fix: normalize and enforce stdin paths as well

See #440 for the last attempt to
do this because (at the time) this "is the file in the tree root?" check
relied upon checking for concrete files in the filesystem. As of
ff3de21,
that's longer the case, so we can make this code a bit more consistent.

cmd/format/format.go

@@ -168,31 +168,31 @@ func Run(v *viper.Viper, statz *stats.Stats, cmd *cobra.Command, paths []string)
 		return fmt.Errorf("invalid walk type: %w", err)
 	}
 
-	if walkType == walk.Stdin {
+	if walkType == walk.Stdin && len(paths) != 1 {
 		// check we have only received one path arg which we use for the file extension / matching to formatters
-		if len(paths) != 1 {
-			return fmt.Errorf("exactly one path should be specified when using the --stdin flag")
+		return fmt.Errorf("exactly one path should be specified when using the --stdin flag")
+	}
+
+	// checks all paths are contained within the tree root and exist
+	// also "normalize" paths so they're relative to cfg.TreeRoot
+	for i, path := range paths {
+		absolutePath, err := filepath.Abs(path)
+		if err != nil {
+			return fmt.Errorf("error computing absolute path of %s: %w", path, err)
 		}
-	} else {
-		// checks all paths are contained within the tree root and exist
-		// also "normalize" paths so they're relative to cfg.TreeRoot
-		for i, path := range paths {
-			absolutePath, err := filepath.Abs(path)
-			if err != nil {
-				return fmt.Errorf("error computing absolute path of %s: %w", path, err)
-			}
 
-			relativePath, err := filepath.Rel(cfg.TreeRoot, absolutePath)
-			if err != nil {
-				return fmt.Errorf("error computing relative path from %s to %s: %s", cfg.TreeRoot, absolutePath, err)
-			}
+		relativePath, err := filepath.Rel(cfg.TreeRoot, absolutePath)
+		if err != nil {
+			return fmt.Errorf("error computing relative path from %s to %s: %s", cfg.TreeRoot, absolutePath, err)
+		}
 
-			if strings.HasPrefix(relativePath, "..") {
-				return fmt.Errorf("path %s not inside the tree root %s", path, cfg.TreeRoot)
-			}
+		if strings.HasPrefix(relativePath, "..") {
+			return fmt.Errorf("path %s not inside the tree root %s", path, cfg.TreeRoot)
+		}
 
-			paths[i] = relativePath
+		paths[i] = relativePath
 
+		if walkType != walk.Stdin {
 			if _, err = os.Stat(absolutePath); err != nil {
 				return fmt.Errorf("path %s not found", path)
 			}

cmd/root_test.go

@@ -748,6 +748,14 @@ func TestStdin(t *testing.T) {
 	as.Equal(`{ ...}: "hello"
 `, string(out))
 
+	// try a file that's outside of the project root
+	contents = `{ foo, ... }: "hello"`
+	os.Stdin = test.TempFile(t, "", "stdin", &contents)
+
+	out, _, err = treefmt(t, "-C", tempDir, "--allow-missing-formatter", "--stdin", "../test.nix")
+	as.Errorf(err, "path ../test.nix not inside the tree root %s", tempDir)
+	as.Equal("", string(out))
+
 	// try some markdown instead
 	contents = `
 | col1 | col2 |