class.new(StandardError)
+  end
+end

lib/gitlab/changelog/eval_state.rb → lib/gitlab/template_parser/eval_state.rb

 # frozen_string_literal: true
 
 module Gitlab
-  module Changelog
+  module TemplateParser
     # A class for tracking state when evaluating a template
     class EvalState
       MAX_LOOPS = 4

lib/gitlab/changelog/parser.rb → lib/gitlab/template_parser/parser.rb

 # frozen_string_literal: true
 
 module Gitlab
-  module Changelog
-    # A parser for the template syntax used for generating changelogs.
+  module TemplateParser
+    # A parser for a simple template syntax, used for example to generate changelogs.
     #
     # As a quick primer on the template syntax, a basic template looks like
     # this:
@@ -166,7 +166,7 @@ module Gitlab
       def parse_and_transform(input)
         AST::Transformer.new.apply(parse(input))
       rescue Parslet::ParseFailed => ex
-        # We raise a custom error so it's easier to catch different changelog
+        # We raise a custom error so it's easier to catch different parser
         # related errors. In addition, this ensures the caller of this method
         # doesn't depend on a Parslet specific error class.
         raise Error, "Failed to parse the template: #{ex.message}"

spec/lib/gitlab/changelog/config_spec.rb

@@ -43,7 +43,7 @@ RSpec.describe Gitlab::Changelog::Config do
 
       expect(config.date_format).to eq('foo')
       expect(config.template)
-        .to be_instance_of(Gitlab::Changelog::AST::Expressions)
+        .to be_instance_of(Gitlab::TemplateParser::AST::Expressions)
 
       expect(config.categories).to eq({ 'foo' => 'bar' })
       expect(config.tag_regex).to eq('foo')
@@ -53,6 +53,16 @@ RSpec.describe Gitlab::Changelog::Config do
       expect { described_class.from_hash(project, 'categories' => 10) }
         .to raise_error(Gitlab::Changelog::Error)
     end
+
+    it 'raises a Gitlab::Changelog::Error when the template is invalid' do
+      invalid_template = <<~TPL
+        {% each {{foo}} %}
+        {% end %}
+      TPL
+
+      expect { described_class.from_hash(project, 'template' => invalid_template) }
+        .to raise_error(Gitlab::Changelog::Error)
+    end
   end
 
   describe '#contributor?' do

spec/lib/gitlab/changelog/ast_spec.rb → spec/lib/gitlab/template_parser/ast_spec.rb

@@ -2,8 +2,8 @@
 
 require 'spec_helper'
 
-RSpec.describe Gitlab::Changelog::AST::Identifier do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Identifier do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
 
   describe '#evaluate' do
     it 'evaluates a selector' do
@@ -26,8 +26,8 @@ RSpec.describe Gitlab::Changelog::AST::Identifier do
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::Integer do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Integer do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
 
   describe '#evaluate' do
     it 'evaluates a selector' do
@@ -44,33 +44,33 @@ RSpec.describe Gitlab::Changelog::AST::Integer do
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::Selector do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Selector do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
   let(:data) { { 'numbers' => [10] } }
 
   describe '#evaluate' do
     it 'evaluates a selector' do
-      ident = Gitlab::Changelog::AST::Identifier.new('numbers')
-      int = Gitlab::Changelog::AST::Integer.new(0)
+      ident = Gitlab::TemplateParser::AST::Identifier.new('numbers')
+      int = Gitlab::TemplateParser::AST::Integer.new(0)
 
       expect(described_class.new([ident, int]).evaluate(state, data)).to eq(10)
     end
 
     it 'evaluates a selector that returns nil' do
-      int = Gitlab::Changelog::AST::Integer.new(0)
+      int = Gitlab::TemplateParser::AST::Integer.new(0)
 
       expect(described_class.new([int]).evaluate(state, data)).to be_nil
     end
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::Variable do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Variable do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
   let(:data) { { 'numbers' => [10] } }
 
   describe '#evaluate' do
     it 'evaluates a variable' do
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{{numbers.0}}')
         .nodes[0]
@@ -80,26 +80,26 @@ RSpec.describe Gitlab::Changelog::AST::Variable do
 
