Fix race condition with :sender_pool_size (#718)

Closes #709.

Author: whatyouhide

lib/sentry/transport/sender.ex

@@ -19,8 +19,7 @@ defmodule Sentry.Transport.Sender do
 
   @spec send_async(module(), Event.t()) :: :ok
   def send_async(client, %Event{} = event) when is_atom(client) do
-    pool_size = Application.fetch_env!(:sentry, :sender_pool_size)
-    random_index = Enum.random(1..pool_size)
+    random_index = Enum.random(1..Transport.SenderPool.pool_size())
 
     GenServer.cast({:via, Registry, {@registry, random_index}}, {:send, client, event})
   end

lib/sentry/transport/sender_pool.ex

@@ -10,17 +10,24 @@ defmodule Sentry.Transport.SenderPool do
 
   @impl true
   def init([]) do
-    pool_size = max(System.schedulers_online(), 8)
-
-    Application.put_env(:sentry, :sender_pool_size, pool_size)
-
     children =
-      for index <- 1..pool_size do
+      for index <- 1..pool_size() do
         Supervisor.child_spec({Sentry.Transport.Sender, index: index},
           id: {Sentry.Transport.Sender, index}
         )
       end
 
     Supervisor.init(children, strategy: :one_for_one)
   end
+
+  @spec pool_size() :: pos_integer()
+  def pool_size do
+    if pool_size = :persistent_term.get({:sentry, :sender_pool_size}, nil) do
+      pool_size
+    else
+      value = max(System.schedulers_online(), 8)
+      :persistent_term.put({:sentry, :sender_pool_size}, value)
+      value
+    end
+  end
 end