Feat: just use a Repo with pool=1 for PG locks

Problem: 
- the sandbox config has some test-specific params, like timing out, cleaning up the conn, etc. 
- this prevents proper usage

Solution: 
- we just stick to a normal repo with a pool of size=1, that way we always get the same connection
- this is only use for locking, so there should be no performance issues

lib/essig/application.ex

@@ -7,7 +7,7 @@ defmodule Essig.Application do
   def start(_type, _args) do
     children = [
       Essig.Repo,
-      Essig.SandRepo,
+      Essig.RepoSingleConn,
       {Phoenix.PubSub, name: Essig.PubSub},
       {Registry, keys: :unique, name: Essig.Scopes.Registry},
       Essig.Scopes.DynamicSupervisor

lib/pg_lock.ex

@@ -1,13 +1,20 @@
 defmodule Essig.PGLock do
   @moduledoc """
   A simple wrapper around pg_try_advisory_lock and pg_advisory_unlock.
-  To get consistent PG connection, it uses SandRepo (which is configured with a Sandbox as pool)
+  To get consistent PG connection, it uses second Repo with pool_size=1
 
   This makes it possible to use the same connection for locking and releasing the lock!
   Because releasing the lock on a different connection than locking it will fail.
   This is the best workaround I could come up with.
+
+
+  - Check locks with:
+
+  ```sql
+  SELECT locktype, transactionid, virtualtransaction, mode FROM pg_locks;
+  ```
   """
-  use Essig.SandRepo
+  use Essig.RepoSingleConn
 
   def with_lock(kind, fun) do
     lock_key = :erlang.phash2("#{kind}-#{Essig.Context.current_scope()}")
@@ -26,10 +33,10 @@ defmodule Essig.PGLock do
   end
 
   def get_lock(key) do
-    Ecto.Adapters.SQL.query(SandRepo, "SELECT pg_try_advisory_lock($1)", [key], [])
+    Ecto.Adapters.SQL.query(RepoSingleConn, "SELECT pg_try_advisory_lock($1)", [key], [])
   end
 
   def release_lock(key) do
-    Ecto.Adapters.SQL.query(SandRepo, "SELECT pg_advisory_unlock($1)", [key], [])
+    Ecto.Adapters.SQL.query(RepoSingleConn, "SELECT pg_advisory_unlock($1)", [key], [])
   end
 end

lib/sand_repo.ex → lib/repo_single_conn.ex

@@ -1,10 +1,10 @@
-defmodule Essig.SandRepo do
+defmodule Essig.RepoSingleConn do
   @moduledoc """
-  This is special SandRepo, that allows checking out a single connection.
+  This is special RepoSingleConn, with a single connection.
   We use this when getting a DB advisory lock and releasing it afterwards.
   This is not possible with the standard Ecto.Repo outside of a transaction.
 
-  To keep the configuration overhead low, we use dynamic config (init -callback) and
+  To keep the configuration overhead low, we use dynamic config (init-callback) and
   copy the main config for the Essig.Repo with a few tweaks.
   This means the DB config stays unchanged.
   """
@@ -19,7 +19,7 @@ defmodule Essig.SandRepo do
   def init(_type, _config) do
     special_config = [
       telemetry_prefix: [:essig, :sand_repo],
-      pool: Ecto.Adapters.SQL.Sandbox
+      pool_size: 1
     ]
 
     main_config = Application.get_env(:essig, Essig.Repo)
@@ -30,7 +30,7 @@ defmodule Essig.SandRepo do
 
   defmacro __using__(_) do
     quote do
-      alias Essig.SandRepo
+      alias Essig.RepoSingleConn
       require Ecto.Query
       import Ecto.Query
       import Ecto.Changeset