Avoid double-free

backup.go

@@ -112,10 +112,13 @@ func (b *Backup) Close() error {
 	if b == nil {
 		return errors.New("nil sqlite backup")
 	}
-	rv := C.sqlite3_backup_finish(b.sb)
+	if b.sb == nil {
+		return nil
+	}
+	rv := C.sqlite3_backup_finish(b.sb) // must be called only once
+	b.sb = nil
 	if rv != C.SQLITE_OK {
 		return b.dst.error(rv, "backup finish failed")
 	}
-	b.sb = nil
 	return nil
 }

blob.go

@@ -84,11 +84,14 @@ func (r *BlobReader) Close() error {
 	if r == nil {
 		return errors.New("nil sqlite blob")
 	}
-	rv := C.sqlite3_blob_close(r.bl)
+	if r.bl == nil {
+		return nil
+	}
+	rv := C.sqlite3_blob_close(r.bl) // must be called only once
+	r.bl = nil
 	if rv != C.SQLITE_OK {
 		return r.c.error(rv, "BlobReader.Close")
 	}
-	r.bl = nil
 	return nil
 }
 

stmt.go

@@ -1167,7 +1167,7 @@ func (s *Stmt) finalize() error {
 		Log(C.SQLITE_MISUSE, "sqlite statement with already closed database connection")
 		return errors.New("sqlite statement with already closed database connection")
 	}
-	rv := C.sqlite3_finalize(s.stmt)
+	rv := C.sqlite3_finalize(s.stmt) // must be called only once
 	s.stmt = nil
 	if rv != C.SQLITE_OK {
 		Log(int32(rv), "error while finalizing Stmt")