Add request pool to improve delivery performance (#10353)

* Add request pool to improve delivery performance

Fix #7909

* Ensure connection is closed when exception interrupts execution

* Remove Timeout#timeout from socket connection

* Fix infinite retrial loop on HTTP::ConnectionError

* Close sockets on failure, reduce idle time to 90 seconds

* Add MAX_REQUEST_POOL_SIZE option to limit concurrent connections to the same server

* Use a shared pool size, 512 by default, to stay below open file limit

* Add some tests

* Add more tests

* Reduce MAX_IDLE_TIME from 90 to 30 seconds, reap every 30 seconds

* Use a shared pool that returns preferred connection but re-purposes other ones when needed

* Fix wrong connection being returned on subsequent calls within the same thread

* Reduce mutex calls on flushes from 2 to 1 and add test for reaping

4 files changed, 323 insertions, 19 deletions

diff --git a/app/lib/connection_pool/shared_connection_pool.rb b/app/lib/connection_pool/shared_connection_pool.rb
new file mode 100644
index 000000000..2865a4108
--- /dev/null
+++ b/app/lib/connection_pool/shared_connection_pool.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+require 'connection_pool'
+require_relative './shared_timed_stack'
+
+class ConnectionPool::SharedConnectionPool < ConnectionPool
+  def initialize(options = {}, &block)
+    super(options, &block)
+
+    @available = ConnectionPool::SharedTimedStack.new(@size, &block)
+  end
+
+  delegate :size, :flush, to: :@available
+
+  def with(preferred_tag, options = {})
+    Thread.handle_interrupt(Exception => :never) do
+      conn = checkout(preferred_tag, options)
+
+      begin
+        Thread.handle_interrupt(Exception => :immediate) do
+          yield conn
+        end
+      ensure
+        checkin(preferred_tag)
+      end
+    end
+  end
+
+  def checkout(preferred_tag, options = {})
+    if ::Thread.current[key(preferred_tag)]
+      ::Thread.current[key_count(preferred_tag)] += 1
+      ::Thread.current[key(preferred_tag)]
+    else
+      ::Thread.current[key_count(preferred_tag)] = 1
+      ::Thread.current[key(preferred_tag)] = @available.pop(preferred_tag, options[:timeout] || @timeout)
+    end
+  end
+
+  def checkin(preferred_tag)
+    if ::Thread.current[key(preferred_tag)]
+      if ::Thread.current[key_count(preferred_tag)] == 1
+        @available.push(::Thread.current[key(preferred_tag)])
+        ::Thread.current[key(preferred_tag)] = nil
+      else
+        ::Thread.current[key_count(preferred_tag)] -= 1
+      end
+    else
+      raise ConnectionPool::Error, 'no connections are checked out'
+    end
+
+    nil
+  end
+
+  private
+
+  def key(tag)
+    :"#{@key}-#{tag}"
+  end
+
+  def key_count(tag)
+    :"#{@key_count}-#{tag}"
+  end
+end
diff --git a/app/lib/connection_pool/shared_timed_stack.rb b/app/lib/connection_pool/shared_timed_stack.rb
new file mode 100644
index 000000000..14a5285c4
--- /dev/null
+++ b/app/lib/connection_pool/shared_timed_stack.rb
@@ -0,0 +1,95 @@
+# frozen_string_literal: true
+
+class ConnectionPool::SharedTimedStack
+  def initialize(max = 0, &block)
+    @create_block = block
+    @max          = max
+    @created      = 0
+    @queue        = []
+    @tagged_queue = Hash.new { |hash, key| hash[key] = [] }
+    @mutex        = Mutex.new
+    @resource     = ConditionVariable.new
+  end
+
+  def push(connection)
+    @mutex.synchronize do
+      store_connection(connection)
+      @resource.broadcast
+    end
+  end
+
+  alias << push
+
+  def pop(preferred_tag, timeout = 5.0)
+    deadline = current_time + timeout
+
+    @mutex.synchronize do
+      loop do
+        return fetch_preferred_connection(preferred_tag) unless @tagged_queue[preferred_tag].empty?
+
+        connection = try_create(preferred_tag)
+        return connection if connection
+
+        to_wait = deadline - current_time
+        raise Timeout::Error, "Waited #{timeout} sec" if to_wait <= 0
+
+        @resource.wait(@mutex, to_wait)
+      end
+    end
+  end
+
+  def empty?
+    size.zero?
+  end
+
+  def size
+    @mutex.synchronize do
+      @queue.size
+    end
+  end
+
+  def flush
+    @mutex.synchronize do
+      @queue.delete_if do |connection|
+        delete = !connection.in_use && (connection.dead || connection.seconds_idle >= RequestPool::MAX_IDLE_TIME)
+
+        if delete
+          @tagged_queue[connection.site].delete(connection)
+          connection.close
+          @created -= 1
+        end
+
+        delete
+      end
+    end
+  end
+
+  private
+
+  def try_create(preferred_tag)
+    if @created == @max && !@queue.empty?
+      throw_away_connection = @queue.pop
+      @tagged_queue[throw_away_connection.site].delete(throw_away_connection)
+      @create_block.call(preferred_tag)
+    elsif @created != @max
+      connection = @create_block.call(preferred_tag)
+      @created += 1
+      connection
+    end
+  end
+
+  def fetch_preferred_connection(preferred_tag)
+    connection = @tagged_queue[preferred_tag].pop
+    @queue.delete(connection)
+    connection
+  end
+
+  def current_time
+    Process.clock_gettime(Process::CLOCK_MONOTONIC)
+  end
+
+  def store_connection(connection)
+    @tagged_queue[connection.site].push(connection)
+    @queue.push(connection)
+  end
+end
diff --git a/app/lib/request.rb b/app/lib/request.rb
index e555ae6a1..af49d6c77 100644
--- a/app/lib/request.rb
+++ b/app/lib/request.rb
@@ -17,15 +17,21 @@ end
 class Request
   REQUEST_TARGET = '(request-target)'
 
