Harden chunk plan cache storage and reuse

internal/framework/chunkplan/store.go

@@ -17,7 +17,8 @@ import (
 const SchemaVersion = pipeline.ChunkPlanSchemaVersion
 
 type filesystemStore struct {
-	root string
+	root  string
+	write func(string, []byte) error
 }
 
 func NewFilesystemStore(root string) (pipeline.ChunkPlanStore, error) {
@@ -28,7 +29,7 @@ func NewFilesystemStore(root string) (pipeline.ChunkPlanStore, error) {
 	if strings.ContainsRune(root, '\x00') {
 		return nil, fmt.Errorf("chunk plan root must not contain NUL")
 	}
-	return &filesystemStore{root: filepath.Clean(root)}, nil
+	return &filesystemStore{root: filepath.Clean(root), write: writeAtomic}, nil
 }
 
 func (s *filesystemStore) Load(sourceDigest string) (pipeline.ChunkPlanRecord, pipeline.ChunkPlanDecision, error) {
@@ -77,7 +78,11 @@ func (s *filesystemStore) Save(record pipeline.ChunkPlanRecord) error {
 		return fmt.Errorf("encode chunk plan record: %w", err)
 	}
 	data = append(data, '\n')
-	if err := writeAtomic(target, data); err != nil {
+	writer := s.write
+	if writer == nil {
+		writer = writeAtomic
+	}
+	if err := writer(target, data); err != nil {
 		return fmt.Errorf("write chunk plan: %w", err)
 	}
 	return nil
@@ -164,7 +169,16 @@ func invalidDecision(reason string) (pipeline.ChunkPlanRecord, pipeline.ChunkPla
 	return pipeline.ChunkPlanRecord{}, pipeline.ChunkPlanDecision{Status: pipeline.ChunkPlanInvalid, Reason: reason}, nil
 }
 
+type atomicWriteHooks struct {
+	BeforeCreateTemp func() error
+	BeforeRename     func() error
+}
+
 func writeAtomic(target string, data []byte) error {
+	return writeAtomicWithHooks(target, data, atomicWriteHooks{})
+}
+
+func writeAtomicWithHooks(target string, data []byte, hooks atomicWriteHooks) error {
 	dir := filepath.Dir(target)
 	if err := os.MkdirAll(dir, 0o700); err != nil {
 		return err
@@ -173,6 +187,11 @@ func writeAtomic(target string, data []byte) error {
 		return err
 	}
 
+	if hooks.BeforeCreateTemp != nil {
+		if err := hooks.BeforeCreateTemp(); err != nil {
+			return err
+		}
+	}
 	temp, err := os.CreateTemp(dir, ".plan.json.tmp-*")
 	if err != nil {
 		return err
@@ -199,6 +218,11 @@ func writeAtomic(target string, data []byte) error {
 	if err := temp.Close(); err != nil {
 		return err
 	}
+	if hooks.BeforeRename != nil {
+		if err := hooks.BeforeRename(); err != nil {
+			return err
+		}
+	}
 	if err := os.Rename(tempPath, target); err != nil {
 		return err
 	}