backup: Refuse to create a new FileBakend if the backing file exists

backup/backup-cli

@@ -12,7 +12,7 @@ import sys
 @click.argument("backend-url")
 def init(lightning_dir, backend_url):
     destination = backend_url
-    backend = get_backend(destination)
+    backend = get_backend(destination, create=True)
     backend.version, backend.prev_version = 0, 0
     backend.offsets = [512, 0]
     backend.version_count = 0

backup/backup.py

@@ -121,7 +121,7 @@ class Backend(object):
 
 
 class FileBackend(Backend):
-    def __init__(self, destination: str):
+    def __init__(self, destination: str, create: bool):
         self.version = None
         self.prev_version = None
         self.destination = destination
@@ -129,6 +129,9 @@ class FileBackend(Backend):
         self.version_count = 0
         self.url = urlparse(self.destination)
 
+        if os.path.exists(self.url.path) and create:
+            raise ValueError("Attempted to create a FileBackend, but file already exists.")
+
     def initialize(self) -> bool:
         if not os.path.exists(self.url.path):
             self.version = 0
@@ -232,13 +235,13 @@ def resolve_backend_class(backend_url):
     return backend_cl
 
 
-def get_backend(destination):
+def get_backend(destination, create=False):
     backend_cl = resolve_backend_class(destination)
     if backend_cl is None:
         raise ValueError("No backend implementation found for {destination}".format(
             destination=destination,
         ))
-    backend = backend_cl(destination)
+    backend = backend_cl(destination, create=create)
     backend.initialize()
     assert(backend.version is not None)
     assert(backend.prev_version is not None)