command: Change 0.13upgrade default to versions.tf

Instead of using providers.tf as the default output file for the
upgrader, we now default to versions.tf. This means that if the
configuration has no `required_providers` blocks at all, or has
multiple, the provider version requirements will be stored in the
versions.tf file.

We now also update the versions.tf file to set a `required_version`
attribute in the first `terraform` block, with value ">= 0.13". This
is similar to the behaviour of the 0.12upgrade command, and signals that
the configuration should not be used with older versions of Terraform.

command/013_config_upgrade.go

@@ -247,8 +247,9 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 			return 1
 		}
 
-		// Default output filename is "providers.tf"
-		filename := path.Join(dir, "providers.tf")
+		// Default output filename is "versions.tf", which is also where the
+		// 0.12upgrade command added the required_version constraint.
+		filename := path.Join(dir, "versions.tf")
 
 		// Special case: if we only have one file with a required providers
 		// block, output to that file instead.
@@ -322,14 +323,22 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 		var first *hclwrite.Block
 		var rest []*hclwrite.Block
 
+		// First terraform block in the first file. Declared at this scope so
+		// that it can be used to write the version constraint later, if this
+		// is the "versions.tf" file.
+		var tfBlock *hclwrite.Block
+
 		if len(requiredProviderBlocks) > 0 {
 			// If we already have one or more required provider blocks, we'll rewrite
 			// the first one, and remove the rest.
 			first, rest = requiredProviderBlocks[0], requiredProviderBlocks[1:]
+
+			// Set the terraform block here for later use to update the
+			// required version constraint.
+			tfBlock = parentBlocks[first]
 		} else {
 			// Otherwise, find or a create a terraform block, and add a new
 			// empty required providers block to it.
-			var tfBlock *hclwrite.Block
 			for _, rootBlock := range root.Blocks() {
 				if rootBlock.Type() == "terraform" {
 					tfBlock = rootBlock
@@ -404,6 +413,14 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 			}
 		}
 
+		// If this is the "versions.tf" file, add a new version constraint to
+		// the first terraform block. If this isn't the "versions.tf" file,
+		// we'll update that file separately.
+		versionsFilename := path.Join(dir, "versions.tf")
+		if filename == versionsFilename {
+			tfBlock.Body().SetAttributeValue("required_version", cty.StringVal(">= 0.13"))
+		}
+
 		// Remove the rest of the blocks (and the parent block, if it's empty)
 		for _, rpBlock := range rest {
 			tfBlock := parentBlocks[rpBlock]
@@ -440,6 +457,84 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 			return 1
 		}
 
+		// If the file we just updated was not a "versions.tf" file, add or
+		// update that file to set the required version constraint in the first
+		// terraform block.
+		if filename != versionsFilename {
+			var file *hclwrite.File
+
+			// If the versions file doesn't exist, just create a new empty file
+			if _, err := os.Stat(versionsFilename); os.IsNotExist(err) {
+				file = hclwrite.NewEmptyFile()
+			} else if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to read configuration file",
+					fmt.Sprintf("Error when reading configuration file %q: %s", versionsFilename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			} else {
+				// Versions file already exists, so load and parse it
+				config, err := ioutil.ReadFile(versionsFilename)
+				if err != nil {
+					diags = diags.Append(tfdiags.Sourceless(
+						tfdiags.Error,
+						"Unable to read configuration file",
+						fmt.Sprintf("Error when reading configuration file %q: %s", versionsFilename, err),
+					))
+					c.showDiagnostics(diags)
+					return 1
+				}
+				var parseDiags hcl.Diagnostics
+				file, parseDiags = hclwrite.ParseConfig(config, filename, hcl.InitialPos)
+				diags = diags.Append(parseDiags)
+			}
+
+			if diags.HasErrors() {
+				c.showDiagnostics(diags)
+				return 1
+			}
+
+			// Find or create a terraform block
+			root := file.Body()
+			var tfBlock *hclwrite.Block
+			for _, rootBlock := range root.Blocks() {
+				if rootBlock.Type() == "terraform" {
+					tfBlock = rootBlock
+					break
+				}
+			}
+			if tfBlock == nil {
+				tfBlock = root.AppendNewBlock("terraform", nil)
+			}
+
+			// Set the required version attribute
+			tfBlock.Body().SetAttributeValue("required_version", cty.StringVal(">= 0.13"))
+
+			// Write the config back to the file
+			f, err := os.OpenFile(versionsFilename, os.O_TRUNC|os.O_CREATE|os.O_WRONLY, 0644)
+			if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to open configuration file for writing",
+					fmt.Sprintf("Error when reading configuration file %q: %s", filename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			}
+			_, err = file.WriteTo(f)
+			if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to rewrite configuration file",
+					fmt.Sprintf("Error when rewriting configuration file %q: %s", filename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			}
+		}
+
 		// After successfully writing the new configuration, remove all other
 		// required provider blocks from remaining configuration files.
 		for _, path := range rewritePaths {
@@ -600,8 +695,8 @@ func (c *ZeroThirteenUpgradeCommand) Help() string {
 	helpText := `
 Usage: terraform 0.13upgrade [module-dir]
 
