Enable test for producing in forked process

This used to crash, but does not anymore. Seems like this is now
supported in librdkafka, although I could not find a pull request that
changes the behaviour. The spec is working flawlessly, so as long as it
stays green we can assume this works.

Fixes #19

README.md

@@ -72,13 +72,6 @@ end
 delivery_handles.each(&:wait)
 ```
 
-## Known issues
-
-When using forked process such as when using Unicorn you currently need
-to make sure that you create rdkafka instances after forking. Otherwise
-they will not work and crash your Ruby process when they are garbage
-collected. See https://github.com/appsignal/rdkafka-ruby/issues/19
-
 ## Development
 
 A Docker Compose file is included to run Kafka and Zookeeper. To run

spec/rdkafka/producer_spec.rb

@@ -280,55 +280,49 @@
     end
   end
 
-# TODO this spec crashes if you create and use the producer before
-# forking like so:
-#
-# @producer = producer
-#
-# This will be added as part of https://github.com/appsignal/rdkafka-ruby/issues/19
-  #it "should produce a message in a forked process" do
-  #  # Fork, produce a message, send the report of a pipe and
-  #  # wait for it in the main process.
-
-  #  reader, writer = IO.pipe
-
-  #  fork do
-  #    reader.close
-
-  #    handle = producer.produce(
-  #      topic:   "produce_test_topic",
-  #      payload: "payload",
-  #      key:     "key"
-  #    )
-
-  #    report = handle.wait(5)
-  #    producer.close
-
-  #    report_json = JSON.generate(
-  #      "partition" => report.partition,
-  #      "offset" => report.offset
-  #    )
-
-  #    writer.write(report_json)
-  #  end
-
-  #  writer.close
-
-  #  report_hash = JSON.parse(reader.read)
-  #  report = Rdkafka::Producer::DeliveryReport.new(
-  #    report_hash["partition"],
-  #    report_hash["offset"]
-  #  )
-
-  #  # Consume message and verify it's content
-  #  message = wait_for_message(
-  #    topic: "produce_test_topic",
-  #    delivery_report: report
-  #  )
-  #  expect(message.partition).to eq 1
-  #  expect(message.payload).to eq "payload"
-  #  expect(message.key).to eq "key"
-  #end
+  it "should produce a message in a forked process" do
+    # Fork, produce a message, send the report over a pipe and
+    # wait for and check the message in the main process.
+
+    reader, writer = IO.pipe
+
+    fork do
+      reader.close
+
+      handle = producer.produce(
+        topic:   "produce_test_topic",
+        payload: "payload-forked",
+        key:     "key-forked"
+      )
+
+      report = handle.wait(5)
+      producer.close
+
+      report_json = JSON.generate(
+        "partition" => report.partition,
+        "offset" => report.offset
+      )
+
+      writer.write(report_json)
+    end
+
+    writer.close
+
+    report_hash = JSON.parse(reader.read)
+    report = Rdkafka::Producer::DeliveryReport.new(
+      report_hash["partition"],
+      report_hash["offset"]
+    )
+
+    # Consume message and verify it's content
+    message = wait_for_message(
+      topic: "produce_test_topic",
+      delivery_report: report
+    )
+    expect(message.partition).to eq 0
+    expect(message.payload).to eq "payload-forked"
+    expect(message.key).to eq "key-forked"
+  end
 
   it "should raise an error when producing fails" do
     expect(Rdkafka::Bindings).to receive(:rd_kafka_producev).and_return(20)