From 095b7e78312aa85bb9ca55d235675501bbd684c2 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Sun, 8 Jan 2017 15:39:57 -0800
Subject: [PATCH] config/module: disallow root modules named "root"

Fixes #11038

This is a **short term fix**.

Terraform core doesn't currently handle root modules named "root" well
because the prefix `[]string{"root"}` has special meaning and Terraform
core [currently] can't disambiguate between the root module and a module
named "root" in the root module.

This PR introduces a short term fix by simply disallowing root modules
named "root". This shouldn't break any BC because since 0.8.0 this
didn't work at all in many broken ways (including crashes).

Longer term, this should be fixed by removing the special prefix at all
and having empty paths be root. I started down this path but the core
changes necessary are far too scary for a patch release. We can aim for
0.9.
---
 .../child/child/main.tf                              |  1 +
 .../validate-module-root-grandchild/child/main.tf    |  1 +
 .../validate-module-root-grandchild/main.tf          |  3 +++
 .../test-fixtures/validate-module-root/child/main.tf |  1 +
 .../test-fixtures/validate-module-root/main.tf       |  3 +++
 config/module/tree.go                                |  8 ++++++++
 config/module/tree_test.go                           | 12 ++++++++++++
 7 files changed, 29 insertions(+)
 create mode 100644 config/module/test-fixtures/validate-module-root-grandchild/child/child/main.tf
 create mode 100644 config/module/test-fixtures/validate-module-root-grandchild/child/main.tf
 create mode 100644 config/module/test-fixtures/validate-module-root-grandchild/main.tf
 create mode 100644 config/module/test-fixtures/validate-module-root/child/main.tf
 create mode 100644 config/module/test-fixtures/validate-module-root/main.tf

diff --git a/config/module/test-fixtures/validate-module-root-grandchild/child/child/main.tf b/config/module/test-fixtures/validate-module-root-grandchild/child/child/main.tf
new file mode 100644
index 000000000..b7db25411
--- /dev/null
+++ b/config/module/test-fixtures/validate-module-root-grandchild/child/child/main.tf
@@ -0,0 +1 @@
+# Empty
diff --git a/config/module/test-fixtures/validate-module-root-grandchild/child/main.tf b/config/module/test-fixtures/validate-module-root-grandchild/child/main.tf
new file mode 100644
index 000000000..4f3d72385
--- /dev/null
+++ b/config/module/test-fixtures/validate-module-root-grandchild/child/main.tf
@@ -0,0 +1 @@
+module "root" { source = "./child" }
diff --git a/config/module/test-fixtures/validate-module-root-grandchild/main.tf b/config/module/test-fixtures/validate-module-root-grandchild/main.tf
new file mode 100644
index 000000000..0f6991c53
--- /dev/null
+++ b/config/module/test-fixtures/validate-module-root-grandchild/main.tf
@@ -0,0 +1,3 @@
+module "child" {
+    source = "./child"
+}
diff --git a/config/module/test-fixtures/validate-module-root/child/main.tf b/config/module/test-fixtures/validate-module-root/child/main.tf
new file mode 100644
index 000000000..b7db25411
--- /dev/null
+++ b/config/module/test-fixtures/validate-module-root/child/main.tf
@@ -0,0 +1 @@
+# Empty
diff --git a/config/module/test-fixtures/validate-module-root/main.tf b/config/module/test-fixtures/validate-module-root/main.tf
new file mode 100644
index 000000000..fd5d8aff4
--- /dev/null
+++ b/config/module/test-fixtures/validate-module-root/main.tf
@@ -0,0 +1,3 @@
+module "root" {
+    source = "./child"
+}
diff --git a/config/module/tree.go b/config/module/tree.go
index 777389bdc..d20f163a4 100644
--- a/config/module/tree.go
+++ b/config/module/tree.go
@@ -261,6 +261,14 @@ func (t *Tree) Validate() error {
 	// If something goes wrong, here is our error template
 	newErr := &TreeError{Name: []string{t.Name()}}
 
+	// Terraform core does not handle root module children named "root".
+	// We plan to fix this in the future but this bug was brought up in
+	// the middle of a release and we don't want to introduce wide-sweeping
+	// changes at that time.
+	if len(t.path) == 1 && t.name == "root" {
+		return fmt.Errorf("root module cannot contain module named 'root'")
+	}
+
 	// Validate our configuration first.
 	if err := t.config.Validate(); err != nil {
 		newErr.Err = err
diff --git a/config/module/tree_test.go b/config/module/tree_test.go
index 4df3f8cff..6ca5f2a72 100644
--- a/config/module/tree_test.go
+++ b/config/module/tree_test.go
@@ -288,6 +288,18 @@ func TestTreeValidate_table(t *testing.T) {
 			"validate-alias-bad",
 			"alias must be defined",
 		},
+
+		{
+			"root module named root",
+			"validate-module-root",
+			"cannot contain module",
+		},
+
+		{
+			"grandchild module named root",
+			"validate-module-root-grandchild",
+			"",
+		},
 	}
 
 	for i, tc := range cases {