storage: remember file length when writing to file

- make append fail if the actual file length differs
 - consolidate when file length > 2 * last consolidated length

electrum/json_db.py

@@ -381,10 +381,12 @@ class JsonDB(Logger):
 
     @locked
     def write(self):
-        if self.storage.file_exists() and not self.storage.is_encrypted():
-            self._append_pending_changes()
-        else:
+        if not self.storage.file_exists()\
+           or self.storage.is_encrypted()\
+           or self.storage.needs_consolidation():
             self._write()
+        else:
+            self._append_pending_changes()
 
     @locked
     def _append_pending_changes(self):

electrum/storage.py

@@ -71,10 +71,14 @@ class WalletStorage(Logger):
         if self.file_exists():
             with open(self.path, "r", encoding='utf-8') as f:
                 self.raw = f.read()
+                self.pos = f.seek(0, os.SEEK_END)
+                self.init_pos = self.pos
             self._encryption_version = self._init_encryption_version()
         else:
             self.raw = ''
             self._encryption_version = StorageEncryptionVersion.PLAINTEXT
+            self.pos = 0
+            self.init_pos = 0
 
     def read(self):
         return self.decrypted if self.is_encrypted() else self.raw
@@ -83,7 +87,7 @@ class WalletStorage(Logger):
         s = self.encrypt_before_writing(data)
         temp_path = "%s.tmp.%s" % (self.path, os.getpid())
         with open(temp_path, "w", encoding='utf-8') as f:
-            f.write(s)
+            self.pos = f.write(s)
             f.flush()
             os.fsync(f.fileno())
         try:
@@ -101,12 +105,16 @@ class WalletStorage(Logger):
     def append(self, data: str) -> None:
         """ append data to file. for the moment, only non-encrypted file"""
         assert not self.is_encrypted()
-        with open(self.path, "r+") as f:
-            f.seek(0, os.SEEK_END)
-            f.write(data)
+        with open(self.path, "r+", encoding='utf-8') as f:
+            pos = f.seek(0, os.SEEK_END)
+            assert pos == self.pos, (self.pos, pos)
+            self.pos += f.write(data)
             f.flush()
             os.fsync(f.fileno())
 
+    def needs_consolidation(self):
+        return self.pos > 2 * self.init_pos
+
     def file_exists(self) -> bool:
         return self._file_exists