backup: Add PRAGMA foreign_keys=on when restoring so cascades work

We heavily rely on foreign key constraints to keep the DB consistent and drop
dependent objects such as UTXO set entries when we roll back a block. We also
need to enable the foreign key constraints when restoring otherwise these
dependents are not removed and we run into constraint violations.

backup/backup.py

@@ -80,26 +80,44 @@ class Backend(object):
         """
         raise NotImplementedError
 
-    def restore(self, dest: str, remove_existing: bool = False) -> bool:
-        """Restore the backup in this backend to its former glory.
-        """
+    def _db_open(self, dest: str) -> sqlite3.Connection:
+        db = sqlite3.connect(dest)
+        db.execute("PRAGMA foreign_keys = 1")
+        return db
+
+    def _restore_snapshot(self, snapshot: bytes, dest: str):
         if os.path.exists(dest):
             os.unlink(dest)
-        db = sqlite3.connect(dest)
-        for c in tqdm(self.stream_changes(), total=self.version):
-            if c.snapshot is not None:
-                if os.path.exists(dest):
-                    os.unlink(dest)
-                with open(dest, 'wb') as f:
-                    f.write(c.snapshot)
-                db = sqlite3.connect(dest)
-            if c.transaction is not None:
-                cur = db.cursor()
-                for q in c.transaction:
-                    cur.execute(q.decode('UTF-8'))
-                db.commit()
+        with open(dest, 'wb') as f:
+            f.write(snapshot)
+        self.db = self._db_open(dest)
 
-        return True
+    def _restore_transaction(self, tx: Iterator[bytes]):
+        assert(self.db)
+        cur = self.db.cursor()
+        for q in tx:
+            cur.execute(q.decode('UTF-8'))
+        self.db.commit()
+
+    def restore(self, dest: str, remove_existing: bool = False):
+        """Restore the backup in this backend to its former glory.
+        """
+
+        if os.path.exists(dest):
+            if not remove_existing:
+                raise ValueError(
+                    "Destination for backup restore exists: {dest}".format(
+                        dest=dest
+                    )
+                )
+            os.unlink(dest)
+
+        self.db = self._db_open(dest)
+        for c in tqdm(self.stream_changes()):
+            if c.snapshot is not None:
+                self._restore_snapshot(c.snapshot, dest)
+            if c.transaction is not None:
+                self._restore_transaction(c.transaction)
 
 
 class FileBackend(Backend):