Ensure missing protocol dependencies are discarded, closes #4842

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/mix/lib/mix/tasks/compile.protocols.ex

@@ -122,10 +122,24 @@ defmodule Mix.Tasks.Compile.Protocols do
   defp consolidate(protocol, paths, output, opts) do
     impls = Protocol.extract_impls(protocol, paths)
     reload(protocol)
-    {:ok, binary} = Protocol.consolidate(protocol, impls)
-    File.write!(Path.join(output, "#{protocol}.beam"), binary)
-    if opts[:verbose] do
-      Mix.shell.info "Consolidated #{inspect protocol}"
+    case Protocol.consolidate(protocol, impls) do
+      {:ok, binary} ->
+        File.write!(Path.join(output, "#{protocol}.beam"), binary)
+        if opts[:verbose] do
+          Mix.shell.info "Consolidated #{inspect protocol}"
+        end
+
+      # If we remove a dependency and we have implemented one of its
+      # protocols locally, we will mark the protocol as needing to be
+      # reconsolidated when the implementation is removed even though
+      # the protocol no longer exists. Although most times removing a
+      # dependency will trigger a full recompilation, such won't happen
+      # in umbrella apps with shared build.
+      {:error, :no_beam_info} ->
+        remove_consolidated(protocol, output)
+        if opts[:verbose] do
+          Mix.shell.info "Unavailable #{inspect protocol}"
+        end
     end
   end
 

lib/mix/test/mix/umbrella_test.exs

@@ -364,7 +364,7 @@ defmodule Mix.UmbrellaTest do
     end)
   end
 
-  test "reconsolidates using umbrella information even on shared _build" do
+  test "reconsolidates using umbrella parent information on shared _build" do
     in_fixture("umbrella_dep/deps/umbrella", fn ->
       File.write!("apps/bar/lib/bar.ex", """
       defprotocol Bar do
@@ -388,6 +388,29 @@ defmodule Mix.UmbrellaTest do
     end)
   end
 
+  test "reconsolidates using umbrella child information on shared _build" do
+    in_fixture("umbrella_dep/deps/umbrella", fn ->
+      File.write!("apps/bar/lib/bar.ex", """
+      defprotocol Bar do
+        def foo(arg)
+      end
+      defimpl Bar, for: List do
+        def foo(list), do: list
+      end
+      """)
+
+      Mix.Project.in_project(:umbrella, ".", fn _ ->
+        Mix.Task.run("compile.protocols")
+      end)
+
+      # Emulate the dependency being removed
+      Mix.Project.in_project(:foo, "apps/foo", [build_path: "../../_build", deps: []], fn _ ->
+        File.rm_rf "../../_build/dev/lib/bar"
+        Mix.Task.run("compile.protocols")
+      end)
+    end)
+  end
+
   defmodule Selective do
     def project do
       [apps_path: "apps",