[Chore] Add danger checks

Problem: it would be helpful to have checks for common rules on styling
commits and stuff, to avoid checking those manually every time.

Solution: add Danger checks, mostly the same we had in Morley.

.github/workflows/danger.yml

@@ -0,0 +1,33 @@
+# SPDX-FileCopyrightText: 2022 Serokell <https://serokell.io/>
+#
+# SPDX-License-Identifier: MPL-2.0
+
+name: Danger
+
+on: [pull_request]
+
+jobs:
+  run-danger-checks:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: '2.7'
+        bundler-cache: true
+    - uses: MeilCli/danger-action@v5
+      name: Instant checks
+      with:
+        install_path: 'vendor/bundle'
+        danger_file: './danger/instant-checks.rb'
+        danger_id: 'instant-checks'
+      env:
+        DANGER_GITHUB_API_TOKEN: ${{ secrets.DANGER_BOT_TOKEN }}
+    - uses: MeilCli/danger-action@v5
+      name: Premerge checks
+      with:
+        install_path: 'vendor/bundle'
+        danger_file: './danger/premerge-checks.rb'
+        danger_id: 'premerge-checks'
+      env:
+        DANGER_GITHUB_API_TOKEN: ${{ secrets.DANGER_BOT_TOKEN }}

danger/branch-name.rb

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+# SPDX-FileCopyrightText: 2022 Oxhead Alpha
+# SPDX-License-Identifier: LicenseRef-MIT-OA
+
+require_relative 'helpers'
+
+def check_branch_name
+  # Proper branch name
+  if branch_match = githost.branch_for_head.match(/([^\/]+)\/([^\-]+)-(.+)/)
+    nick, issue_id, desc = branch_match.captures
+
+    # We've decided not to put any restrictions on nickname for now
+
+    unless /^(#\d+|chore)$/.match?(issue_id)
+      warn(
+        "Bad issue ID in branch name.\n"\
+        "Valid format for issue IDs: `#123` or `chore`."
+      )
+    end
+
+    weird_chars = desc.scan(/[^a-zA-Z\-\d]/)
+    unless weird_chars.empty?
+      warn(
+        "Please, only use letters, digits and dashes in the branch name.
+        Found: #{weird_chars}"
+      )
+    end
+  elsif
+    warn(
+      "Please use `<nickname>/<issue-id>-<brief-description>`` format for branch names.`\n"\
+      "Example: `lazyman/#123-my-commit`"
+    )
+  end
+end

