safer os.chmod for wallet files and config: set perms before write

Set unix file permissions first, before writing data.

electrum/gui/qt/main_window.py

@@ -2332,7 +2332,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger, QtEventListener):
 
     def do_export_privkeys(self, fileName, pklist, is_csv):
         with open(fileName, "w+") as f:
-            os_chmod(fileName, 0o600)
+            os_chmod(fileName, 0o600)  # set restrictive perms *before* we write data
             if is_csv:
                 transaction = csv.writer(f)
                 transaction.writerow(["address", "private_key"])

electrum/simple_config.py

@@ -421,8 +421,8 @@ class SimpleConfig(Logger):
         s = json.dumps(self.user_config, indent=4, sort_keys=True)
         try:
             with open(path, "w", encoding='utf-8') as f:
+                os_chmod(path, stat.S_IREAD | stat.S_IWRITE)  # set restrictive perms *before* we write data
                 f.write(s)
-            os_chmod(path, stat.S_IREAD | stat.S_IWRITE)
         except OSError:
             # datadir probably deleted while running... e.g. portable exe running on ejected USB drive
             # (in which case it is typically either FileNotFoundError or PermissionError,

electrum/storage.py

@@ -87,22 +87,22 @@ class WalletStorage(Logger):
         return self.decrypted if self.is_encrypted() else self.raw
 
     def write(self, data: str) -> None:
+        try:
+            mode = os.stat(self.path).st_mode
+        except FileNotFoundError:
+            mode = stat.S_IREAD | stat.S_IWRITE
         s = self.encrypt_before_writing(data)
         temp_path = "%s.tmp.%s" % (self.path, os.getpid())
         with open(temp_path, "wb") as f:
+            os_chmod(temp_path, mode)  # set restrictive perms *before* we write data
             f.write(s.encode("utf-8"))
             self.pos = f.seek(0, os.SEEK_END)
             f.flush()
             os.fsync(f.fileno())
-        try:
-            mode = os.stat(self.path).st_mode
-        except FileNotFoundError:
-            mode = stat.S_IREAD | stat.S_IWRITE
         # assert that wallet file does not exist, to prevent wallet corruption (see issue #5082)
         if not self.file_exists():
             assert not os.path.exists(self.path)
         os.replace(temp_path, self.path)
-        os_chmod(self.path, mode)
         self._file_exists = True
         self.logger.info(f"saved {self.path}")