command/init: return an error with invalid -backend-config files (#25411)

* command/init: return an error with invalid -backend-config files

The -backend-config flag expects a set of key-value pairs or a file
containing key-value pairs. If the file instead contains a full backend
configuration block, it was silently ignored. This commit adds a check
for blocks in the file and returns an error if they are encountered.

Fixes #24845

* emphasize backend configuration file in docs

command/init.go

@@ -803,6 +803,18 @@ func (c *InitCommand) backendConfigOverrideBody(flags rawFlags, schema *configsc
 			// The value is interpreted as a filename.
 			newBody, fileDiags := c.loadHCLFile(item.Value)
 			diags = diags.Append(fileDiags)
+			// Verify that the file contains only key-values pairs, and not a
+			// full backend config block. JustAttributes() will return an error
+			// if blocks are found
+			_, attrDiags := newBody.JustAttributes()
+			if attrDiags.HasErrors() {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Invalid backend configuration file",
+					fmt.Sprintf("The backend configuration file %q given on the command line must contain key-value pairs only, and not configuration blocks.", item.Value),
+				))
+				continue
+			}
 			flushVals() // deal with any accumulated individual values first
 			mergeBody(newBody)
 		} else {

command/init_test.go

@@ -337,24 +337,43 @@ func TestInit_backendConfigFile(t *testing.T) {
 	defer os.RemoveAll(td)
 	defer testChdir(t, td)()
 
-	ui := new(cli.MockUi)
-	c := &InitCommand{
-		Meta: Meta{
-			testingOverrides: metaOverridesForProvider(testProvider()),
-			Ui:               ui,
-		},
-	}
+	t.Run("good-config-file", func(t *testing.T) {
+		ui := new(cli.MockUi)
+		c := &InitCommand{
+			Meta: Meta{
+				testingOverrides: metaOverridesForProvider(testProvider()),
+				Ui:               ui,
+			},
+		}
+		args := []string{"-backend-config", "input.config"}
+		if code := c.Run(args); code != 0 {
+			t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+		}
 
-	args := []string{"-backend-config", "input.config"}
-	if code := c.Run(args); code != 0 {
-		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
-	}
+		// Read our saved backend config and verify we have our settings
+		state := testDataStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+		if got, want := normalizeJSON(t, state.Backend.ConfigRaw), `{"path":"hello","workspace_dir":null}`; got != want {
+			t.Errorf("wrong config\ngot:  %s\nwant: %s", got, want)
+		}
+	})
 
-	// Read our saved backend config and verify we have our settings
-	state := testDataStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
-	if got, want := normalizeJSON(t, state.Backend.ConfigRaw), `{"path":"hello","workspace_dir":null}`; got != want {
-		t.Errorf("wrong config\ngot:  %s\nwant: %s", got, want)
-	}
+	// the backend config file must be a set of key-value pairs and not a full backend {} block
+	t.Run("invalid-config-file", func(t *testing.T) {
+		ui := new(cli.MockUi)
+		c := &InitCommand{
+			Meta: Meta{
+				testingOverrides: metaOverridesForProvider(testProvider()),
+				Ui:               ui,
+			},
+		}
+		args := []string{"-backend-config", "backend.config"}
+		if code := c.Run(args); code != 1 {
+			t.Fatalf("expected error, got success\n")
+		}
+		if !strings.Contains(ui.ErrorWriter.String(), "Invalid backend configuration file") {
+			t.Fatalf("wrong error: %s", ui.ErrorWriter)
+		}
+	})
 }
 
 func TestInit_backendConfigFilePowershellConfusion(t *testing.T) {

command/testdata/init-backend-config-file/backend.config

@@ -0,0 +1,7 @@
+// the -backend-config flag on init cannot be used to point to a "full" backend
+// block, only key-value pairs (like terraform.tfvars) 
+terraform {
+    backend "local" {
+        path = "hello"
+    }
+}

website/docs/backends/config.html.md

@@ -63,12 +63,12 @@ There are several ways to supply the remaining arguments:
     values, unless interactive input is disabled. Terraform will not prompt for
     optional values.
 
-  * **File**: A configuration file may be specified via the `init` command line.
-    To specify a file, use the `-backend-config=PATH` option when running
-    `terraform init`. If the file contains secrets it may be kept in
-    a secure data store, such as
-    [Vault](https://www.vaultproject.io/), in which case it must be downloaded
-    to the local disk before running Terraform.
+  * **File**: A [backend configuration file](#backend-configuration-file) may be specified via the
+    `init` command line. To specify a file, use the `-backend-config=PATH`
+    option when running `terraform init`. If the file contains secrets it may be
+    kept in a secure data store, such as [Vault](https://www.vaultproject.io/),
+    in which case it must be downloaded to the local disk before running
+    Terraform.
 
   * **Command-line key/value pairs**: Key/value pairs can be specified via the
     `init` command line. Note that many shells retain command-line flags in a
@@ -96,6 +96,7 @@ terraform {
 }
 ```
 
+## Backend Configuration File
 A backend configuration file has the contents of the `backend` block as
 top-level attributes, without the need to wrap it in another `terraform`
 or `backend` block: