Ensure build per environment in umbrella children do not clash

Author: José Valim

lib/mix/lib/mix/tasks/deps.check.ex

@@ -37,9 +37,14 @@ defmodule Mix.Tasks.Deps.Check do
   # If the build is per environment, we should be able to look
   # at all dependencies and remove the builds that no longer
   # has a dependnecy defined for them.
+  #
+  # Notice we require the build_path to be nil. If the build_path
+  # is not nil, it means it was set by a parent application and
+  # the parent application should be the one to do the pruning.
   defp prune_deps(all) do
     config = Mix.project
-    if config[:build_per_environment] do
+
+    if nil?(config[:build_path]) && config[:build_per_environment] do
       paths = Mix.Project.build_path(config)
               |> Path.join("lib/*/ebin")
               |> Path.wildcard

lib/mix/test/mix/umbrella_test.exs

@@ -29,13 +29,39 @@ defmodule Mix.UmbrellaTest do
         assert_received { :mix_shell, :info, ["* bar (apps/bar)"] }
         assert_received { :mix_shell, :info, ["* foo (apps/foo)"] }
 
-        # Ensure we can't compile and run checks
+        # Ensure we can compile and run checks
         Mix.Task.run "deps.compile"
         Mix.Task.run "deps.check"
       end)
     end
   end
 
+  test "dependencies in umbrella with build per environment" do
+    in_fixture "umbrella_dep/deps/umbrella", fn ->
+      Mix.Project.in_project(:umbrella, ".", [build_per_environment: true], fn _ ->
+        File.write "apps/foo/mix.exs", """
+        defmodule Foo.Mix do
+          use Mix.Project
+
+          def project do
+            [ app: :foo, build_per_environment: true, version: "0.1.0" ]
+          end
+        end
+        """
+
+        Mix.Task.run "deps"
+        assert_received { :mix_shell, :info, ["* bar (apps/bar)"] }
+        assert_received { :mix_shell, :info, ["* foo (apps/foo)"] }
+
+        # Ensure we can also start each app and
+        # they won't remove each other build
+        Mix.Task.run "compile"
+        Mix.Task.clear
+        Mix.Task.run "app.start", ["--no-compile"]
+      end)
+    end
+  end
+
   defmodule UmbrellaDeps do
     def project do
       [ apps_path: "apps",