command: Fix 0.13upgrade bug with multiple blocks

If a configuration had multiple blocks in the versions.tf file, it would
be added to the `rewritePaths` list multiple times. We would then remove
it from this slice, but only once, and so the output file would later be
rewritten to remove the required providers block.

This commit uses a set instead of a list to prevent this case, and adds
a regression test.

command/013_config_upgrade.go

@@ -164,12 +164,12 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 
 	// Build up a list of required providers, uniquely by local name
 	requiredProviders := make(map[string]*configs.RequiredProvider)
-	var rewritePaths []string
+	rewritePaths := make(map[string]bool)
 
 	// Step 1: copy all explicit provider requirements across
 	for path, file := range files {
 		for _, rps := range file.RequiredProviders {
-			rewritePaths = append(rewritePaths, path)
+			rewritePaths[path] = true
 			for _, rp := range rps.RequiredProviders {
 				if previous, exist := requiredProviders[rp.Name]; exist {
 					diags = diags.Append(&hcl.Diagnostic{
@@ -254,18 +254,16 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 		// Special case: if we only have one file with a required providers
 		// block, output to that file instead.
 		if len(rewritePaths) == 1 {
-			filename = rewritePaths[0]
+			for path := range rewritePaths {
+				filename = path
+				break
+			}
 		}
 
 		// Remove the output file from the list of paths we want to rewrite
 		// later. Otherwise we'd delete the required providers block after
 		// writing it.
-		for i, path := range rewritePaths {
-			if path == filename {
-				rewritePaths = append(rewritePaths[:i], rewritePaths[i+1:]...)
-				break
-			}
-		}
+		delete(rewritePaths, filename)
 
 		// Open or create the output file
 		out, openDiags := c.openOrCreateFile(filename)
@@ -456,7 +454,7 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 
 		// After successfully writing the new configuration, remove all other
 		// required provider blocks from remaining configuration files.
-		for _, path := range rewritePaths {
+		for path := range rewritePaths {
 			// Read and parse the existing file
 			config, err := ioutil.ReadFile(path)
 			if err != nil {

command/testdata/013upgrade-multiple-blocks/expected/bar-baz.tf

@@ -1,5 +1,2 @@
 resource bar_instance a {}
 resource baz_instance b {}
-terraform {
-  required_version = "> 0.12.0"
-}

command/testdata/013upgrade-multiple-blocks/expected/foo.tf

@@ -1 +1,4 @@
+terraform {
+  required_version = ">= 0.12"
+}
 resource foo_instance c {}

command/testdata/013upgrade-multiple-blocks/expected/versions.tf

@@ -1,5 +1,9 @@
 terraform {
   required_providers {
+    foo = {
+      source  = "hashicorp/foo"
+      version = "0.5"
+    }
     bar = {
       source = "registry.acme.corp/acme/bar"
     }
@@ -7,10 +11,6 @@ terraform {
       source  = "terraform-providers/baz"
       version = "~> 2.0.0"
     }
-    foo = {
-      source  = "hashicorp/foo"
-      version = "0.5"
-    }
   }
   required_version = ">= 0.13"
 }

command/testdata/013upgrade-multiple-blocks/input/bar-baz.tf

@@ -1,10 +1,2 @@
 resource bar_instance a {}
 resource baz_instance b {}
-terraform {
-  required_version = "> 0.12.0"
-  required_providers {
-    bar = {
-      source = "registry.acme.corp/acme/bar"
-    }
-  }
-}

command/testdata/013upgrade-multiple-blocks/input/foo.tf

@@ -1,11 +1,7 @@
 terraform {
+  required_version = ">= 0.12"
   required_providers {
     baz = "~> 2.0.0"
   }
 }
-terraform {
-  required_providers {
-    foo = "0.5"
-  }
-}
 resource foo_instance c {}

command/testdata/013upgrade-multiple-blocks/input/versions.tf

@@ -0,0 +1,12 @@
+terraform {
+  required_providers {
+    foo = "0.5"
+  }
+}
+terraform {
+  required_providers {
+    bar = {
+      source = "registry.acme.corp/acme/bar"
+    }
+  }
+}