Prevent recovering to wrong devices or paths

Before starting a recovery run verify the source and destination have
not changed.  This will prevent issues on some extreme edge cases but
the main goal is for disappearing source drives with heavy damage.

e.g. A very damaged source drive disappears mid-recovery, drops off and
before would need a restart, or unplug/replug, to continue.  Now we can
attempt to re-detect the drive and resume recovery without leaving the
script.  If for some reason the drive order were to change then we'll
avoid using the wrong source or destination device.

scripts/wk/hw/ddrescue.py

@@ -1719,6 +1719,8 @@ def is_missing_source_or_destination(state):
     'Source': state.source,
     'Destination': state.destination,
     }
+
+  # Check items
   for name, item in items.items():
     if not item:
       continue
@@ -1733,9 +1735,38 @@ def is_missing_source_or_destination(state):
     else:
       LOG.error('Unknown %s type: %s', name, item)
 
+  # Update top panes
+  state.update_top_panes()
+
+  # Done
   return missing
 
 
+def source_or_destination_changed(state):
+  """Verify the source and destination objects are still valid."""
+  changed = False
+
+  # Compare objects
+  for obj in (state.source, state.destination):
+    if not obj:
+      changed = True
+    elif hasattr(obj, 'exists'):
+      # Assuming dest path
+      changed = changed or not obj.exists()
+    elif isinstance(obj, hw_obj.Disk):
+      compare_dev = hw_obj.Disk(obj.path)
+      for key in ('model', 'serial'):
+        changed = changed or obj.details[key] != compare_dev.details[key]
+
+  # Update top panes
+  state.update_top_panes()
+
+  # Done
+  if changed:
+    std.print_error('Source and/or Destination changed')
+  return changed
+
+
 def main():
   # pylint: disable=too-many-branches
   """Main function for ddrescue TUI."""
@@ -1760,7 +1791,6 @@ def main():
 
   # Show menu
   while True:
-    state.update_top_panes()
     selection = main_menu.advanced_select()
 
     # Change settings
@@ -1781,6 +1811,8 @@ def main():
         std.print_standard('Forcing controllers to rescan for devices...')
         cmd = 'echo "- - -" | sudo tee /sys/class/scsi_host/host*/scan'
         exe.run_program(cmd, check=False, shell=True)
+        if source_or_destination_changed(state):
+          std.abort()
 
     # Start recovery
     if 'Start' in selection:
@@ -2022,10 +2054,12 @@ def run_recovery(state, main_menu, settings_menu, dry_run=True):
 
   # Bail early
   if is_missing_source_or_destination(state):
-    state.update_top_panes()
     std.print_standard('')
     std.pause('Press Enter to return to main menu...')
     return
+  if source_or_destination_changed(state):
+    std.print_standard('')
+    std.abort()
 
   # Get settings
   for name, details in main_menu.toggles.items():