discourse/script/backport.rb

# frozen_string_literal: true

require "json"
require "open3"

RunResult = Data.define(:success, :stdout, :stderr)

def run(*cmd, allow_failure: false)
  puts "Running: #{cmd.join(" ")}"
  stdout, stderr, status = Open3.capture3(*cmd)
  puts stdout unless stdout.empty?
  puts stderr unless stderr.empty?
  raise "Command failed: #{cmd.join(" ")}\n#{stderr}" unless status.success? || allow_failure
  RunResult.new(success: status.success?, stdout: stdout.strip, stderr: stderr.strip)
end

def gh(*args, allow_failure: false)
  run("gh", *args, allow_failure: allow_failure)
end

# Quote a string as a single-line bash argument using ANSI-C ($'...') quoting,
# so embedded newlines become a literal \n instead of wrapping the command.
def bash_quote(str)
  escaped = str.gsub(/\r\n?/, "\n").gsub(/[\\'\n]/, "\\" => "\\\\", "'" => "\\'", "\n" => "\\n")
  "$'#{escaped}'"
end

pr_number = ENV.fetch("PR_NUMBER")

# Resolve the repo so manual instructions target the same one this script runs against
# (it runs in both discourse/discourse and discourse/discourse-private-mirror).
repo = gh("repo", "view", "--json", "nameWithOwner", "-q", ".nameWithOwner").stdout.strip
repo_url = "git@github.com:#{repo}"

# Get PR details (title, body, base branch)
pr = JSON.parse(gh("pr", "view", pr_number, "--json", "title,body,baseRefName,mergeCommit").stdout)

pr_title = pr["title"]
pr_body = pr["body"] || ""
base_branch = pr["baseRefName"]
merge_commit = pr.dig("mergeCommit", "oid")

puts "PR ##{pr_number}: #{pr_title}"
puts "Base branch: #{base_branch}"

# Fetch the PR head using GitHub's magic ref (works for forks too)
pr_ref = "refs/pull/#{pr_number}/head"
run("git", "fetch", "origin", "#{pr_ref}:pr-head")

# Fetch the base branch
run("git", "fetch", "origin", base_branch)

# Find merge base between PR head and base branch
merge_base = run("git", "merge-base", "pr-head", "origin/#{base_branch}").stdout
pr_head = run("git", "rev-parse", "pr-head").stdout
puts "Merge base: #{merge_base}"

# Read versions.json from main branch to find backport targets
versions = JSON.parse(run("git", "show", "origin/main:versions.json").stdout)

# Optional target versions can be supplied, space- or comma-separated, each with
# an optional "v" prefix: "@discoursebot backport 2026.5, v2026.4"
target_versions =
  ENV["COMMENT_BODY"].to_s[/@discoursebot\s+backport\b(.*)/i, 1]
    .to_s
    .split(/[\s,]+/)
    .reject(&:empty?)
    .map { |v| v.sub(/\Av/i, "") }

backport_versions =
  if target_versions.any?
    unknown = target_versions.reject { |v| versions.key?(v) }
    if unknown.any?
      gh(
        "pr",
        "comment",
        pr_number,
        "--body",
        "Unknown version(s): #{unknown.join(", ")}. Known versions: #{versions.keys.sort.reverse.join(", ")}.",
      )
      exit 0
    end
    target_versions.uniq.sort.reverse
  else
    # Find versions that are both supported and released
    versions
      .select { |version, info| info["supported"] == true && info["released"] == true }
      .keys
      .sort
      .reverse
  end

puts "Will backport to versions: #{backport_versions.join(", ")}"

if backport_versions.empty?
  gh(
    "pr",
    "comment",
    pr_number,
    "--body",
    "No supported and released versions found to backport to.",
  )
  exit 0
end

results = []

backport_versions.each do |version|
  release_branch = "release/#{version}"
  backport_branch = "backport/#{version}/#{pr_number}"
  backport_title = "#{pr_title} [backport #{version}]"
  backport_body = <<~BODY
    Backport of ##{pr_number} to #{release_branch}.