+  # We enforce a 5s timeout on DNS resolving, 5s timeout on socket opening
+  # and 5s timeout on the TLS handshake, meaning the worst case should take
+  # about 15s in total
+  TIMEOUT = { connect: 5, read: 10, write: 10 }.freeze
+
   include RoutingHelper
 
   def initialize(verb, url, **options)
     raise ArgumentError if url.blank?
 
-    @verb    = verb
-    @url     = Addressable::URI.parse(url).normalize
-    @options = options.merge(use_proxy? ? Rails.configuration.x.http_client_proxy : { socket_class: Socket })
-    @headers = {}
+    @verb        = verb
+    @url         = Addressable::URI.parse(url).normalize
+    @http_client = options.delete(:http_client)
+    @options     = options.merge(use_proxy? ? Rails.configuration.x.http_client_proxy : { socket_class: Socket })
+    @headers     = {}
 
     raise Mastodon::HostValidationError, 'Instance does not support hidden service connections' if block_hidden_service?
 
@@ -50,15 +56,24 @@ class Request
 
   def perform
     begin
-      response = http_client.headers(headers).public_send(@verb, @url.to_s, @options)
+      response = http_client.public_send(@verb, @url.to_s, @options.merge(headers: headers))
     rescue => e
       raise e.class, "#{e.message} on #{@url}", e.backtrace[0]
     end
 
     begin
-      yield response.extend(ClientLimit) if block_given?
+      response = response.extend(ClientLimit)
+
+      # If we are using a persistent connection, we have to
+      # read every response to be able to move forward at all.
+      # However, simply calling #to_s or #flush may not be safe,
+      # as the response body, if malicious, could be too big
+      # for our memory. So we use the #body_with_limit method
+      response.body_with_limit if http_client.persistent?
+
+      yield response if block_given?
     ensure
-      http_client.close
+      http_client.close unless http_client.persistent?
     end
   end
 
@@ -76,6 +91,10 @@ class Request
 
       %w(http https).include?(parsed_url.scheme) && parsed_url.host.present?
     end
+
+    def http_client
+      HTTP.use(:auto_inflate).timeout(:per_operation, TIMEOUT.dup).follow(max_hops: 2)
+    end
   end
 
   private
@@ -116,16 +135,8 @@ class Request
     end
   end
 
-  def timeout
-    # We enforce a 1s timeout on DNS resolving, 10s timeout on socket opening
-    # and 5s timeout on the TLS handshake, meaning the worst case should take
-    # about 16s in total
-
-    { connect: 5, read: 10, write: 10 }
-  end
-
   def http_client
-    @http_client ||= HTTP.use(:auto_inflate).timeout(:per_operation, timeout).follow(max_hops: 2)
+    @http_client ||= Request.http_client
   end
 
   def use_proxy?
@@ -169,20 +180,41 @@ class Request
         return super(host, *args) if thru_hidden_service?(host)
 
         outer_e = nil
+        port    = args.first
 
         Resolv::DNS.open do |dns|
           dns.timeouts = 5
 
           addresses = dns.getaddresses(host).take(2)
-          time_slot = 10.0 / addresses.size
 
           addresses.each do |address|
             begin
               raise Mastodon::HostValidationError if PrivateAddressCheck.private_address?(IPAddr.new(address.to_s))
 
