fix corrupted outputs/.next_prefix file

- Since 2.3.2 invokeai stores the next PNG file's numeric prefix in a
  file named `.next_prefix` in the outputs directory. This avoids the
  overhead of doing a directory listing to find out what file number
  comes next.

- The code uses advisory locking to prevent corruption of this file in
  the event that multiple invokeai's try to access it simultaneously,
  but some users have experienced corruption of the file nevertheless.

- This PR addresses the problem by detecting a potentially corrupted
  `.next_prefix` file and falling back to the directory listing method.
  A fixed version of the file is then written out.

- Closes #3001

ldm/invoke/pngwriter.py

@@ -30,14 +30,17 @@ class PngWriter:
                 prefix = self._unused_prefix()
             else:
                 with open(next_prefix_file,'r') as file:
-                    prefix=int(file.readline() or int(self._unused_prefix())-1)
-                    prefix+=1
+                    prefix = 0
+                    try:
+                        prefix=int(file.readline())
+                    except (TypeError, ValueError):
+                        prefix=self._unused_prefix()
             with open(next_prefix_file,'w') as file:
-                file.write(str(prefix))
+                file.write(str(prefix+1))
         return f'{prefix:06}'
 
     # gives the next unique prefix in outdir
-    def _unused_prefix(self):
+    def _unused_prefix(self)->int:
         # sort reverse alphabetically until we find max+1
         dirlist = sorted(os.listdir(self.outdir), reverse=True)
         # find the first filename that matches our pattern or return 000000.0.png
@@ -45,8 +48,7 @@ class PngWriter:
             (f for f in dirlist if re.match('^(\d+)\..*\.png', f)),
             '0000000.0.png',
         )
-        basecount = int(existing_name.split('.', 1)[0]) + 1
-        return f'{basecount:06}'
+        return int(existing_name.split('.', 1)[0]) + 1
 
     # saves image named _image_ to outdir/name, writing metadata from prompt
     # returns full path of output