Ensure change tracking is enabled when traversing keyed comprehensions (#3904)

When rendering live component that shared statics, we'd traverse with change tracking
disabled. The following situation could happen:

LiveComponent 1 renders a keyed comprehension with one entry.
LiveComponent 2 renders a keyed comprehension with more entries, but the first fingerprint
still matches. Now, when rendering the second entry (which is new), we'd build a new
template map, which could be incompatible with the first component. Then, when rendering,
the first entry of the keyed comprehension could point to invalid statics.

The solution is to always consider elements as new when change tracking is disabled.

Author: SteffenDE

lib/phoenix_live_view/diff.ex

@@ -675,8 +675,8 @@ defmodule Phoenix.LiveView.Diff do
     end
   end
 
-  # it's an existing entry
-  defp process_keyed({key, new_vars, render}, previous_prints, changed?, stream?, acc)
+  # it's an existing entry and we are change tracking
+  defp process_keyed({key, new_vars, render}, previous_prints, true = changed?, stream?, acc)
        when is_map_key(previous_prints, key) and not stream? do
     {diff, index, new_prints, pending, components, template} = acc
 

test/phoenix_live_view/diff_test.exs

@@ -2479,5 +2479,69 @@ defmodule Phoenix.LiveView.DiffTest do
                }
              }
     end
+
+    defmodule LiveComponentForIssue3904 do
+      # https://github.com/phoenixframework/phoenix_live_view/pull/3904
+      use Phoenix.LiveComponent
+
+      def render(assigns) do
+        ~H"""
+        <div>
+          <div :for={i <- 1..assigns.count}>
+            Shared
+            <%= if i == 2 do %>
+              1 {2} 3
+            <% end %>
+            <%= if i > 0 do %>
+              Always
+            <% end %>
+          </div>
+        </div>
+        """
+      end
+    end
+
+    test "considers element rendered in shared live component trees as new" do
+      assigns = %{}
+
+      template = ~H"""
+      <.live_component id={0} module={LiveComponentForIssue3904} count={1} />
+      <.live_component id={1} module={LiveComponentForIssue3904} count={2} />
+      """
+
+      {full_render, _fingerprints, _components} = render(template)
+
+      assert full_render == %{
+               0 => 1,
+               1 => 2,
+               :c => %{
+                 1 => %{
+                   0 => %{
+                     p: %{0 => ["\n      Always\n    "]},
+                     s: ["<div>\n    Shared\n    ", "\n    ", "\n  </div>"],
+                     k: %{0 => %{0 => "", 1 => %{s: 0}}, :kc => 1}
+                   },
+                   :r => 1,
+                   :s => ["<div>\n  ", "\n</div>"]
+                 },
+                 2 => %{
+                   0 => %{
+                     p: %{0 => ["\n      Always\n    "], 1 => ["\n      1 ", " 3\n    "]},
+                     k: %{
+                       0 => %{0 => "", 1 => %{s: 0}},
+                       1 => %{0 => %{0 => "2", :s => 1}, 1 => %{s: 0}},
+                       :kc => 2
+                     }
+                   },
+                   :s => 1
+                 }
+               },
+               :p => %{0 => ["", "\n", ""]},
+               :s => 0
+             }
+
+      assert rendered_to_binary(full_render) ==
+               "<div>\n  <div>\n    Shared\n    \n    \n      Always\n    \n  </div>\n</div>\n<div>\n  <div>\n    Shared\n    \n    \n      Always\n    \n  </div><div>\n    Shared\n    \n      1 2 3\n    \n    \n      Always\n    \n  </div>\n</div>"
+    end
   end
 end