    ---

    #{pr_body}
  BODY

  puts "\n--- Backporting to #{release_branch} ---"

  # Check if release branch exists
  if !run("git", "ls-remote", "--heads", "origin", release_branch).stdout.include?(release_branch)
    puts "Release branch #{release_branch} does not exist, skipping"
    results << { version: version, success: false, error: "Release branch does not exist" }
    next
  end

  # Fetch the release branch
  run("git", "fetch", "origin", release_branch)

  # Create backport branch from release branch
  run("git", "checkout", "-B", backport_branch, "origin/#{release_branch}")

  # Cherry-pick the commits
  cherry_pick_range =
    if merge_commit
      merge_commit
    else
      "#{merge_base}..#{pr_head}"
    end

  puts "Cherry-picking #{cherry_pick_range}..."
  result = run("git", "cherry-pick", cherry_pick_range, allow_failure: true)

  unless result.success
    puts "Failed to backport to #{version}:\n#{result.stderr}"
    results << {
      version: version,
      success: false,
      error: result.stderr,
      release_branch: release_branch,
      backport_branch: backport_branch,
      cherry_pick_range: cherry_pick_range,
      backport_title: backport_title,
      backport_body: backport_body,
    }
    run("git", "cherry-pick", "--abort", allow_failure: true)
    run("git", "checkout", "main", allow_failure: true)
    next
  end

  # Push the backport branch
  run("git", "push", "-f", "origin", backport_branch)

  # Create or update PR
  # Try to create the PR
  created =
    gh(
      "pr",
      "create",
      "--base",
      release_branch,
      "--head",
      backport_branch,
      "--title",
      backport_title,
      "--body",
      backport_body,
      allow_failure: true,
    ).success

  if created
    # Get the PR URL for the newly created PR
    pr_url = gh("pr", "view", backport_branch, "--json", "url", "-q", ".url").stdout.strip
    puts "Created PR: #{pr_url}"
  else
    # PR already exists, update it
    puts "PR already exists, updating..."
    gh("pr", "edit", backport_branch, "--title", backport_title, "--body", backport_body)
    pr_url = gh("pr", "view", backport_branch, "--json", "url", "-q", ".url").stdout.strip
    puts "Updated PR: #{pr_url}"
  end

  results << { version: version, success: true, pr_url: pr_url }

  # Return to main branch for next iteration
  run("git", "checkout", "main", allow_failure: true)
end

# Post summary comment
successful = results.select { |r| r[:success] }
failed = results.reject { |r| r[:success] }

comment_lines = ["## Backport Results\n"]

if successful.any?
  comment_lines << "### Successful backports"
  successful.each { |r| comment_lines << "- #{r[:version]}: #{r[:pr_url]}" }
  comment_lines << ""
end

if failed.any?
  comment_lines << "### Failed backports"
  failed.each do |r|
    if r[:cherry_pick_range]
      gh_create =
        "gh pr create --repo #{repo} --base #{r[:release_branch]} --head #{r[:backport_branch]} " \
          "--title #{bash_quote(r[:backport_title])} --body #{bash_quote(r[:backport_body])}"

      comment_lines << <<~MSG
        #### #{r[:version]}
        ```
        #{r[:error]}
        ```

        To resolve manually:
        ```bash
        git fetch #{repo_url} #{r[:release_branch]}
        git checkout -B #{r[:backport_branch]} FETCH_HEAD
        git cherry-pick #{r[:cherry_pick_range]}

        # Resolve the conflicts, then push the branch and open the PR:
        git push -f #{repo_url} #{r[:backport_branch]}:#{r[:backport_branch]}
        #{gh_create}
        ```
      MSG
    else
      comment_lines << "- **#{r[:version]}**: #{r[:error]}"
    end
  end
end

comment_lines << "No backports were attempted." if successful.empty? && failed.empty?

gh("pr", "comment", pr_number, "--body", comment_lines.join("\n"))
puts "\nBackport complete!"