Add a configuration option that prevents puma from queueing requests.

With queue_requests set to true (the default), workers accept all
requests and queue them before passing them to the handlers.
With it set to false, each worker process accepts exactly as
many requests as it is configured to simultaneously handle.

In combination with threads 1, 1 this ensures that requests
are balanced across workers in a single threaded application.
This can avoid deadlocks when a single threaded app sends a
request to itself. (For example, to generate a PDF.)

lib/puma/client.rb

@@ -222,6 +222,14 @@ module Puma
       end
     end # IS_JRUBY
 
+    def finish
+      return true if @ready
+      until try_to_finish
+        IO.select([@to_io], nil, nil)
+      end
+      true
+    end
+
     def read_body
       # Read an odd sized chunk so we can read even sized ones
       # after this

lib/puma/configuration.rb

@@ -27,6 +27,7 @@ module Puma
       @options[:after_worker_boot] ||= []
       @options[:worker_timeout] ||= DefaultWorkerTimeout
       @options[:worker_shutdown_timeout] ||= DefaultWorkerShutdownTimeout
+      @options[:queue_requests] ||= true
     end
 
     attr_reader :options
@@ -402,6 +403,23 @@ module Puma
       def worker_shutdown_timeout(timeout)
         @options[:worker_shutdown_timeout] = timeout
       end
+
+      # When set to true (the default), workers accept all requests
+      # and queue them before passing them to the handlers.
+      # When set to false, each worker process accepts exactly as
+      # many requests as it is configured to simultaneously handle.
+      #
+      # Queueing requests generally improves performance. In some
+      # cases, such as a single threaded application, it may be
+      # better to ensure requests get balanced across workers.
+      #
+      # Note that setting this to false disables HTTP keepalive and
+      # slow clients will occupy a handler thread while the request
+      # is being sent. A reverse proxy, such as nginx, can handle
+      # slow clients and queue requests before they reach puma.
+      def queue_requests(answer=true)
+        @options[:queue_requests] = answer
+      end
     end
   end
 end

lib/puma/server.rb

@@ -241,7 +241,12 @@ module Puma
         process_now = false
 
         begin
-          process_now = client.eagerly_finish
+          if @options[:queue_requests]
+            process_now = client.eagerly_finish
+          else
+            client.finish
+            process_now = true
+          end
         rescue HttpParserError => e
           client.write_400
           client.close
@@ -261,9 +266,10 @@ module Puma
 
       @thread_pool.clean_thread_locals = @options[:clean_thread_locals]
 
-      @reactor = Reactor.new self, @thread_pool
+      if @options[:queue_requests]
-
+        @reactor = Reactor.new self, @thread_pool
-      @reactor.run_in_thread
+        @reactor.run_in_thread
+      end
 
       if @auto_trim_time
         @thread_pool.auto_trim!(@auto_trim_time)
@@ -296,6 +302,7 @@ module Puma
                   if io = sock.accept_nonblock
                     client = Client.new io, @binder.env(sock)
                     pool << client
+                    pool.wait_until_not_full unless @options[:queue_requests]
                   end
                 rescue SystemCallError
                 end
@@ -312,9 +319,10 @@ module Puma
         @events.fire :state, @status
 
         graceful_shutdown if @status == :stop || @status == :restart
-        @reactor.clear! if @status == :restart
+        if @options[:queue_requests]
-
+          @reactor.clear! if @status == :restart
-        @reactor.shutdown
+          @reactor.shutdown
+        end
       rescue Exception => e
         STDERR.puts "Exception handling servers: #{e.message} (#{e.class})"
         STDERR.puts e.backtrace
@@ -367,6 +375,7 @@ module Puma
             close_socket = false
             return
           when true
+            return unless @options[:queue_requests]
             buffer.reset
 
             unless client.reset(@status == :run)

lib/puma/thread_pool.rb

@@ -12,7 +12,8 @@ module Puma
     # thread.
     #
     def initialize(min, max, *extra, &block)
-      @cond = ConditionVariable.new
+      @not_empty = ConditionVariable.new
+      @not_full = ConditionVariable.new
       @mutex = Mutex.new
 
       @todo = []
@@ -60,7 +61,8 @@ module Puma
         todo  = @todo
         block = @block
         mutex = @mutex
-        cond  = @cond
+        not_empty = @not_empty
+        not_full = @not_full
 
         extra = @extra.map { |i| i.new }
 
@@ -83,10 +85,11 @@ module Puma
               end
 
               @waiting += 1
-              cond.wait mutex
+              not_full.signal
-              @waiting -= 1
+              not_empty.wait mutex
             end
 
+            @waiting -= 1
             work = todo.shift if continue
           end
 
@@ -125,9 +128,18 @@ module Puma
 
         if @waiting < @todo.size and @spawned < @max
           spawn_thread
+          @waiting+=1
         end
 
-        @cond.signal
+        @not_empty.signal
+      end
+    end
+
+    def wait_until_not_full
+      @mutex.synchronize do
+        until @todo.size - @waiting < @max - @spawned or @shutdown
+          @not_full.wait @mutex
+        end
       end
     end
 
@@ -139,7 +151,7 @@ module Puma
       @mutex.synchronize do
         if (force or @waiting > 0) and @spawned - @trim_requested > @min
           @trim_requested += 1
-          @cond.signal
+          @not_empty.signal
         end
       end
     end
@@ -178,7 +190,8 @@ module Puma
     def shutdown
       @mutex.synchronize do
         @shutdown = true
-        @cond.broadcast
+        @not_empty.broadcast
+        @not_full.broadcast
 
         @auto_trim.stop if @auto_trim
       end