Ensure we don't hang in Socket#accept due to spurious readiness.

It's possible, especially on dual stack, to have issues where a server
may become readable, but by the time accept is called, the connection is
gone. This can cause a deadlock between the semaphore and the accept call,
which can hang indefinitely.

Author: ioquatix

examples/limited/config.ru

@@ -20,8 +20,6 @@ run do |env|
 	# There is no guarantee that there is a connection or that the connection has a token:
 	token = limited_semaphore_token(request)
 
-	Console.info(self, "Sleeping 10 seconds", token: token)
-	
 	if env["PATH_INFO"] == "/fast"
 		if token
 			# Keeping the connection alive here is problematic because if the next request is slow, it will "block the server" since we have relinquished the token already.
@@ -30,10 +28,10 @@ run do |env|
 		end
 
 		# Simulated "fast / non-blocking" request:
-		sleep(1)
+		sleep(0.01)
 	else
 		# Simulated "slow / blocking" request:
-		sleep(10)
+		sleep(0.1)
 	end
 
 	[200, {}, ["Hello World"]]

examples/limited/limited.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Limited
 	# Thread local storage for the semaphore (per-worker):
 	Thread.attr_accessor :limited_semaphore
@@ -12,21 +14,34 @@ def self.instance
 		# Create a new semaphore with the given limit.
 		def initialize(limit = 1)
 			@queue = Thread::Queue.new
+			
+			Console.debug(self, "Initializing queue...", limit: limit)
 			limit.times{release}
 		end
 
 		# Release the semaphore.
 		def release
+			Console.debug(self, "Releasing semaphore...")
 			@queue.push(true)
 		end
 
 		# Acquire the semaphore. May block until the semaphore is available.
 		def acquire
+			Console.debug(self, "Acquiring semaphore...")
 			@queue.pop
-			
+			Console.debug(self, "Acquired semaphore...")
+
 			return Token.new(self)
 		end
 
+		def try_acquire
+			if @queue.empty?
+				return nil
+			else
+				return acquire
+			end
+		end
+		
 		# A token that can be used to release the semaphore once and once only.
 		class Token
 			def initialize(semaphore)
@@ -44,19 +59,42 @@ def release
 
 	# A wrapper implementation for the endpoint that limits the number of connections that can be accepted.
 	class Wrapper < IO::Endpoint::Wrapper
-		def socket_accept(server)
+		# Wait for an inbound connection to be ready to be accepted.
+		def wait_for_inbound_connection(server)
 			semaphore = Semaphore.instance
 
 			# Wait until there is a connection ready to be accepted:
-			server.wait_readable
+			while true
+				server.wait_readable
 
-			# Acquire the semaphore:
-			Console.info(self, "Acquiring semaphore...")
-			token = semaphore.acquire
-			
-			# Accept the connection:
-			socket, address = super
-			Console.info(self, "Accepted connection from #{address.inspect}", socket: socket)
+				# Acquire the semaphore:
+				if token = semaphore.try_acquire
+					return token
+				end
+			end
+		end
+		
+		# Once the server is readable and we've acquired the token, we can accept the connection (if it's still there).
+		def socket_accept_nonblock(server, token)
+			return server.accept_nonblock
+		rescue IO::WaitReadable, Errno::EINTR
+			token.release
+			return nil
+		end
+		
+		# Accept a connection from the server, limited by the per-worker (thread or process) semaphore.
+		def socket_accept(server)
+			while true
+				if token = wait_for_inbound_connection(server)
+					# In principle, there is a connection ready to be accepted:
+					socket, address = socket_accept_nonblock(server, token)
+					
+					if socket
+						Console.debug(self, "Accepted connection from #{address.inspect}", socket: socket)
+						break
+					end
+				end
+			end
 
 			# Provide access to the token, so that the connection limit could be released prematurely if it is determined that the request will not overload the server:
 			socket.define_singleton_method :token do
@@ -67,11 +105,14 @@ def socket_accept(server)
 			socket.define_singleton_method :close do
 				super()
 			ensure
-				Console.info(self, "Closing connection from #{address.inspect}", socket: socket)
+				Console.debug(self, "Releasing connection from #{address.inspect}", socket: socket)
 				token.release
 			end
 
+			success = true
 			return socket, address
+		ensure
+			token&.release unless success
 		end
 	end
 end