     it 'evaluates an undefined variable' do
       node =
-        Gitlab::Changelog::Parser.new.parse_and_transform('{{foobar}}').nodes[0]
+        Gitlab::TemplateParser::Parser.new.parse_and_transform('{{foobar}}').nodes[0]
 
       expect(node.evaluate(state, data)).to eq('')
     end
 
     it 'evaluates the special variable "it"' do
       node =
-        Gitlab::Changelog::Parser.new.parse_and_transform('{{it}}').nodes[0]
+        Gitlab::TemplateParser::Parser.new.parse_and_transform('{{it}}').nodes[0]
 
       expect(node.evaluate(state, data)).to eq(data.to_s)
     end
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::Expressions do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Expressions do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
 
   describe '#evaluate' do
     it 'evaluates all expressions' do
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{{number}}foo')
 
@@ -108,8 +108,8 @@ RSpec.describe Gitlab::Changelog::AST::Expressions do
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::Text do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Text do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
 
   describe '#evaluate' do
     it 'returns the text' do
@@ -118,12 +118,12 @@ RSpec.describe Gitlab::Changelog::AST::Text do
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::If do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::If do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
 
   describe '#evaluate' do
     it 'evaluates a truthy if expression without an else clause' do
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{% if thing %}foo{% end %}')
         .nodes[0]
@@ -132,7 +132,7 @@ RSpec.describe Gitlab::Changelog::AST::If do
     end
 
     it 'evaluates a falsy if expression without an else clause' do
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{% if thing %}foo{% end %}')
         .nodes[0]
@@ -141,7 +141,7 @@ RSpec.describe Gitlab::Changelog::AST::If do
     end
 
     it 'evaluates a falsy if expression with an else clause' do
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{% if thing %}foo{% else %}bar{% end %}')
         .nodes[0]
@@ -177,13 +177,13 @@ RSpec.describe Gitlab::Changelog::AST::If do
   end
 end
 
-RSpec.describe Gitlab::Changelog::AST::Each do
-  let(:state) { Gitlab::Changelog::EvalState.new }
+RSpec.describe Gitlab::TemplateParser::AST::Each do
+  let(:state) { Gitlab::TemplateParser::EvalState.new }
 
   describe '#evaluate' do
     it 'evaluates the expression' do
       data = { 'animals' => [{ 'name' => 'Cat' }, { 'name' => 'Dog' }] }
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{% each animals %}{{name}}{% end %}')
         .nodes[0]
@@ -193,7 +193,7 @@ RSpec.describe Gitlab::Changelog::AST::Each do
 
     it 'returns an empty string when the input is not a collection' do
       data = { 'animals' => 10 }
-      node = Gitlab::Changelog::Parser
+      node = Gitlab::TemplateParser::Parser
         .new
         .parse_and_transform('{% each animals %}{{name}}{% end %}')
         .nodes[0]
@@ -237,10 +237,10 @@ RSpec.describe Gitlab::Changelog::AST::Each do
       TPL
 
       node =
-        Gitlab::Changelog::Parser.new.parse_and_transform(template).nodes[0]
+        Gitlab::TemplateParser::Parser.new.parse_and_transform(template).nodes[0]
 
       expect { node.evaluate(state, data) }
-        .to raise_error(Gitlab::Changelog::Error)
+        .to raise_error(Gitlab::TemplateParser::Error)
     end
   end
 end

spec/lib/gitlab/changelog/parser_spec.rb → spec/lib/gitlab/template_parser/parser_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Gitlab::Changelog::Parser do
+RSpec.describe Gitlab::TemplateParser::Parser do
   let(:parser) { described_class.new }
 
   describe '#root' do
@@ -67,12 +67,12 @@ RSpec.describe Gitlab::Changelog::Parser do
     it 'parses and transforms a template' do
       node = parser.parse_and_transform('foo')
 
-      expect(node).to be_instance_of(Gitlab::Changelog::AST::Expressions)
+      expect(node).to be_instance_of(Gitlab::TemplateParser::AST::Expressions)
     end
 
     it 'raises parsing errors using a custom error class' do
       expect { parser.parse_and_transform('{% each') }
-        .to raise_error(Gitlab::Changelog::Error)
+        .to raise_error(Gitlab::TemplateParser::Error)
     end
   end
 end