Fix SpanCaptureReporter test spillage (#22)

* Discard spans started before `attach/0` was called

* Trigger `report/2` and wait 1ms before `collect/0`

* Set and advise `send_interval_ms: 100` to reduce unpredictability
  of `report/2` time, which with `send_interval_ms: 1` would sometimes
  wait until after `detach/0`

Author: Garth Kidd

config/config.exs

@@ -9,5 +9,5 @@ if Mix.env() == :test do
 
   config :opencensus,
     reporters: [{Opencensus.TestSupport.SpanCaptureReporter, []}],
-    send_interval_ms: 0
+    send_interval_ms: 100
 end

lib/opencensus/span.ex

@@ -34,4 +34,10 @@ defmodule Opencensus.Span do
   def load(span_ctx) when is_tuple(span_ctx) do
     span_ctx |> SpanContext.from() |> Map.get(:span_id) |> load()
   end
+
+  @doc false
+  def began_monotonic(record) when Record.is_record(record, :span) do
+    {native_time, _native_offset} = span(record, :start_time)
+    native_time
+  end
 end

lib/opencensus/test_support/span_capture_reporter.ex

@@ -8,7 +8,7 @@ defmodule Opencensus.TestSupport.SpanCaptureReporter do
   ```elixir
   if Mix.env() == :test do
     config :opencensus,
-      send_interval_ms: 1,
+      send_interval_ms: 100,
       reporters: [{Opencensus.TestSupport.SpanCaptureReporter, []}]
   end
   ```
@@ -52,25 +52,52 @@ defmodule Opencensus.TestSupport.SpanCaptureReporter do
   end
 
   @doc false
-  def handler([__MODULE__], %{}, %{spans: spans}, pid), do: send(pid, {:spans, spans})
+  def handler([__MODULE__], %{}, %{spans: spans}, {pid, attached_monotonic}) do
+    started_after_attach? = fn span -> span |> Span.began_monotonic() >= attached_monotonic end
+    filtered = spans |> Enum.filter(started_after_attach?)
+    send(pid, {:spans, filtered})
+  end
 
   @doc "Attach the reporter to deliver spans to your process inbox."
-  def attach, do: :telemetry.attach(__MODULE__, [__MODULE__], &handler/4, self())
+  def attach do
+    :telemetry.attach(__MODULE__, [__MODULE__], &handler/4, {self(), :erlang.monotonic_time()})
+  end
+
+  @doc """
+  Detach the reporter to stop delivering spans to your process inbox.
+
+  If still attached, triggers span delivery before detaching.
+  """
+  def detach do
+    if handler_attached?(), do: trigger_and_wait_for_span_delivery()
+    :telemetry.detach(__MODULE__)
+  end
 
-  @doc "Detach the reporter to stop delivering spans to your process inbox."
-  def detach, do: :telemetry.detach(__MODULE__)
+  @doc """
+  Collect spans from your process inbox.
 
-  @doc "Collect spans from your process inbox."
+  If still attached, triggers span delivery before collection.
+  """
   @spec collect() :: list(%Span{})
   def collect do
+    if handler_attached?(), do: trigger_and_wait_for_span_delivery()
     collect_span_records([]) |> Enum.map(&Span.from/1)
   end
 
+  defp trigger_and_wait_for_span_delivery do
+    send(:oc_reporter, :report_spans)
+    :timer.sleep(1)
+  end
+
+  defp handler_attached? do
+    :telemetry.list_handlers([__MODULE__]) != []
+  end
+
   defp collect_span_records(acc) when is_list(acc) do
     receive do
       {:spans, spans} -> collect_span_records(acc ++ spans)
     after
-      10 -> acc
+      1 -> acc
     end
   end
 end

test/opencensus_test_support_span_capture_reporter_test.exs

@@ -0,0 +1,43 @@
+defmodule Opencensus.TestSupport.SpanCaptureReporterTest do
+  use ExUnit.Case, async: false
+  import Opencensus.Trace
+
+  alias Opencensus.TestSupport.SpanCaptureReporter
+
+  describe "Opencensus.TestSupport.SpanCaptureReporter.collect/3" do
+    defp loop_count do
+      case System.get_env("CI") do
+        nil -> 10
+        _ -> 1000
+      end
+    end
+
+    test "before detach, gets the just-finished span" do
+      for _ <- 1..loop_count() do
+        SpanCaptureReporter.attach()
+
+        with_child_span "inner" do
+          [0, 1, 1, 2, 2, 2, 2, 3, 3, 4] |> Enum.random() |> :timer.sleep()
+          :...
+        end
+
+        assert SpanCaptureReporter.collect() |> length() == 1
+        SpanCaptureReporter.detach()
+      end
+    end
+
+    test "after detach, gets the just-finished span" do
+      for _ <- 1..loop_count() do
+        SpanCaptureReporter.attach()
+
+        with_child_span "inner" do
+          [0, 1, 1, 2, 2, 2, 2, 3, 3, 4] |> Enum.random() |> :timer.sleep()
+          :...
+        end
+
+        SpanCaptureReporter.detach()
+        assert SpanCaptureReporter.collect() |> length() == 1
+      end
+    end
+  end
+end