danger/commit-style.rb

@@ -0,0 +1,93 @@
+# SPDX-FileCopyrightText: 2022 Oxhead Alpha
+# SPDX-License-Identifier: LicenseRef-MIT-OA
+
+require_relative 'helpers'
+
+def wrap_workarounds(fun)
+  return lambda { |msg|
+    method(fun).call(msg + "\nSee also \\[Note\\].")
+    markdown(
+      "\\[Note\\]: Skip this check by adding `wip`, `tmp` or `[temporary]` to the commit subject. "\
+      "Fixup commits (marked with `fixup!` or `squash!`) are also exempt from this check.")
+  }
+end
+
+def check_commit_style (mywarn = wrap_workarounds(:warn), myfail = wrap_workarounds(:fail))
+  # Proper commit style
+  # Note: we do not use commit_lint plugin because it triggers on fixup commits
+  git.commits.each { |commit|
+    if commit.fixup? || commit.wip?
+      next
+    end
+
+    subject = commit.subject
+    subject_payload = subject.sub(issue_tags_pattern, "")
+    subject_ticked = commit.subject_ticked
+
+    unless has_valid_issue_tags(subject)
+      # If any of these substrings is included into commit message,
+      # we are fine with issue tag absence.
+      exclusions = [
+        # In lower-case
+        "changelog"
+      ]
+      if exclusions.none? { |exc| subject.downcase.include?(exc) }
+        mywarn.call("In #{commit.sha} message lacks issue id: #{subject_ticked}.")
+      end
+    end
+
+    if subject_payload.start_with?(" ")
+      mywarn.call("Extra space in commit #{commit.sha} subject after the issue tags: #{subject_ticked}.")
+    elsif !subject_payload.start_with?(/[A-Z]/)
+      mywarn.call("In #{commit.sha} subject does not begin with an uppercase letter: #{subject_ticked}.")
+    end
+
+    if subject[-1..-1] == '.'
+      mywarn.call("In #{commit.sha} message ends with a dot: #{subject_ticked} :fire_engine:")
+    end
+
+    if subject.length > 90
+      myfail.call("Nooo, such long commit message names do not work (#{commit.sha}).")
+    elsif subject.length > 72
+      mywarn.call("In commit #{commit.sha} message is too long (#{subject.length} chars), "\
+        "please keep its length within 72 characters.")
+    end
+
+    if commit.message_body.empty?
+      # If any of these substrings is included into commit message,
+      # we are fine with commit description absence.
+      exclusions = [
+        # In lower-case
+        "changelog"
+      ]
+      unless commit.chore? || exclusions.any? { |exc| subject.downcase.include?(exc) }
+        myfail.call(
+          "Commit #{commit.sha} lacks description :unamused:\n"\
+          "Commits marked as `[Chore]` are exempt from this check."
+          )
+      end
+    else
+      # Checks on description
+
+      if !commit.blank_line_after_subject?
+        mywarn.call("In #{commit.sha} blank line is missing after the commit's subject.")
+      end
+
+      if !commit.chore?
+        description_patterns = [
+          /^Problem:[ \n].*^Solution:[ \n]/m,
+          /And yes, I don't care about templates/
+        ]
+        unless description_patterns.any? { |pattern| pattern.match?(commit.description) }
+          mywarn.call(
+            "Description of #{commit.sha} does not follow the template.\n"\
+            "Try `Problem:`/`Solution:` structure.\n"\
+            "If you really have to, you can add `And yes, I don't care about templates` "\
+            "to the commit message body."
+          )
+        end
+      end
+    end
+
+  }
+end

danger/helpers.rb

@@ -0,0 +1,105 @@
+# SPDX-FileCopyrightText: 2022 Oxhead Alpha
+# SPDX-License-Identifier: LicenseRef-MIT-OA
+
+### Some convenient extensions and helpers ###
+
+class Danger::Dangerfile
+  # Unified `github`/`gitlab` variable.
+  def githost
+    if defined?(github)
+      githost = github
+    elsif defined?(gitlab)
+      githost = gitlab
+    else
+      error "Failed to figure out which service are we running from."
+    end
+  end
+
+  # The original PR title (excludes "Draft" tags).
+  def pr_title_payload
+    githost.mr_title
+      .sub(/^(Draft|WIP): /, "")
+      .sub(/^\[Draft\] /, "")
+  end
+  alias_method :mr_title_payload, :pr_title_payload
+end
+
+class Git::Diff::DiffFile
+  # When a file is renamed (e.g. with `git mv`) 'path' will return the old
+  # path, this is true even if the file was modified a little.
+  # However we'd probably like to access the destination path instead, so
+  # this parses the new path from the 'file.patch'.
+  def destination_path
+    rename_match = /(?<=(\nrename to ))(\S)*/.match(self.patch)
+    if rename_match.nil?
+      self.path
+    else
+      rename_match.to_s
+    end
+  end
+end
+
+# Add some helpers to Commit class.
+class Git::Object::Commit
+  # Commit subject (unlike the 'message' field which includes description).
+  def subject
+    self.message.lines.first.rstrip
+  end
+  alias_method :message_subject, :subject
+
+  def subject_ticked
+    "`" + self.subject.gsub("`", "'") + "`"
+  end
+
+  # Commit description.
+  # If absent, set to empty string.
+  def description
+    self.message.lines.drop(1).drop_while{ |s| s == "\n" }.join
+  end
+  alias_method :message_body, :description
+
+  # Whether there is a blank line between commit subject and body.
+  def blank_line_after_subject?
+    self.message.lines[1] == "\n"
+  end
+
+  # Whether this commit is fixup commit.
+  def fixup?
+    return /\bfixup!|\bsquash!/.match?(subject)
+  end
+
+  # Whether this commit is a temporary commit.
+  def wip?
+    return /\bwip\b|\btmp\b|\[temporary\]/i.match?(subject)
+  end
+
+  # Whether this commit is a minor chore commit.
+  # Such commits usually have an obvious purpose and are not related to the
+  # business logic.
+  def chore?
+    return subject.include?("[Chore]")
+  end
+
+end
+
+module Danger::Helpers::CommentsHelper
+  # By default, every comment for a particular source code also includes
+  # the name of the referred file.
+  #
+  # We don't need this feature.
+  # The source code welcomes us to override the respective method, and this is
+  # exactly what we do.
+  def markdown_link_to_message(_, _)
+    ""
+  end
+end
+
+# Example: `[Chore][#123] My commit`
+def issue_tags_pattern
+  /^(\[(#\d+|Chore)\])+ (?=\w)/
+end
+
+# Whether a string starts with an appropriate ticket tag.
+def has_valid_issue_tags(name)
+  return name.start_with?(issue_tags_pattern)
+end

