Redact credential headers from machines API errors

lib/flame/fly_backend.ex

@@ -373,10 +373,26 @@ defmodule FLAME.FlyBackend do
         http_post!(url, remaining_tries - 1, opts)
 
       {:ok, {{_, status, reason}, _, resp_body}} ->
-        raise "failed POST #{url} with #{inspect(status)} (#{inspect(reason)}): #{inspect(resp_body)} #{inspect(headers)}"
+        raise "failed POST #{url} with #{inspect(status)} (#{inspect(reason)}): #{inspect(resp_body)} #{inspect(redact(headers))}"
 
       {:error, reason} ->
-        raise "failed POST #{url} with #{inspect(reason)} #{inspect(headers)}"
+        raise "failed POST #{url} with #{inspect(reason)} #{inspect(redact(headers))}"
+    end
+  end
+
+  @redacted_headers ~w(authorization proxy-authorization)
+
+  # The machines API is called with the Fly API token as a bearer credential, and
+  # the errors above are raised into logs and error trackers. Keep the header
+  # names, which are what you want when debugging a failed request, but drop the
+  # values of the ones that carry credentials.
+  defp redact(headers) do
+    for {name, value} <- headers do
+      if String.downcase(to_string(name)) in @redacted_headers do
+        {name, "[REDACTED]"}
+      else
+        {name, value}
+      end
     end
   end
 

test/fly_backend_test.exs

@@ -82,6 +82,22 @@ defmodule FLAME.FlyBackendTest do
     assert Runner.new(backend: FLAME.FlyBackend)
   end
 
+  test "boot failures do not leak the API token" do
+    # Nothing is listening on this port, so the POST fails and remote_boot raises.
+    opts = [token: "super-secret", image: "img", app: "app", host: "http://127.0.0.1:1"]
+    runner = new({FlyBackend, opts})
+    assert {:ok, init} = runner.backend_init
+
+    err =
+      assert_raise RuntimeError, fn ->
+        FlyBackend.remote_boot(%{init | parent_ref: make_ref()})
+      end
+
+    refute err.message =~ "super-secret"
+    assert err.message =~ "Authorization"
+    assert err.message =~ "[REDACTED]"
+  end
+
   test "parent backend attributes" do
     assert %FLAME.Parent{
              pid: _,