Ensure docs gen inserts at correct place in file (#18250)

The regex for finding the latest release (as a fallback when current is not in
the file) had a bug. This caused the generated docs to be inserted at the wrong
place in the file (the end of the file) instead of the top. This commit fixes
the logic such that we find the last release when the current is not found.

(cherry picked from commit a3228e2)

# Conflicts:
#	tools/release/generate_release_notes_md.rb

Author: donoghuc

tools/release/generate_release_notes_md.rb

@@ -0,0 +1,136 @@
+#!/usr/bin/env ruby
+# Licensed to Elasticsearch B.V. under one or more contributor
+# license agreements. See the NOTICE file distributed with
+# this work for additional information regarding copyright
+# ownership. Elasticsearch B.V. licenses this file to you under
+# the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#  http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# Example:
+# ruby generate_release_notes.rb 6.4 6.4.1
+#
+# This:
+# * compares the lock file of two commits
+# * for each plugin version bumped show CHANGELOG.md of the bumped version
+require 'tempfile'
+require 'yaml'
+require 'json'
+require 'net/http'
+
+RELEASE_NOTES_PATH = "docs/release-notes/index.md"
+release_branch = ARGV[0]
+previous_release_tag = ARGV[1]
+user = ARGV[2]
+token = ARGV[3]
+report = []
+
+`git checkout #{release_branch}`
+
+current_release = YAML.load(IO.read("versions.yml"))["logstash"]
+
+release_notes = IO.read(RELEASE_NOTES_PATH).split("\n")
+
+coming_tag_index = release_notes.find_index {|line| line.match(/^## #{current_release} \[logstash-#{current_release}-release-notes\]$/) }
+coming_tag_index += 1 if coming_tag_index
+release_notes_entry_index = coming_tag_index || release_notes.find_index {|line| line.match(/^## .*\[logstash-.*-release-notes\]$/) }
+
+unless coming_tag_index
+  report << "## #{current_release} [logstash-#{current_release}-release-notes]\n\n"
+  report << "### Features and enhancements [logstash-#{current_release}-features-enhancements]\n"
+end
+
+plugin_changes = {}
+
+report <<  "---------- GENERATED CONTENT STARTS HERE ------------"
+report <<  "=== Logstash Pull Requests with label v#{current_release}\n"
+
+uri = URI.parse("https://api.github.com/search/issues?q=repo:elastic/logstash+is:pr+is:closed+label:v#{current_release}&sort=created&order=asc")
+pull_requests = JSON.parse(Net::HTTP.get(uri))
+pull_requests['items'].each do |prs|
+  report << "* #{prs['title']} #{prs['html_url']}[##{prs['number']}]"
+end
+report << ""
+
+report <<  "=== Logstash Commits between #{release_branch} and #{previous_release_tag}\n"
+report <<  "Computed with \"git log --pretty=format:'%h -%d %s (%cr) <%an>' --abbrev-commit --date=relative v#{previous_release_tag}..#{release_branch}\""
+report <<  ""
+logstash_prs = `git log --pretty=format:'%h -%d %s (%cr) <%an>' --abbrev-commit --date=relative v#{previous_release_tag}..#{release_branch}`
+report << logstash_prs
+report << "\n=== Logstash Plugin Release Changelogs ==="
+report << "Computed from \"git diff v#{previous_release_tag}..#{release_branch} *.release\""
+result = `git diff v#{previous_release_tag}..#{release_branch} *.release`.split("\n")
+
+result.each do |line|
+  # example "+    logstash-input-syslog (3.4.1)"
+  if match = line.match(/\+\s+(?<plugin>logstash-.+?-.+?)\s+\((?<version>\d+\.\d+.\d+).*?\)/)
+    plugin_changes[match[:plugin]] ||= []
+    plugin_changes[match[:plugin]] << match[:version]
+  elsif match = line.match(/\-\s+(?<plugin>logstash-.+?-.+?)\s+\((?<version>\d+\.\d+.\d+).*?\)/)
+    plugin_changes[match[:plugin]] ||= []
+    plugin_changes[match[:plugin]].unshift(match[:version])
+  else
+    # ..
+  end
+end
+report << "Changed plugin versions:"
+plugin_changes.each {|p, v| report << "#{p}: #{v.first} -> #{v.last}" }
+report << "---------- GENERATED CONTENT ENDS HERE ------------\n"
+
+report << "### Plugins [logstash-plugin-#{current_release}-changes]\n"
+
+plugin_changes.each do |plugin, versions|
+  _, type, name = plugin.split("-")
+  header = "**#{name.capitalize} #{type.capitalize} - #{versions.last}**"
+  start_changelog_file = Tempfile.new(plugin + 'start')
+  end_changelog_file = Tempfile.new(plugin + 'end')
+  changelog = `curl https://raw.githubusercontent.com/logstash-plugins/#{plugin}/v#{versions.last}/CHANGELOG.md`.split("\n")
+  report << "#{header}\n"
+  changelog.each do |line|
+    break if line.match(/^## #{versions.first}/)
+    next if line.match(/^##/)
+    line.gsub!(/^\+/, "")
+    line.gsub!(/ #(?<number>\d+)\s*$/, " https://github.com/logstash-plugins/#{plugin}/issues/\\k<number>[#\\k<number>]")
+    line.gsub!(/\[#(?<number>\d+)\]\((?<url>[^)]*)\)/, "[#\\k<number>](\\k<url>)")
+    line.gsub!(/^\s+-/, "*")
+    report << line
+  end
+  report << ""
+  start_changelog_file.unlink
+  end_changelog_file.unlink
+end
+
+release_notes.insert(release_notes_entry_index, report.join("\n").gsub(/\n{3,}/, "\n\n"))
+
+IO.write(RELEASE_NOTES_PATH, release_notes.join("\n"))
+
+if token.nil?
+  puts "No token provided, skipping commit and push"
+  exit
+end
+
+puts "Creating commit.."
+branch_name = "update_release_notes_#{Time.now.to_i}"
+`git checkout -b #{branch_name}`
+`git commit #{RELEASE_NOTES_PATH} -m "Update release notes for #{current_release}"`
+
+puts "Pushing commit.."
+`git remote set-url origin https://x-access-token:#{token}@github.com/elastic/logstash.git`
+`git push origin #{branch_name}`
+
+puts "Creating Pull Request"
+pr_title = "Release notes for #{current_release}"
+result = `curl -H "Authorization: token #{token}" -d '{"title":"#{pr_title}","base":"#{release_branch}", "head":"#{branch_name}", "draft": true}' https://api.github.com/repos/elastic/logstash/pulls`
+puts result
+pr_number = JSON.parse(result)["number"]
+puts `curl -X POST -H "Accept: application/vnd.github.v3+json" -H "Authorization: token #{token}" https://api.github.com/repos/elastic/logstash/issues/#{pr_number}/assignees -d '{"assignees":["#{user}"]}'`
+puts "Done"