op-program: kv store add pre-image overwriting error

op-program/host/kvstore/disk.go

@@ -30,9 +30,11 @@ func (d *DiskKV) pathKey(k common.Hash) string {
 }
 
 func (d *DiskKV) Put(k common.Hash, v []byte) error {
-	// no O_EXCL, the pre-image may already exist. It's fine to overwrite it.
-	f, err := os.OpenFile(d.pathKey(k), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, diskPermission)
+	f, err := os.OpenFile(d.pathKey(k), os.O_WRONLY|os.O_CREATE|os.O_EXCL|os.O_TRUNC, diskPermission)
 	if err != nil {
+		if os.IsExist(err) {
+			return ErrAlreadyExists
+		}
 		return fmt.Errorf("failed to open new pre-image file %s: %w", k, err)
 	}
 	if _, err := f.Write([]byte(hex.EncodeToString(v))); err != nil {
@@ -49,7 +51,7 @@ func (d *DiskKV) Get(k common.Hash) ([]byte, error) {
 	f, err := os.OpenFile(d.pathKey(k), os.O_RDONLY, diskPermission)
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, NotFoundErr
+			return nil, ErrNotFound
 		}
 		return nil, fmt.Errorf("failed to open pre-image file %s: %w", k, err)
 	}

op-program/host/kvstore/kv.go

@@ -6,11 +6,21 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 )
 
-// NotFoundErr is returned when a pre-image cannot be found in the KV store.
-var NotFoundErr = errors.New("not found")
+// ErrNotFound is returned when a pre-image cannot be found in the KV store.
+var ErrNotFound = errors.New("not found")
+
+// ErrAlreadyExists is returned when a pre-image already exists in the KV store.
+var ErrAlreadyExists = errors.New("already exists")
 
 // KV is a Key-Value store interface for pre-image data.
 type KV interface {
+	// Put puts the pre-image value v in the key-value store with key k.
+	// It returns ErrAlreadyExists when the key already exists.
+	// KV store implementations may return additional errors specific to the KV storage.
 	Put(k common.Hash, v []byte) error
+
+	// Get retrieves the pre-image with key k from the key-value store.
+	// It returns ErrNotFound when the pre-image cannot be found.
+	// KV store implementations may return additional errors specific to the KV storage.
 	Get(k common.Hash) ([]byte, error)
 }

op-program/host/kvstore/kv_test.go

@@ -11,7 +11,7 @@ func kvTest(t *testing.T, kv KV) {
 	t.Run("roundtrip", func(t *testing.T) {
 		t.Parallel()
 		_, err := kv.Get(common.Hash{0xaa})
-		require.Equal(t, err, NotFoundErr, "file (in new tmp dir) does not exist yet")
+		require.Equal(t, err, ErrNotFound, "file (in new tmp dir) does not exist yet")
 
 		require.NoError(t, kv.Put(common.Hash{0xaa}, []byte("hello world")))
 		dat, err := kv.Get(common.Hash{0xaa})
@@ -44,4 +44,10 @@ func kvTest(t *testing.T, kv KV) {
 		require.NoError(t, err, "pre-image must exist now")
 		require.Equal(t, []byte{4, 2}, dat, "pre-image must match")
 	})
+
+	t.Run("not overwriting pre-image", func(t *testing.T) {
+		t.Parallel()
+		require.NoError(t, kv.Put(common.Hash{0xdd}, []byte{4, 2}))
+		require.ErrorIs(t, kv.Put(common.Hash{0xdd}, []byte{4, 2}), ErrAlreadyExists)
+	})
 }

op-program/host/kvstore/mem.go

@@ -23,6 +23,9 @@ func NewMemKV() *MemKV {
 func (m *MemKV) Put(k common.Hash, v []byte) error {
 	m.Lock()
 	defer m.Unlock()
+	if _, ok := m.m[k]; ok {
+		return ErrAlreadyExists
+	}
 	m.m[k] = v
 	return nil
 }
@@ -32,7 +35,7 @@ func (m *MemKV) Get(k common.Hash) ([]byte, error) {
 	defer m.RUnlock()
 	v, ok := m.m[k]
 	if !ok {
-		return nil, NotFoundErr
+		return nil, ErrNotFound
 	}
 	return v, nil
 }