restrict file writes for --multi to the specified output directory

cmd/jsonnet/cmd.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -293,6 +294,48 @@ func processArgs(givenArgs []string, config *config, vm *jsonnet.VM) (processArg
 	return processArgsStatusContinue, nil
 }
 
+// (*Root) ReadFile() is added in Go 1.25; switch to that in the future.
+func rootReadFile(root *os.Root, name string) ([]byte, error) {
+	f, err := root.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return io.ReadAll(f)
+}
+
+// (*Root) WriteFile() is added in Go 1.25; switch to that in the future.
+func rootWriteFile(root *os.Root, name string, data []byte, perm os.FileMode) error {
+	f, err := root.OpenFile(name, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	_, finalErr := f.Write(data)
+	if err := f.Close(); err != nil {
+		finalErr = err
+	}
+	return finalErr
+}
+
+// (*Root) MkdirAll() is added in Go 1.25; switch to that in the future.
+func rootMkdirAll(root *os.Root, name string, perm os.FileMode) error {
+	var dirs []string
+	rest := name
+	for rest != "." && rest != "/" && rest != "" {
+		dirs = append(dirs, rest)
+		rest = path.Dir(rest)
+	}
+	for i := range dirs {
+		if err := root.Mkdir(dirs[len(dirs)-i-1], perm); err != nil {
+			if os.IsExist(err) {
+				continue
+			}
+			return err
+		}
+	}
+	return nil
+}
+
 func writeMultiOutputFiles(output map[string]string, outputDir, outputFile string, createDirs bool) (err error) {
 	// If multiple file output is used, then iterate over each string from
 	// the sequence of strings returned by jsonnet_evaluate_snippet_multi,
@@ -320,22 +363,43 @@ func writeMultiOutputFiles(output map[string]string, outputDir, outputFile strin
 		keys = append(keys, k)
 	}
 	sort.Strings(keys)
+
+	if len(keys) == 0 {
+		return nil
+	}
+
+	outputDir = filepath.Clean(outputDir)
+	root, err := os.OpenRoot(outputDir)
+	if err != nil {
+		if os.IsNotExist(err) && createDirs {
+			if err := os.MkdirAll(outputDir, 0755); err != nil {
+				return err
+			}
+			root, err = os.OpenRoot(outputDir)
+		}
+		if err != nil {
+			return err
+		}
+	}
+
 	for _, key := range keys {
 		newContent := output[key]
-		filename := outputDir + key
+		relpath := filepath.Clean(key)
+		if filepath.IsAbs(key) {
+			relpath, err = filepath.Rel("/", key)
+			if err != nil {
+				return err
+			}
+		}
+		abspath := filepath.Join(outputDir, relpath)
 
-		_, err := manifest.WriteString(filename)
+		_, err := manifest.WriteString(abspath + "\n")
 		if err != nil {
 			return err
 		}
 
-		_, err = manifest.WriteString("\n")
-		if err != nil {
-			return err
-		}
-
-		if _, err := os.Stat(filename); !os.IsNotExist(err) {
-			existingContent, err := os.ReadFile(filename)
+		if _, err := root.Stat(relpath); !os.IsNotExist(err) {
+			existingContent, err := rootReadFile(root, relpath)
 			if err != nil {
 				return err
 			}
@@ -347,12 +411,12 @@ func writeMultiOutputFiles(output map[string]string, outputDir, outputFile strin
 			}
 		}
 		if createDirs {
-			if err := os.MkdirAll(filepath.Dir(filename), 0755); err != nil {
+			if err := rootMkdirAll(root, filepath.Dir(relpath), 0755); err != nil {
 				return err
 			}
 		}
 
-		err = os.WriteFile(filename, []byte(newContent), 0666)
+		err = rootWriteFile(root, relpath, []byte(newContent), 0666)
 		if err != nil {
 			return err
 		}