-  Generates a "providers.tf" configuration file which includes source
-  configuration for every non-default provider.
+  Updates module configuration files to add provider source attributes and
+  merge multiple required_providers blocks into one.
 `
 	return strings.TrimSpace(helpText)
 }

command/013_config_upgrade_test.go

@@ -93,7 +93,7 @@ func TestZeroThirteenUpgrade_success(t *testing.T) {
 		"preserves comments":    "013upgrade-preserves-comments",
 		"multiple blocks":       "013upgrade-multiple-blocks",
 		"multiple files":        "013upgrade-multiple-files",
-		"existing providers.tf": "013upgrade-existing-providers-tf",
+		"existing versions.tf":  "013upgrade-existing-versions-tf",
 		"skipped files":         "013upgrade-skipped-files",
 	}
 	for name, testPath := range testCases {

command/testdata/013upgrade-existing-providers-tf/input/providers.tf

@@ -1,3 +0,0 @@
-# This is a file called providers.tf which does not originally have a
-# required_providers block. 
-resource foo_resource a {}

command/testdata/013upgrade-existing-versions-tf/expected/versions.tf

@@ -1,7 +1,9 @@
-# This is a file called providers.tf which does not originally have a
+# This is a file called versions.tf which does not originally have a
 # required_providers block. 
 resource foo_resource a {}
+
 terraform {
+  required_version = ">= 0.13"
   required_providers {
     bar = {
       source = "hashicorp/bar"

command/testdata/013upgrade-existing-versions-tf/input/versions.tf

@@ -0,0 +1,7 @@
+# This is a file called versions.tf which does not originally have a
+# required_providers block. 
+resource foo_resource a {}
+
+terraform {
+  required_version = ">= 0.12"
+}

command/testdata/013upgrade-explicit-providers/expected/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.13"
+}

command/testdata/013upgrade-file-exists/expected/versions.tf

@@ -9,4 +9,5 @@ terraform {
       source = "hashicorp/foo"
     }
   }
+  required_version = ">= 0.13"
 }

command/testdata/013upgrade-implicit-not-found/expected/versions.tf

@@ -13,4 +13,5 @@ terraform {
       # https://www.terraform.io/docs/configuration/providers.html#provider-source
     }
   }
+  required_version = ">= 0.13"
 }

command/testdata/013upgrade-implicit-providers/expected/versions.tf

@@ -10,4 +10,5 @@ terraform {
       source = "hashicorp/foo"
     }
   }
+  required_version = ">= 0.13"
 }

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

@@ -12,4 +12,5 @@ terraform {
       version = "0.5"
     }
   }
+  required_version = ">= 0.13"
 }

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

@@ -13,4 +13,5 @@ terraform {
       version = "1.0.0"
     }
   }
+  required_version = ">= 0.13"
 }

command/testdata/013upgrade-preserves-comments/expected/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.13"
+}

command/testdata/013upgrade-provider-not-found/expected/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.13"
+}

command/testdata/013upgrade-providers-with-source/expected/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.13"
+}

command/testdata/013upgrade-skipped-files/expected/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.13"
+}

command/testdata/013upgrade-submodule/expected-module/module/versions.tf

@@ -4,4 +4,5 @@ terraform {
       source = "hashicorp/foo"
     }
   }
+  required_version = ">= 0.13"
 }

command/testdata/013upgrade-submodule/expected/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.13"
+}