Improve error handling and reporting for AuthzPolicy

[Ryan J Ollos]

We can improve error handling and reporting for 3 scenarios that may be seen when using the AuthzPolicy​.

1. When an authz file has duplicated sections, Trac generates an internal error with a traceback:

   Traceback (most recent call last):
     File "/home/user/Workspace/t8976/teo-rjollos.git/trac/web/main.py", line 497, in _dispatch_request
       dispatcher.dispatch(req)
     File "/home/user/Workspace/t8976/teo-rjollos.git/trac/web/main.py", line 214, in dispatch
       resp = chosen_handler.process_request(req)
     File "/home/user/Workspace/t8976/teo-rjollos.git/trac/wiki/web_ui.py", line 126, in process_request
       req.perm(page.resource).require('WIKI_VIEW')
     File "/home/user/Workspace/t8976/teo-rjollos.git/trac/perm.py", line 578, in require
       if not self._has_permission(action, resource):
     File "/home/user/Workspace/t8976/teo-rjollos.git/trac/perm.py", line 570, in _has_permission
       check_permission(action, perm.username, resource, perm)
     File "/home/user/Workspace/t8976/teo-rjollos.git/trac/perm.py", line 462, in check_permission
       perm)
     File "/home/user/Workspace/t8976/teo-rjollos.git/tracopt/perm/authz_policy.py", line 147, in check_permission
       self.parse_authz()
     File "/home/user/Workspace/t8976/teo-rjollos.git/tracopt/perm/authz_policy.py", line 176, in parse_authz
       self.authz = ConfigObj(self.get_authz_file(), encoding='utf8')
     File "/usr/lib/python2.7/dist-packages/configobj.py", line 1230, in __init__
       self._load(infile, configspec)
     File "/usr/lib/python2.7/dist-packages/configobj.py", line 1320, in _load
       raise error
   ConfigObjError: Parsing failed with several errors.
   First error at line 4.
   

   Though the duplicate sections error is easy to reproduce, the proposed change should trap and present a user-friendly error message when any exception is thrown while ConfigObj is parsing the file (by trapping all ​ConfigObjErrors).
2. When the authz file is not found, Trac generates an internal error with a traceback:

   Traceback (most recent call last):
     File "/home/user/Workspace/t11260/teo-rjollos.git/trac/web/main.py", line 497, in _dispatch_request
       dispatcher.dispatch(req)
     File "/home/user/Workspace/t11260/teo-rjollos.git/trac/web/main.py", line 214, in dispatch
       resp = chosen_handler.process_request(req)
     File "/home/user/Workspace/t11260/teo-rjollos.git/trac/wiki/web_ui.py", line 126, in process_request
       req.perm(page.resource).require('WIKI_VIEW')
     File "/home/user/Workspace/t11260/teo-rjollos.git/trac/perm.py", line 578, in require
       if not self._has_permission(action, resource):
     File "/home/user/Workspace/t11260/teo-rjollos.git/trac/perm.py", line 570, in _has_permission
       check_permission(action, perm.username, resource, perm)
     File "/home/user/Workspace/t11260/teo-rjollos.git/trac/perm.py", line 462, in check_permission
       perm)
     File "/home/user/Workspace/t11260/teo-rjollos.git/tracopt/perm/authz_policy.py", line 149, in check_permission
       self.parse_authz()
     File "/home/user/Workspace/t11260/teo-rjollos.git/tracopt/perm/authz_policy.py", line 211, in parse_authz
       self.authz_mtime = self.get_authz_file_mtime
     File "/home/user/Workspace/t11260/teo-rjollos.git/tracopt/perm/authz_policy.py", line 178, in get_authz_file_mtime
       return os.path.getmtime(self.get_authz_file())
     File "/home/user/Workspace/t11260/lib/python2.7/genericpath.py", line 54, in getmtime
       return os.stat(filename).st_mtime
   OSError: [Errno 2] No such file or directory: '/home/user/Workspace/t11260/tracdev/conf/authzpolicy?.conf'
   

3. If there is an error in the configuration of permission policies, for example: permission_policies = AuthzPermissionPolicy, DefaultPermissionPolicy, LegacyAttachmentPolicy (the first entry should be AuthzPolicy), no error is reported. There should at least be a warning in the logs.

While ExtensionOption​ throw an exception when the specified implementation of the interface can't be found, OrderedExtensionOption​ does not. Changing the behavior of OrderedExtensionOption will have to be carefully considered.