Close Hackney connections after use

Make sure that Hackney connections are closed after use. This fixes
an issue where the Hackney pool does not correctly claim back
connections, which fixes #970.

Implement a `transmit_and_close/3` convenience method in the
transmitter for use cases where the body is not of interest, meaning
that the connection can be immediately closed.

.changesets/fix-check-ins-not-being-sent-after-some-time.md

@@ -0,0 +1,8 @@
+---
+bump: patch
+type: fix
+---
+
+Fix an issue where, after a certain amount of time, check-ins would no longer be sent.
+
+This issue also caused the default Hackney connection pool to be saturated, affecting other code that uses the default Hackney connection pool.

lib/appsignal/check_in/scheduler.ex

@@ -98,11 +98,11 @@ defmodule Appsignal.CheckIn.Scheduler do
     config = Appsignal.Config.config()
     endpoint = "#{config[:logging_endpoint]}/check_ins/json"
 
-    case @transmitter.transmit(endpoint, {Enum.reverse(events), :ndjson}, config) do
-      {:ok, status_code, _, _} when status_code in 200..299 ->
+    case @transmitter.transmit_and_close(endpoint, {Enum.reverse(events), :ndjson}, config) do
+      {:ok, status_code, _} when status_code in 200..299 ->
         @integration_logger.trace("Transmitted #{description}")
 
-      {:ok, status_code, _, _} ->
+      {:ok, status_code, _} ->
         @integration_logger.error(
           "Failed to transmit #{description}: status code was #{status_code}"
         )

lib/appsignal/diagnose/report.ex

@@ -13,6 +13,7 @@ defmodule Appsignal.Diagnose.Report do
     case Transmitter.transmit(config[:diagnose_endpoint], {%{diagnose: report}, :json}, config) do
       {:ok, 200, _, reference} ->
         {:ok, body} = :hackney.body(reference)
+        :hackney.close(reference)
 
         case Jason.decode(body) do
           {:ok, response} -> {:ok, response["token"]}
@@ -21,6 +22,7 @@ defmodule Appsignal.Diagnose.Report do
 
       {:ok, status_code, _, reference} ->
         {:ok, body} = :hackney.body(reference)
+        :hackney.close(reference)
         {:error, %{status_code: status_code, body: body}}
 
       {:error, reason} ->

lib/appsignal/transmitter.ex

@@ -13,6 +13,12 @@ defmodule Appsignal.Transmitter do
   def transmit(url, payload_and_format \\ {nil, nil}, config \\ nil)
   def transmit(url, nil, config), do: transmit(url, {nil, nil}, config)
 
+  # This function calls `:hackney.request/5` -- it is the
+  # caller's responsibility to ensure that `:hackney.close/1` is
+  # called afterwards.
+  #
+  # If you're not interested in the body, only in the status code
+  # and headers, use `transmit_and_close/3` instead.
   def transmit(url, {payload, format}, config) do
     config = config || Appsignal.Config.config()
 
@@ -32,6 +38,17 @@ defmodule Appsignal.Transmitter do
     request(:post, url, headers, body)
   end
 
+  def transmit_and_close(url, payload_and_format \\ {nil, nil}, config \\ nil) do
+    case transmit(url, payload_and_format, config) do
+      {:ok, status, headers, reference} ->
+        :hackney.close(reference)
+        {:ok, status, headers}
+
+      {:error, reason} ->
+        {:error, reason}
+    end
+  end
+
   defp encode_body(nil, _), do: ""
   defp encode_body(payload, :json), do: Jason.encode!(payload)
 

lib/appsignal/utils/push_api_key_validator.ex

@@ -5,10 +5,10 @@ defmodule Appsignal.Utils.PushApiKeyValidator do
   def validate(config) do
     url = "#{config[:endpoint]}/1/auth"
 
-    case Transmitter.transmit(url, nil, config) do
-      {:ok, 200, _, _} -> :ok
-      {:ok, 401, _, _} -> {:error, :invalid}
-      {:ok, status_code, _, _} -> {:error, status_code}
+    case Transmitter.transmit_and_close(url, nil, config) do
+      {:ok, 200, _} -> :ok
+      {:ok, 401, _} -> {:error, :invalid}
+      {:ok, status_code, _} -> {:error, status_code}
       {:error, reason} -> {:error, reason}
     end
   end

test/support/appsignal/fake_transmitter.ex

@@ -31,6 +31,16 @@ defmodule Appsignal.FakeTransmitter do
     Agent.get(__MODULE__, & &1[:response]).()
   end
 
+  def transmit_and_close(url, payload, config) do
+    case transmit(url, payload, config) do
+      {:ok, status, headers, _reference} ->
+        {:ok, status, headers}
+
+      {:error, reason} ->
+        {:error, reason}
+    end
+  end
+
   def transmitted do
     Agent.get(__MODULE__, &Enum.reverse(&1[:transmitted]))
   end