Copy out atx blob from the db query cache

sql/atxs/atxs.go

@@ -301,10 +301,6 @@ func GetBlobSizes(db sql.Executor, ids [][]byte) (sizes []int, err error) {
 }
 
 // LoadBlob loads ATX as an encoded blob, ready to be sent over the wire.
-//
-// SAFETY: The contents of the returned blob MUST NOT be modified.
-// They might point to the inner sql cache and modifying them would
-// corrupt the cache.
 func LoadBlob(ctx context.Context, db sql.Executor, id []byte, blob *sql.Blob) (types.AtxVersion, error) {
 	if sql.IsCached(db) {
 		type cachedBlob struct {
@@ -325,8 +321,8 @@ func LoadBlob(ctx context.Context, db sql.Executor, id []byte, blob *sql.Blob) (
 		if err != nil {
 			return 0, err
 		}
-		// Here we return the cached slice, hence the safety warning.
-		blob.Bytes = cached.buf
+		blob.Resize(len(cached.buf))
+		copy(blob.Bytes, cached.buf)
 		return cached.version, nil
 	}
 

sql/atxs/atxs_test.go

@@ -647,6 +647,26 @@ func TestGetBlobCached_CacheEntriesAreDistinct(t *testing.T) {
 	require.Equal(t, atx.AtxBlob.Blob, blob.Bytes)
 }
 
+// Test that the cached blob is not shared with the caller
+// but copied into the provided blob.
+func TestGetBlobCached_OverwriteSafety(t *testing.T) {
+	db := sql.InMemory(sql.WithQueryCache(true))
+	atx := types.ActivationTx{}
+	atx.SetID(types.RandomATXID())
+	atx.AtxBlob.Blob = []byte("original blob")
+	require.NoError(t, atxs.Add(db, &atx))
+	require.Equal(t, 2, db.QueryCount()) // insert atx + blob
+
+	var b sql.Blob // we will reuse the blob between queries
+	_, err := atxs.LoadBlob(context.Background(), db, atx.ID().Bytes(), &b)
+	require.NoError(t, err)
+	require.Equal(t, atx.AtxBlob.Blob, b.Bytes)
+	b.Bytes[0] = 'X' // modify the blob
+	_, err = atxs.LoadBlob(context.Background(), db, atx.ID().Bytes(), &b)
+	require.NoError(t, err)
+	require.Equal(t, atx.AtxBlob.Blob, b.Bytes)
+}
+
 func TestCachedBlobEviction(t *testing.T) {
 	db := sql.InMemory(
 		sql.WithQueryCache(true),

sql/database.go

@@ -505,10 +505,10 @@ type Blob struct {
 	Bytes []byte
 }
 
-// resize the underlying byte slice to the specified size.
+// Resize the underlying byte slice to the specified size.
 // The returned slice has length equal n, but it might have a larger capacity.
 // Warning: it is not guaranteed to keep the old data.
-func (b *Blob) resize(n int) {
+func (b *Blob) Resize(n int) {
 	if cap(b.Bytes) < n {
 		b.Bytes = make([]byte, n)
 	}
@@ -517,10 +517,10 @@ func (b *Blob) resize(n int) {
 
 func (b *Blob) FromColumn(stmt *Statement, col int) {
 	if l := stmt.ColumnLen(col); l != 0 {
-		b.resize(l)
+		b.Resize(l)
 		stmt.ColumnBytes(col, b.Bytes)
 	} else {
-		b.resize(0)
+		b.Resize(0)
 	}
 }