Merge pull request #262 from koic/stdio_client_transport
Add `MCP::Client::Stdio` transport
This commit is contained in:
Koichi ITO
GitHub
commit
09b4e30da6
2 changed files with 1079 additions and 4 deletions
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@ -1,5 +1,6 @@
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# frozen_string_literal: true
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require_relative "client/stdio"
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require_relative "client/http"
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require_relative "client/tool"
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222
lib/mcp/client/stdio.rb
Normal file
222
lib/mcp/client/stdio.rb
Normal file
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@ -0,0 +1,222 @@
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# frozen_string_literal: true
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require "json"
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require "open3"
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require "securerandom"
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require "timeout"
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require_relative "../../json_rpc_handler"
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require_relative "../configuration"
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require_relative "../methods"
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require_relative "../version"
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module MCP
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class Client
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class Stdio
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# Seconds to wait for the server process to exit before sending SIGTERM.
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# Matches the Python and TypeScript SDKs' shutdown timeout:
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# https://github.com/modelcontextprotocol/python-sdk/blob/v1.26.0/src/mcp/client/stdio/__init__.py#L48
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# https://github.com/modelcontextprotocol/typescript-sdk/blob/v1.27.1/src/client/stdio.ts#L221
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CLOSE_TIMEOUT = 2
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STDERR_READ_SIZE = 4096
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attr_reader :command, :args, :env
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def initialize(command:, args: [], env: nil, read_timeout: nil)
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@command = command
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@args = args
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@env = env
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@read_timeout = read_timeout
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@stdin = nil
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@stdout = nil
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@stderr = nil
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@wait_thread = nil
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@stderr_thread = nil
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@started = false
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@initialized = false
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end
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def send_request(request:)
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start unless @started
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initialize_session unless @initialized
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write_message(request)
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read_response(request)
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end
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def start
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raise "MCP::Client::Stdio already started" if @started
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spawn_env = @env || {}
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@stdin, @stdout, @stderr, @wait_thread = Open3.popen3(spawn_env, @command, *@args)
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@stdout.set_encoding("UTF-8")
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@stdin.set_encoding("UTF-8")
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# Drain stderr in the background to prevent the pipe buffer from filling up,
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# which would cause the server process to block and deadlock.
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@stderr_thread = Thread.new do
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loop do
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@stderr.readpartial(STDERR_READ_SIZE)
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end
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rescue IOError
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nil
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end
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@started = true
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rescue Errno::ENOENT, Errno::EACCES, Errno::ENOEXEC => e
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raise RequestHandlerError.new(
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"Failed to spawn server process: #{e.message}",
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{},
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error_type: :internal_error,
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original_error: e,
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)
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end
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def close
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return unless @started
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@stdin.close
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@stdout.close
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@stderr.close
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begin
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Timeout.timeout(CLOSE_TIMEOUT) { @wait_thread.value }
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rescue Timeout::Error
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begin
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Process.kill("TERM", @wait_thread.pid)
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Timeout.timeout(CLOSE_TIMEOUT) { @wait_thread.value }
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rescue Timeout::Error
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begin
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Process.kill("KILL", @wait_thread.pid)
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rescue Errno::ESRCH
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nil
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end
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rescue Errno::ESRCH
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nil
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end
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end
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@stderr_thread.join(CLOSE_TIMEOUT)
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@started = false
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@initialized = false
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end
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private
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# The client MUST send a protocol version it supports. This SHOULD be the latest version.
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# https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#version-negotiation
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#
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# Always sends `LATEST_STABLE_PROTOCOL_VERSION`, matching the Python and TypeScript SDKs:
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# https://github.com/modelcontextprotocol/python-sdk/blob/v1.26.0/src/mcp/client/session.py#L175
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# https://github.com/modelcontextprotocol/typescript-sdk/blob/v1.27.1/src/client/index.ts#L495
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def initialize_session
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init_request = {
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jsonrpc: JsonRpcHandler::Version::V2_0,
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id: SecureRandom.uuid,
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method: MCP::Methods::INITIALIZE,
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params: {
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protocolVersion: MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION,
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capabilities: {},
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clientInfo: { name: "mcp-ruby-client", version: MCP::VERSION },
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},
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}
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write_message(init_request)
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response = read_response(init_request)
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if response.key?("error")
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error = response["error"]
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raise RequestHandlerError.new(
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"Server initialization failed: #{error["message"]}",
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{ method: MCP::Methods::INITIALIZE },
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error_type: :internal_error,
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)
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end
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unless response.key?("result")
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raise RequestHandlerError.new(
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"Server initialization failed: missing result in response",
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{ method: MCP::Methods::INITIALIZE },
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error_type: :internal_error,
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)
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end
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notification = {
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jsonrpc: JsonRpcHandler::Version::V2_0,
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method: MCP::Methods::NOTIFICATIONS_INITIALIZED,
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}
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write_message(notification)
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@initialized = true
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end
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def write_message(message)
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ensure_running!
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json = JSON.generate(message)
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@stdin.puts(json)
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@stdin.flush
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rescue IOError, Errno::EPIPE => e
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raise RequestHandlerError.new(
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"Failed to write to server process",
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{},
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error_type: :internal_error,
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original_error: e,
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)
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end
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def read_response(request)
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request_id = request[:id] || request["id"]
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method = request[:method] || request["method"]
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params = request[:params] || request["params"]
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loop do
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ensure_running!
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wait_for_readable!(method, params) if @read_timeout
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line = @stdout.gets
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raise_connection_error!(method, params) if line.nil?
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parsed = JSON.parse(line.strip)
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next unless parsed.key?("id")
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return parsed if parsed["id"] == request_id
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end
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rescue JSON::ParserError => e
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raise RequestHandlerError.new(
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"Failed to parse server response",
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{ method: method, params: params },
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error_type: :internal_error,
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original_error: e,
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)
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end
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def ensure_running!
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return if @wait_thread.alive?
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raise RequestHandlerError.new(
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"Server process has exited",
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{},
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error_type: :internal_error,
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)
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end
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def wait_for_readable!(method, params)
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ready = @stdout.wait_readable(@read_timeout)
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return if ready
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raise RequestHandlerError.new(
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"Timed out waiting for server response",
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{ method: method, params: params },
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error_type: :internal_error,
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)
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end
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def raise_connection_error!(method, params)
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raise RequestHandlerError.new(
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"Server process closed stdout unexpectedly",
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{ method: method, params: params },
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error_type: :internal_error,
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)
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end
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end
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end
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end
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