1 files changed, 150 insertions, 0 deletions
diff --git a/tooling/danger/commit_linter.rb b/tooling/danger/commit_linter.rb
new file mode 100644
index 00000000000..905031ec881
--- /dev/null
+++ b/tooling/danger/commit_linter.rb
@@ -0,0 +1,150 @@
+# frozen_string_literal: true
+
+require_relative 'base_linter'
+require_relative 'emoji_checker'
+
+module Tooling
+  module Danger
+    class CommitLinter < BaseLinter
+      MAX_CHANGED_FILES_IN_COMMIT = 3
+      MAX_CHANGED_LINES_IN_COMMIT = 30
+      SHORT_REFERENCE_REGEX = %r{([\w\-\/]+)?(?<!`)(#|!|&|%)\d+(?<!`)}.freeze
+
+      def self.problems_mapping
+        super.merge(
+          {
+            separator_missing: "The commit subject and body must be separated by a blank line",
+            details_too_many_changes: "Commits that change #{MAX_CHANGED_LINES_IN_COMMIT} or more lines across " \
+          "at least #{MAX_CHANGED_FILES_IN_COMMIT} files must describe these changes in the commit body",
+            details_line_too_long: "The commit body should not contain more than #{MAX_LINE_LENGTH} characters per line",
+            message_contains_text_emoji: "Avoid the use of Markdown Emoji such as `:+1:`. These add limited value " \
+          "to the commit message, and are displayed as plain text outside of GitLab",
+            message_contains_unicode_emoji: "Avoid the use of Unicode Emoji. These add no value to the commit " \
+          "message, and may not be displayed properly everywhere",
+            message_contains_short_reference: "Use full URLs instead of short references (`gitlab-org/gitlab#123` or " \
+          "`!123`), as short references are displayed as plain text outside of GitLab"
+          }
+        )
+      end
+
+      def initialize(commit)
+        super
+
+        @linted = false
+      end
+
+      def fixup?
+        commit.message.start_with?('fixup!', 'squash!')
+      end
+
+      def suggestion?
+        commit.message.start_with?('Apply suggestion to')
+      end
+
+      def merge?
+        commit.message.start_with?('Merge branch')
+      end
+
+      def revert?
+        commit.message.start_with?('Revert "')
+      end
+
+      def multi_line?
+        !details.nil? && !details.empty?
+      end
+
+      def lint
+        return self if @linted
+
+        @linted = true
+        lint_subject
+        lint_separator
+        lint_details
+        lint_message
+
+        self
+      end
+
+      private
+
+      def lint_separator
+        return self unless separator && !separator.empty?
+
+        add_problem(:separator_missing)
+
+        self
+      end
+
+      def lint_details
+        if !multi_line? && many_changes?
+          add_problem(:details_too_many_changes)
+        end
+
+        details&.each_line do |line|
+          line_without_urls = line.strip.gsub(%r{https?://\S+}, '')
+
+          # If the line includes a URL, we'll allow it to exceed MAX_LINE_LENGTH characters, but
+          # only if the line _without_ the URL does not exceed this limit.
+          next unless line_too_long?(line_without_urls)
+
+          add_problem(:details_line_too_long)
+          break
+        end
+
+        self
+      end
+
+      def lint_message
+        if message_contains_text_emoji?
+          add_problem(:message_contains_text_emoji)
+        end
+
+        if message_contains_unicode_emoji?
+          add_problem(:message_contains_unicode_emoji)
+        end
+
+        if message_contains_short_reference?
+          add_problem(:message_contains_short_reference)
+        end
+
+        self
+      end
+
+      def files_changed
+        commit.diff_parent.stats[:total][:files]
+      end
+
+      def lines_changed
+        commit.diff_parent.stats[:total][:lines]
+      end
+
+      def many_changes?
+        files_changed > MAX_CHANGED_FILES_IN_COMMIT && lines_changed > MAX_CHANGED_LINES_IN_COMMIT
+      end
+
+      def separator
+        message_parts[1]
+      end
+
+      def details
+        message_parts[2]&.gsub(/^Signed-off-by.*$/, '')
+      end
+
+      def message_contains_text_emoji?
+        emoji_checker.includes_text_emoji?(commit.message)
+      end
+
+      def message_contains_unicode_emoji?
+        emoji_checker.includes_unicode_emoji?(commit.message)
+      end
+
+      def message_contains_short_reference?
+        commit.message.match?(SHORT_REFERENCE_REGEX)
+      end
+
+      def emoji_checker
+        @emoji_checker ||= Tooling::Danger::EmojiChecker.new
+      end
+    end
+  end
+end