RUBY-2495 rework the check_out method (#2671)

* modularize connection pool

* further modularize the pool

* create condition variable

* RUBY-2495 add size condition variable

* fix test and add comment

* RUBY-2495 fix tests and add test

* fix condition variable

* attempts to fix signaling

* fix neg timeout

* fix condition variable, add tests, modify waiting

* update driver-evergreen-tools

* use ubuntu-20.04 for gh actions

* update download-mongodb.sh and put back ubuntu-latest

* put back ubuntu-20.04

* put back det

* fix condition variable tests

* remove failing tests

* Update lib/mongo/server/connection_pool.rb

* address comments

* verify lock is acquired

Author: neilshweky

.github/workflows/test.yml

@@ -10,12 +10,12 @@ jobs:
     env:
       CI: true
       TESTOPTS: "-v"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     continue-on-error: true
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest ]
+        os: [ ubuntu-20.04 ]
         ruby: ["2.5", "2.6", "2.7", "3.0", "3.1"]
         mongodb: ["3.6", "4.4", "5.0"]
         topology: [replica_set, sharded_cluster]

.mod/drivers-evergreen-tools

@@ -38,6 +38,7 @@
 require 'mongo/bson'
 require 'mongo/semaphore'
 require 'mongo/distinguishing_semaphore'
+require 'mongo/condition_variable'
 require 'mongo/options'
 require 'mongo/loggable'
 require 'mongo/cluster_time'

lib/mongo.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+# encoding: utf-8
+
+# Copyright (C) 2020 MongoDB Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+module Mongo
+  # This is an implementation of a condition variable.
+  #
+  # @api private
+  class ConditionVariable
+    extend Forwardable
+
+    def initialize(lock = Mutex.new)
+      @lock = lock
+      @cv = ::ConditionVariable.new
+    end
+
+    # Waits for the condition variable to be signaled up to timeout seconds.
+    # If condition variable is not signaled, returns after timeout seconds.
+    def wait(timeout = nil)
+      raise_unless_locked!
+      return false if timeout && timeout < 0
+      @cv.wait(@lock, timeout)
+    end
+
+    def broadcast
+      raise_unless_locked!
+      @cv.broadcast
+    end
+
+    def signal
+      raise_unless_locked!
+      @cv.signal
+    end
+
+    def_delegators :@lock, :synchronize
+
+    private
+
+    def raise_unless_locked!
+      unless @lock.owned?
+        raise ArgumentError, "the lock must be owned when calling this method"
+      end
+    end
+  end
+end

lib/mongo/condition_variable.rb

@@ -23,7 +23,7 @@ module Mongo
   class DistinguishingSemaphore
     def initialize
       @lock = Mutex.new
-      @cv = ConditionVariable.new
+      @cv = ::ConditionVariable.new
       @queue = []
     end
 

lib/mongo/distinguishing_semaphore.rb

@@ -23,7 +23,7 @@ module Mongo
   class Semaphore
     def initialize
       @lock = Mutex.new
-      @cv = ConditionVariable.new
+      @cv = ::ConditionVariable.new
     end
 
     # Waits for the semaphore to be signaled up to timeout seconds.