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Class: ActionDispatch::DebugLocks

Relationships & Source Files
Inherits: Object
Defined in: actionpack/lib/action_dispatch/middleware/debug_locks.rb

Overview

This middleware can be used to diagnose deadlocks in the autoload interlock.

To use it, insert it near the top of the middleware stack, using config/application.rb:

config.middleware.insert_before Rack::Sendfile, ActionDispatch::DebugLocks

After restarting the application and re-triggering the deadlock condition, the route /rails/locks will show a summary of all threads currently known to the interlock, which lock level they are holding or awaiting, and their current backtrace.

Generally a deadlock will be caused by the interlock conflicting with some other external lock or blocking I/O call. These cannot be automatically identified, but should be visible in the displayed backtraces.

NOTE: The formatting and content of this middleware’s output is intended for human consumption, and should be expected to change between releases.

This middleware exposes operational details of the server, with no access control. It should only be enabled when in use, and removed thereafter.

Class Method Summary

Instance Method Summary

Constructor Details

.new(app, path = "/rails/locks") ⇒ DebugLocks

[ GitHub ] 

  


# File 'actionpack/lib/action_dispatch/middleware/debug_locks.rb', line 30



def initialize(app, path = "/rails/locks")
  @app = app
  @path = path
end


  







Instance Method Details



  

    #blocked_by?(victim, blocker, all_threads)  ⇒ Boolean  (private)
  



  

    
  





  


  [ GitHub ]


  

  


# File 'actionpack/lib/action_dispatch/middleware/debug_locks.rb', line 108



def blocked_by?(victim, blocker, all_threads)
  return false if victim.equal?(blocker)

  case victim[:sleeper]
  when :start_sharing
    blocker[:exclusive] ||
      (!victim[:waiting] && blocker[:compatible] && !blocker[:compatible].include?(false))
  when :start_exclusive
    blocker[:sharing] > 0 ||
      blocker[:exclusive] ||
      (blocker[:compatible] && !blocker[:compatible].include?(victim[:purpose]))
  when :yield_shares
    blocker[:exclusive]
  when :stop_exclusive
    blocker[:exclusive] ||
      victim[:compatible] &&
      victim[:compatible].include?(blocker[:purpose]) &&
      all_threads.all? { |other| !other[:compatible] || blocker.equal?(other) || other[:compatible].include?(blocker[:purpose]) }
  end
end


  








  

    #call(env)  
  

  [ GitHub ]


  

  


# File 'actionpack/lib/action_dispatch/middleware/debug_locks.rb', line 35



def call(env)
  req = ActionDispatch::Request.new env

  if req.get?
    path = req.path_info.chomp("/")
    if path == @path
      return render_details(req)
    end
  end

  @app.call(env)
end


  








  

    #render_details(req)   (private)
  

  [ GitHub ]


  

  


# File 'actionpack/lib/action_dispatch/middleware/debug_locks.rb', line 49



def render_details(req)
  threads = ActiveSupport::Dependencies.interlock.raw_state do |raw_threads|
    # The Interlock itself comes to a complete halt as long as this block is
    # executing. That gives us a more consistent picture of everything, but creates
    # a pretty strong Observer Effect.
    #
    # Most directly, that means we need to do as little as possible in this block.
    # More widely, it means this middleware should remain a strictly diagnostic tool
    # (to be used when something has gone wrong), and not for any sort of general
    # monitoring.

    raw_threads.each.with_index do |(thread, info), idx|
      info[:index] = idx
      info[:backtrace] = thread.backtrace
    end

    raw_threads
  end

  str = threads.map do |thread, info|
    if info[:exclusive]
      lock_state = +"Exclusive"
    elsif info[:sharing] > 0
      lock_state = +"Sharing"
      lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
    else
      lock_state = +"No lock"
    end

    if info[:waiting]
      lock_state << " (yielded share)"
    end

    msg = +"Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"

    if info[:sleeper]
      msg << "  Waiting in #{info[:sleeper]}"
      msg << " to #{info[:purpose].to_s.inspect}" unless info[:purpose].nil?
      msg << "\n"

      if info[:compatible]
        compat = info[:compatible].map { |c| c == false ? "share" : c.to_s.inspect }
        msg << "  may be pre-empted for: #{compat.join(', ')}\n"
      end

      blockers = threads.values.select { |binfo| blocked_by?(info, binfo, threads.values) }
      msg << "  blocked by: #{blockers.map { |i| i[:index] }.join(', ')}\n" if blockers.any?
    end

    blockees = threads.values.select { |binfo| blocked_by?(binfo, info, threads.values) }
    msg << "  blocking: #{blockees.map { |i| i[:index] }.join(', ')}\n" if blockees.any?

    msg << "\n#{info[:backtrace].join("\n")}\n" if info[:backtrace]
  end.join("\n\n---\n\n\n")

  [200, { Rack::CONTENT_TYPE => "text/plain; charset=#{ActionDispatch::Response.default_charset}",
          Rack::CONTENT_LENGTH => str.size.to_s }, [str]]
end