-              ::Timeout.timeout(time_slot, HTTP::TimeoutError) do
-                return super(address.to_s, *args)
+              sock     = ::Socket.new(::Socket::AF_INET, ::Socket::SOCK_STREAM, 0)
+              sockaddr = ::Socket.pack_sockaddr_in(port, address.to_s)
+
+              sock.setsockopt(::Socket::IPPROTO_TCP, ::Socket::TCP_NODELAY, 1)
+
+              begin
+                sock.connect_nonblock(sockaddr)
+              rescue IO::WaitWritable
+                if IO.select(nil, [sock], nil, Request::TIMEOUT[:connect])
+                  begin
+                    sock.connect_nonblock(sockaddr)
+                  rescue Errno::EISCONN
+                    # Yippee!
+                  rescue
+                    sock.close
+                    raise
+                  end
+                else
+                  sock.close
+                  raise HTTP::TimeoutError, "Connect timed out after #{Request::TIMEOUT[:connect]} seconds"
+                end
               end
+
+              return sock
             rescue => e
               outer_e = e
             end
diff --git a/app/lib/request_pool.rb b/app/lib/request_pool.rb
new file mode 100644
index 000000000..e5899a79a
--- /dev/null
+++ b/app/lib/request_pool.rb
@@ -0,0 +1,114 @@
+# frozen_string_literal: true
+
+require_relative './connection_pool/shared_connection_pool'
+
+class RequestPool
+  def self.current
+    @current ||= RequestPool.new
+  end
+
+  class Reaper
+    attr_reader :pool, :frequency
+
+    def initialize(pool, frequency)
+      @pool      = pool
+      @frequency = frequency
+    end
+
+    def run
+      return unless frequency&.positive?
+
+      Thread.new(frequency, pool) do |t, p|
+        loop do
+          sleep t
+          p.flush
+        end
+      end
+    end
+  end
+
+  MAX_IDLE_TIME = 30
+  WAIT_TIMEOUT  = 5
+  MAX_POOL_SIZE = ENV.fetch('MAX_REQUEST_POOL_SIZE', 512).to_i
+
+  class Connection
+    attr_reader :site, :last_used_at, :created_at, :in_use, :dead, :fresh
+
+    def initialize(site)
+      @site         = site
+      @http_client  = http_client
+      @last_used_at = nil
+      @created_at   = current_time
+      @dead         = false
+      @fresh        = true
+    end
+
+    def use
+      @last_used_at = current_time
+      @in_use       = true
+
+      retries = 0
+
+      begin
+        yield @http_client
+      rescue HTTP::ConnectionError
+        # It's possible the connection was closed, so let's
+        # try re-opening it once
+
+        close
+
+        if @fresh || retries.positive?
+          raise
+        else
+          @http_client = http_client
+          retries     += 1
+          retry
+        end
+      rescue StandardError
+        # If this connection raises errors of any kind, it's
+        # better if it gets reaped as soon as possible
+
+        close
+        @dead = true
+        raise
+      end
+    ensure
+      @fresh  = false
+      @in_use = false
+    end
+
+    def seconds_idle
+      current_time - (@last_used_at || @created_at)
+    end
+
+    def close
+      @http_client.close
+    end
+
+    private
+
+    def http_client
+      Request.http_client.persistent(@site, timeout: MAX_IDLE_TIME)
+    end
+
+    def current_time
+      Process.clock_gettime(Process::CLOCK_MONOTONIC)
+    end
+  end
+
+  def initialize
+    @pool   = ConnectionPool::SharedConnectionPool.new(size: MAX_POOL_SIZE, timeout: WAIT_TIMEOUT) { |site| Connection.new(site) }
+    @reaper = Reaper.new(self, 30)
+    @reaper.run
+  end
+
+  def with(site, &block)
+    @pool.with(site) do |connection|
+      ActiveSupport::Notifications.instrument('with.request_pool', miss: connection.fresh, host: connection.site) do
+        connection.use(&block)
+      end
+    end
+  end
+
+  delegate :size, :flush, to: :@pool
+end