feat: add telemetry event emission for repo process start

lib/ecto/repo/supervisor.ex

@@ -201,19 +201,26 @@ defmodule Ecto.Repo.Supervisor do
         name = if is_atom(name), do: name, else: nil
         cache = Ecto.Query.Planner.new_query_cache(name)
         meta = Map.merge(meta, %{repo: repo, cache: cache})
-        child_spec = wrap_child_spec(child, [name, adapter, meta])
+        child_spec = wrap_child_spec(child, [name, adapter, meta, opts])
         Supervisor.init([child_spec], strategy: :one_for_one, max_restarts: 0)
 
       :ignore ->
         :ignore
     end
   end
 
-  def start_child({mod, fun, args}, name, adapter, meta) do
+  def start_child({mod, fun, args}, name, adapter, meta, opts) do
     case apply(mod, fun, args) do
       {:ok, pid} ->
         meta = Map.merge(meta, %{pid: pid, adapter: adapter})
         Ecto.Repo.Registry.associate(self(), name, meta)
+
+        :telemetry.execute(
+          [:ecto, :repo, :started],
+          %{system_time: System.system_time()},
+          %{repo: meta.repo, adapter: meta.adapter, config: opts}
+        )
+
         {:ok, pid}
 
       other ->

test/ecto/repo/supervisor_test.exs

@@ -52,6 +52,41 @@ defmodule Ecto.Repo.SupervisorTest do
     :telemetry.detach(:telemetry_test)
   end
 
+  test "emits :started telemetry event after repo process starts" do
+    :telemetry.attach_many(
+      :telemetry_started_test,
+      [[:ecto, :repo, :started]],
+      &__MODULE__.handle_event/4,
+      %{pid: self()}
+    )
+
+    {:ok, _pid} = Ecto.TestRepo.start_link(name: :telemetry_started_test)
+
+    assert_receive {[:ecto, :repo, :started], %{system_time: _}, %{repo: Ecto.TestRepo, adapter: Ecto.TestAdapter, config: config}}
+    assert config[:name] == :telemetry_started_test
+    assert config[:database] == "hello"
+
+    :telemetry.detach(:telemetry_started_test)
+  end
+
+  test "emits :started telemetry event even when repo modifies config in init" do
+    :telemetry.attach_many(
+      :telemetry_started_test_init,
+      [[:ecto, :repo, :started]],
+      &__MODULE__.handle_event/4,
+      %{pid: self()}
+    )
+
+    {:ok, _pid} = Ecto.TestRepoInitModify.start_link(name: :telemetry_started_test_init_modify)
+
+    assert_receive {[:ecto, :repo, :started], %{system_time: _}, %{repo: Ecto.TestRepoInitModify, adapter: Ecto.TestAdapter, config: config}}
+    assert config[:name] == :telemetry_started_test_init_modify
+    assert config[:database] == "hello"
+    refute Keyword.has_key?(config, :telemetry_prefix)
+
+    :telemetry.detach(:telemetry_started_test_init)
+  end
+
   test "reads otp app configuration" do
     put_env(database: "hello")
     {:ok, config} = init_config(:runtime, __MODULE__, :ecto, [])

test/support/test_repo.exs

@@ -213,4 +213,14 @@ defmodule Ecto.TestRepo do
   end
 end
 
+defmodule Ecto.TestRepoInitModify do
+  use Ecto.Repo, otp_app: :ecto, adapter: Ecto.TestAdapter
+
+  def init(_type, opts) do
+    opts = [url: "ecto://user:pass@local/hello"] ++ opts
+    opts = Keyword.drop(opts, [:telemetry_prefix])
+    {:ok, opts}
+  end
+end
+
 Ecto.TestRepo.start_link()