danger/instant-checks.rb

@@ -0,0 +1,34 @@
+# SPDX-FileCopyrightText: 2022 Oxhead Alpha
+# SPDX-License-Identifier: LicenseRef-MIT-OA
+
+# Checks that, when hit, should be fixed as soon as possible.
+
+require_relative 'helpers'
+require_relative 'trailing-whitespaces'
+require_relative 'commit-style'
+require_relative 'branch-name'
+require_relative 'licenses'
+
+check_trailing_whitespaces()
+
+# Clean commits history
+if git.commits.any? { |c| c.subject =~ /^Merge branch/ }
+  fail 'Please, no merge commits. Rebase for the win.'
+end
+
+check_commit_style()
+
+# Proper MR content
+mr_title_payload = githost.mr_title_payload
+
+unless has_valid_issue_tags(mr_title_payload)
+  warn(
+    "Inappropriate title for PR.\n"\
+    "Should start from issue ID (e.g. `[#123]`) or `[Chore]` tag.\n"\
+    "Note: please use `[Chore]` also for tickets tracked internally on YouTrack."
+  )
+end
+
+check_branch_name()
+
+check_licenses()

danger/licenses.rb

@@ -0,0 +1,27 @@
+# SPDX-FileCopyrightText: 2022 Oxhead Alpha
+# SPDX-License-Identifier: LicenseRef-MIT-OA
+
+require_relative 'helpers'
+
+def check_licenses
+  # Licenses
+  # Check that the REUSE license header contains the current year.
+  cur_year = Time.new.year
+  # Only go over new files; see https://gitlab.com/morley-framework/morley/-/merge_requests/1091
+  # for the discussion and rationale for this.
+  git.added_files.each do |file|
+    File.foreach(file).with_index(1).find do |line, line_num|
+      if year_match = line.match(/(^.*SPDX-FileCopyrightText:)\s+(\w+-)?(\w+)\s+(.*)$/)
+        head, start, year, holder = year_match.captures
+        unless (year == cur_year.to_s)
+          markdown(
+            ":warning: The year in this license header is outdated, time to update!\n\n",
+            file: file, line: line_num
+          )
+        end
+        # either way, we return 'true' to stop looking after the first 'match'
+        true
+      end
+    end
+  end
+end

danger/premerge-checks.rb

@@ -0,0 +1,16 @@
+# SPDX-FileCopyrightText: 2022 Oxhead Alpha
+# SPDX-License-Identifier: LicenseRef-MIT-OA
+
+# Checks that are fine to fail during development, but must be fixed before merging.
+
+require_relative 'helpers'
+
+# Fixup commits
+if git.commits.any? &:fixup?
+  fail "Some fixup commits are still there."
+end
+
+# Work-in-progress commits
+if git.commits.any? &:wip?
+  fail "WIP commits are still there."
+end