backup: Implement the restore method for all backends

Also implements the `stream_changes` function in the FileBackend which is used
by `restore` to get all the changes.

backup/backup.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 from collections import namedtuple
 from pyln.client import Plugin
+from tqdm import tqdm
 from typing import Mapping, Type, Iterator
 from urllib.parse import urlparse
 import json
@@ -8,6 +9,7 @@ import logging
 import os
 import struct
 import sys
+import sqlite3
 
 
 plugin = Plugin()
@@ -78,6 +80,27 @@ 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.
+        """
+        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()
+
+        return True
+
 
 class FileBackend(Backend):
     def __init__(self, destination: str):
@@ -160,6 +183,27 @@ class FileBackend(Backend):
         self.prev_version, self.offsets[1] = 0, 0
         return True
 
+    def stream_changes(self) -> Iterator[Change]:
+        self.read_metadata()
+        version = -1
+        with open(self.url.path, 'rb') as f:
+            # Skip the header
+            f.seek(512)
+            while version < self.version:
+                length, version, typ = struct.unpack("!IIb", f.read(9))
+                payload = f.read(length)
+                if typ == 1:
+                    yield Change(version=version, snapshot=None, transaction=payload.split(b'\x00'))
+                elif typ == 2:
+                    yield Change(version=version, snapshot=payload, transaction=None)
+                else:
+                    raise ValueError("Unknown FileBackend entry type {}".format(typ))
+
+            if version != self.version:
+                raise ValueError("Versions do not match up: restored version {}, backend version {}".format(version, self.version))
+            assert(version == self.version)
+
+
 
 def resolve_backend_class(backend_url):
     backend_map: Mapping[str, Type[Backend]] = {