HTTP: radically simplify pool checkin/checkout

Use a custom tesla middleware instead of adapter helper function +
custom redirect middleware.

This will also fix "Client died before releasing the connection"
messages when the request pool is overloaded. Since the checkout is
now done after passing ConcurrentLimiter.

This is technically less efficient, since the connection needs to be
checked in/out every time the middleware is left or entered respectively.
But I don't think the nanoseconds we might lose on redirects
to the same host are worth the complexity.

lib/pleroma/http/adapter_helper.ex

@@ -19,7 +19,6 @@ defmodule Pleroma.HTTP.AdapterHelper do
           | {Connection.proxy_type(), Connection.host(), pos_integer()}
 
   @callback options(keyword(), URI.t()) :: keyword()
-  @callback get_conn(URI.t(), keyword()) :: {:ok, term()} | {:error, term()}
 
   @spec format_proxy(String.t() | tuple() | nil) :: proxy() | nil
   def format_proxy(nil), do: nil
@@ -47,9 +46,6 @@ defmodule Pleroma.HTTP.AdapterHelper do
     |> adapter_helper().options(uri)
   end
 
-  @spec get_conn(URI.t(), keyword()) :: {:ok, keyword()} | {:error, atom()}
-  def get_conn(uri, opts), do: adapter_helper().get_conn(uri, opts)
-
   defp adapter, do: Application.get_env(:tesla, :adapter)
 
   defp adapter_helper do

lib/pleroma/http/adapter_helper/gun.ex

@@ -6,7 +6,6 @@ defmodule Pleroma.HTTP.AdapterHelper.Gun do
   @behaviour Pleroma.HTTP.AdapterHelper
 
   alias Pleroma.Config
-  alias Pleroma.Gun.ConnectionPool
   alias Pleroma.HTTP.AdapterHelper
 
   require Logger
@@ -57,14 +56,6 @@ defmodule Pleroma.HTTP.AdapterHelper.Gun do
     Config.get([:pools, pool, :timeout], default)
   end
 
-  @spec get_conn(URI.t(), keyword()) :: {:ok, keyword()} | {:error, atom()}
-  def get_conn(uri, opts) do
-    case ConnectionPool.get_conn(uri, opts) do
-      {:ok, conn_pid} -> {:ok, Keyword.merge(opts, conn: conn_pid, close_conn: false)}
-      err -> err
-    end
-  end
-
   @prefix Pleroma.Gun.ConnectionPool
   def limiter_setup do
     wait = Config.get([:connections_pool, :connection_acquisition_wait])

lib/pleroma/http/adapter_helper/hackney.ex

@@ -23,7 +23,4 @@ defmodule Pleroma.HTTP.AdapterHelper.Hackney do
   end
 
   defp add_scheme_opts(opts, _), do: opts
-
-  @spec get_conn(URI.t(), keyword()) :: {:ok, keyword()}
-  def get_conn(_uri, opts), do: {:ok, opts}
 end

lib/pleroma/http/http.ex

@@ -62,28 +62,21 @@ defmodule Pleroma.HTTP do
     uri = URI.parse(url)
     adapter_opts = AdapterHelper.options(uri, options[:adapter] || [])
 
-    case AdapterHelper.get_conn(uri, adapter_opts) do
-      {:ok, adapter_opts} ->
-        options = put_in(options[:adapter], adapter_opts)
-        params = options[:params] || []
-        request = build_request(method, headers, options, url, body, params)
-
-        adapter = Application.get_env(:tesla, :adapter)
-
-        client = Tesla.client(adapter_middlewares(adapter), adapter)
-
-        maybe_limit(
-          fn ->
-            request(client, request)
-          end,
-          adapter,
-          adapter_opts
-        )
-
-      # Connection release is handled in a custom FollowRedirects middleware
-      err ->
-        err
-    end
+    options = put_in(options[:adapter], adapter_opts)
+    params = options[:params] || []
+    request = build_request(method, headers, options, url, body, params)
+
+    adapter = Application.get_env(:tesla, :adapter)
+
+    client = Tesla.client(adapter_middlewares(adapter), adapter)
+
+    maybe_limit(
+      fn ->
+        request(client, request)
+      end,
+      adapter,
+      adapter_opts
+    )
   end
 
   @spec request(Client.t(), keyword()) :: {:ok, Env.t()} | {:error, any()}
@@ -110,7 +103,7 @@ defmodule Pleroma.HTTP do
   end
 
   defp adapter_middlewares(Tesla.Adapter.Gun) do
-    [Pleroma.HTTP.Middleware.FollowRedirects]
+    [Tesla.Middleware.FollowRedirects, Pleroma.Tesla.Middleware.ConnectionPool]
   end
 
   defp adapter_middlewares(_), do: []

lib/pleroma/tesla/middleware/connection_pool.ex

@@ -0,0 +1,35 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Tesla.Middleware.ConnectionPool do
+  @moduledoc """
+  Middleware to get/release connections from `Pleroma.Gun.ConnectionPool`
+  """
+
+  @behaviour Tesla.Middleware
+
+  alias Pleroma.Gun.ConnectionPool
+
+  @impl Tesla.Middleware
+  def call(%Tesla.Env{url: url, opts: opts} = env, next, _) do
+    uri = URI.parse(url)
+
+    case ConnectionPool.get_conn(uri, opts[:adapter]) do
+      {:ok, conn_pid} ->
+        adapter_opts = Keyword.merge(opts[:adapter], conn: conn_pid, close_conn: false)
+        opts = Keyword.put(opts, :adapter, adapter_opts)
+        env = %{env | opts: opts}
+        res = Tesla.run(env, next)
+
+        unless opts[:adapter][:body_as] == :chunks do
+          ConnectionPool.release_conn(conn_pid)
+        end
+
+        res
+
+      err ->
+        err
+    end
+  end
+end