auto_review_pr: suggest re-filing PRs from fork branches

PRs filed from branches in ruby/ruby (rather than forks) make it
harder to manage the bi-directional mirror with git.ruby-lang.org.
Add an auto-review comment asking contributors to re-file from a fork.

Refactor #review into separate methods for each review type.

Author: k0kubun

tool/auto_review_pr.rb

@@ -34,46 +34,89 @@ class AutoReviewPR
   REPO = 'ruby/ruby'
 
   COMMENT_USER = 'github-actions[bot]'
-  COMMENT_PREFIX = 'The following files are maintained in the following upstream repositories:'
-  COMMENT_SUFFIX = 'Please file a pull request to the above instead. Thank you!'
+
+  UPSTREAM_COMMENT_PREFIX = 'The following files are maintained in the following upstream repositories:'
+  UPSTREAM_COMMENT_SUFFIX = 'Please file a pull request to the above instead. Thank you!'
+
+  FORK_COMMENT_PREFIX = 'It looks like this pull request was filed from a branch in ruby/ruby.'
+  FORK_COMMENT_BODY = <<~COMMENT
+    #{FORK_COMMENT_PREFIX}
+
+    Since ruby/ruby is bi-directionally mirrored with the official git repository at git.ruby-lang.org, \
+    having topic branches in ruby/ruby makes it harder to manage the mirror.
+
+    Could you please close this pull request and re-file it from a branch in your personal fork instead? \
+    You can fork https://github.com/ruby/ruby, push your branch there, and open a new pull request from it.
+
+    Thank you for your contribution!
+  COMMENT
 
   def initialize(client)
     @client = client
   end
 
   def review(pr_number)
-    # Fetch the list of files changed by the PR
-    changed_files = @client.get("/repos/#{REPO}/pulls/#{pr_number}/files").map { it.fetch(:filename) }
+    existing_comments = fetch_existing_comments(pr_number)
+    review_non_fork_branch(pr_number, existing_comments)
+    review_upstream_repos(pr_number, existing_comments)
+  end
 
-    # Build a Hash: { upstream_repo => files, ... }
-    upstream_repos = SyncDefaultGems::Repository.group(changed_files)
-    upstream_repos.delete(nil) # exclude no-upstream files
-    upstream_repos.delete('prism') if changed_files.include?('prism_compile.c') # allow prism changes in this case
-    if upstream_repos.empty?
-      puts "Skipped: The PR ##{pr_number} doesn't have upstream repositories."
+  private
+
+  def fetch_existing_comments(pr_number)
+    comments = @client.get("/repos/#{REPO}/issues/#{pr_number}/comments")
+    comments.map { [it.fetch(:user).fetch(:login), it.fetch(:body)] }
+  end
+
+  def already_commented?(existing_comments, prefix)
+    existing_comments.any? { |user, comment| user == COMMENT_USER && comment.start_with?(prefix) }
+  end
+
+  def post_comment(pr_number, comment)
+    result = @client.post("/repos/#{REPO}/issues/#{pr_number}/comments", { body: comment })
+    puts "Success: #{JSON.pretty_generate(result)}"
+  end
+
+  # Suggest re-filing from a fork if the PR branch is in ruby/ruby itself
+  def review_non_fork_branch(pr_number, existing_comments)
+    if already_commented?(existing_comments, FORK_COMMENT_PREFIX)
+      puts "Skipped: The PR ##{pr_number} already has a fork branch comment."
       return
     end
 
-    # Check if the PR is already reviewed
-    existing_comments = @client.get("/repos/#{REPO}/issues/#{pr_number}/comments")
-    existing_comments.map! { [it.fetch(:user).fetch(:login), it.fetch(:body)] }
-    if existing_comments.any? { |user, comment| user == COMMENT_USER && comment.start_with?(COMMENT_PREFIX) }
-      puts "Skipped: The PR ##{pr_number} already has an automated review comment."
+    pr = @client.get("/repos/#{REPO}/pulls/#{pr_number}")
+    head_repo = pr.dig(:head, :repo, :full_name)
+    if head_repo != REPO
+      puts "Skipped: The PR ##{pr_number} is already from a fork (#{head_repo})."
       return
     end
 
-    # Post a comment
-    comment = format_comment(upstream_repos)
-    result = @client.post("/repos/#{REPO}/issues/#{pr_number}/comments", { body: comment })
-    puts "Success: #{JSON.pretty_generate(result)}"
+    post_comment(pr_number, FORK_COMMENT_BODY)
   end
 
-  private
+  # Suggest filing PRs to upstream repositories for files that have one
+  def review_upstream_repos(pr_number, existing_comments)
+    if already_commented?(existing_comments, UPSTREAM_COMMENT_PREFIX)
+      puts "Skipped: The PR ##{pr_number} already has an upstream repos comment."
+      return
+    end
+
+    changed_files = @client.get("/repos/#{REPO}/pulls/#{pr_number}/files").map { it.fetch(:filename) }
+
+    upstream_repos = SyncDefaultGems::Repository.group(changed_files)
+    upstream_repos.delete(nil)
+    upstream_repos.delete('prism') if changed_files.include?('prism_compile.c')
+    if upstream_repos.empty?
+      puts "Skipped: The PR ##{pr_number} doesn't have upstream repositories."
+      return
+    end
+
+    post_comment(pr_number, format_upstream_comment(upstream_repos))
+  end
 
-  # upstream_repos: { upstream_repo => files, ... }
-  def format_comment(upstream_repos)
+  def format_upstream_comment(upstream_repos)
     comment = +''
-    comment << "#{COMMENT_PREFIX}\n\n"
+    comment << "#{UPSTREAM_COMMENT_PREFIX}\n\n"
 
     upstream_repos.each do |upstream_repo, files|
       comment << "* https://github.com/ruby/#{upstream_repo}\n"
@@ -82,7 +125,7 @@ def format_comment(upstream_repos)
       end
     end
 
-    comment << "\n#{COMMENT_SUFFIX}"
+    comment << "\n#{UPSTREAM_COMMENT_SUFFIX}"
     comment
   end
 end