RUBY-2132 distinguishing semaphore implementation

lib/mongo.rb

@@ -34,6 +34,7 @@
 require 'mongo/id'
 require 'mongo/bson'
 require 'mongo/semaphore'
+require 'mongo/distinguishing_semaphore'
 require 'mongo/options'
 require 'mongo/loggable'
 require 'mongo/cluster_time'

lib/mongo/distinguishing_semaphore.rb

@@ -0,0 +1,55 @@
+# 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 a semaphore that distinguishes waits ending due to the timeout
+  # being reached from waits ending due to the semaphore being signaled.
+  #
+  # @api private
+  class DistinguishingSemaphore
+    def initialize
+      @lock = Mutex.new
+      @cv = ConditionVariable.new
+      @queue = []
+    end
+
+    # Waits for the semaphore to be signaled up to timeout seconds.
+    # If semaphore is not signaled, returns after timeout seconds.
+    #
+    # @return [ true | false ] true if semaphore was signaled, false if
+    #   timeout was reached.
+    def wait(timeout = nil)
+      @lock.synchronize do
+        @cv.wait(@lock, timeout)
+        (!@queue.empty?).tap do
+          @queue.clear
+        end
+      end
+    end
+
+    def broadcast
+      @lock.synchronize do
+        @queue.push(true)
+        @cv.broadcast
+      end
+    end
+
+    def signal
+      @lock.synchronize do
+        @queue.push(true)
+        @cv.signal
+      end
+    end
+  end
+end

spec/mongo/distinguishing_semaphore_spec.rb

@@ -0,0 +1,63 @@
+require 'lite_spec_helper'
+
+describe Mongo::DistinguishingSemaphore do
+  let(:semaphore) do
+    described_class.new
+  end
+
+  it 'waits until signaled' do
+    result = nil
+
+    consumer = Thread.new do
+      result = semaphore.wait(3)
+    end
+
+    # Context switch to start the thread
+    sleep 0.1
+
+    start_time = Time.now
+    semaphore.signal
+    consumer.join
+
+    (Time.now - start_time).should < 1
+
+    result.should be true
+  end
+
+  it 'waits until broadcast' do
+    result = nil
+
+    consumer = Thread.new do
+      result = semaphore.wait(3)
+    end
+
+    # Context switch to start the thread
+    sleep 0.1
+
+    start_time = Time.now
+    semaphore.broadcast
+    consumer.join
+
+    (Time.now - start_time).should < 1
+
+    result.should be true
+  end
+
+  it 'times out' do
+    result = nil
+
+    consumer = Thread.new do
+      result = semaphore.wait(2)
+    end
+
+    # Context switch to start the thread
+    sleep 0.1
+
+    start_time = Time.now
+    consumer.join
+
+    (Time.now - start_time).should > 1
+
+    result.should be false
+  end
+end

spec/mongo/semaphore_spec.rb

@@ -0,0 +1,51 @@
+require 'lite_spec_helper'
+
+describe Mongo::Semaphore do
+  let(:semaphore) do
+    described_class.new
+  end
+
+  it 'waits until signaled' do
+    consumer = Thread.new do
+      semaphore.wait(3)
+    end
+
+    # Context switch to start the thread
+    sleep 0.1
+
+    start_time = Time.now
+    semaphore.signal
+    consumer.join
+
+    (Time.now - start_time).should < 1
+  end
+
+  it 'waits until broadcast' do
+    consumer = Thread.new do
+      semaphore.wait(3)
+    end
+
+    # Context switch to start the thread
+    sleep 0.1
+
+    start_time = Time.now
+    semaphore.broadcast
+    consumer.join
+
+    (Time.now - start_time).should < 1
+  end
+
+  it 'times out' do
+    consumer = Thread.new do
+      semaphore.wait(2)
+    end
+
+    # Context switch to start the thread
+    sleep 0.1
+
+    start_time = Time.now
+    consumer.join
+
+    (Time.now - start_time).should > 1
+  end
+end