qt gui: more resilient startup: catch more exceptions, better fallback

fixes https://github.com/spesmilo/electrum/issues/7447

Consider this trace for 4.2.0:
```
Traceback (most recent call last):
  File "electrum/gui/qt/__init__.py", line 332, in start_new_window
  File "electrum/gui/qt/__init__.py", line 363, in _start_wizard_to_select_or_create_wallet
  File "electrum/gui/qt/installwizard.py", line 302, in select_storage
  File "electrum/util.py", line 504, in get_new_wallet_name
PermissionError: [Errno 1] Operation not permitted: '/Users/admin/Documents/Peach/MS'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "electrum/gui/qt/__init__.py", line 426, in main
  File "electrum/gui/qt/__init__.py", line 307, in wrapper
  File "electrum/gui/qt/__init__.py", line 349, in start_new_window
  File "electrum/util.py", line 504, in get_new_wallet_name
PermissionError: [Errno 1] Operation not permitted: '/Users/admin/Documents/Peach/MS'
```

Note that `get_new_wallet_name` (os.listdir) can raise OSError,
and we were calling that on the main entrypoint codepath without exception-handling.
We were also calling it in the fallback codepath without exception-handling.
i.e. the GUI errored out on every startup for affected users, and without CLI usage
it was not possible to recover.

electrum/gui/qt/__init__.py

@@ -320,10 +320,20 @@ class ElectrumGui(BaseElectrumGui, Logger):
         return wrapper
 
     @count_wizards_in_progress
-    def start_new_window(self, path, uri, *, app_is_starting=False) -> Optional[ElectrumWindow]:
+    def start_new_window(
+            self,
+            path,
+            uri: Optional[str],
+            *,
+            app_is_starting: bool = False,
+            force_wizard: bool = False,
+    ) -> Optional[ElectrumWindow]:
         '''Raises the window for the wallet if it is open.  Otherwise
         opens the wallet and creates a new window for it'''
         wallet = None
+        # Try to open with daemon first. If this succeeds, there won't be a wizard at all
+        # (the wallet main window will appear directly).
+        if not force_wizard:
             try:
                 wallet = self.daemon.load_wallet(path, None)
             except Exception as e:
@@ -335,6 +345,8 @@ class ElectrumGui(BaseElectrumGui, Logger):
                 # if app is starting, still let wizard appear
                 if not app_is_starting:
                     return
+        # Open a wizard window. This lets the user e.g. enter a password, or select
+        # a different wallet.
         try:
             if not wallet:
                 wallet = self._start_wizard_to_select_or_create_wallet(path)
@@ -353,9 +365,19 @@ class ElectrumGui(BaseElectrumGui, Logger):
                                title=_('Error'),
                                text=_('Cannot load wallet') + '(2) :\n' + repr(e))
             if app_is_starting:
+                # If we raise in this context, there are no more fallbacks, we will shut down.
+                # Worst case scenario, we might have gotten here without user interaction,
+                # in which case, if we raise now without user interaction, the same sequence of
+                # events is likely to repeat when the user restarts the process.
+                # So we play it safe: clear path, clear uri, force a wizard to appear.
+                try:
                     wallet_dir = os.path.dirname(path)
-                path = os.path.join(wallet_dir, get_new_wallet_name(wallet_dir))
-                self.start_new_window(path, uri)
+                    filename = get_new_wallet_name(wallet_dir)
+                except OSError:
+                    path = self.config.get_fallback_wallet_path()
+                else:
+                    path = os.path.join(wallet_dir, filename)
+                self.start_new_window(path, uri=None, force_wizard=True)
             return
         if uri:
             window.pay_to_URI(uri)

electrum/gui/qt/installwizard.py

@@ -202,6 +202,8 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
         self.refresh_gui()  # Need for QT on MacOSX.  Lame.
 
     def select_storage(self, path, get_wallet_from_daemon) -> Tuple[str, Optional[WalletStorage]]:
+        if os.path.isdir(path):
+            raise Exception("wallet path cannot point to a directory")
 
         vbox = QVBoxLayout()
         hbox = QHBoxLayout()
@@ -297,9 +299,7 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
 
         button.clicked.connect(on_choose)
         button_create_new.clicked.connect(
-            partial(
-                name_e.setText,
-                get_new_wallet_name(wallet_folder)))
+            lambda: name_e.setText(get_new_wallet_name(wallet_folder)))  # FIXME get_new_wallet_name might raise
         name_e.textChanged.connect(on_filename)
         name_e.setText(os.path.basename(path))
 

electrum/gui/qt/main_window.py

@@ -676,9 +676,15 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         except FileNotFoundError as e:
             self.show_error(str(e))
             return
+        try:
             filename = get_new_wallet_name(wallet_folder)
-        full_path = os.path.join(wallet_folder, filename)
-        self.gui_object.start_new_window(full_path, None)
+        except OSError as e:
+            self.logger.exception("")
+            self.show_error(repr(e))
+            path = self.config.get_fallback_wallet_path()
+        else:
+            path = os.path.join(wallet_folder, filename)
+        self.gui_object.start_new_window(path, uri=None, force_wizard=True)
 
     def init_menubar(self):
         menubar = QMenuBar()

electrum/simple_config.py

@@ -296,12 +296,7 @@ class SimpleConfig(Logger):
             if path and os.path.exists(path):
                 return path
 
-        # default path
-        util.assert_datadir_available(self.path)
-        dirpath = os.path.join(self.path, "wallets")
-        make_dir(dirpath, allow_symlink=False)
-
-        new_path = os.path.join(self.path, "wallets", "default_wallet")
+        new_path = self.get_fallback_wallet_path()
 
         # default path in pre 1.9 versions
         old_path = os.path.join(self.path, "electrum.dat")
@@ -310,6 +305,13 @@ class SimpleConfig(Logger):
 
         return new_path
 
+    def get_fallback_wallet_path(self):
+        util.assert_datadir_available(self.path)
+        dirpath = os.path.join(self.path, "wallets")
+        make_dir(dirpath, allow_symlink=False)
+        path = os.path.join(self.path, "wallets", "default_wallet")
+        return path
+
     def remove_from_recently_open(self, filename):
         recent = self.get('recently_open', [])
         if filename in recent:

electrum/util.py

@@ -498,6 +498,9 @@ def standardize_path(path):
 
 
 def get_new_wallet_name(wallet_folder: str) -> str:
+    """Returns a file basename for a new wallet to be used.
+    Can raise OSError.
+    """
     i = 1
     while True:
         filename = "wallet_%d" % i