error: make HintException a mix-in class not derived from BaseException (API)

HintException is unrelated to the hierarchy of errors. It is an implementation
detail whether a class inherits from HintException or not, a sort of "private
inheritance" in C++.

New Hint isn't an exception class, which prevents catching error by its type:

    try:
        dosomething()
    except error.Hint:
        pass

Unfortunately, this passes on PyPy 5.3.1, but not on Python 2, and raises more
detailed TypeError on Python 3.

mercurial/error.py

@@ -1,237 +1,242 b''
 # error.py - Mercurial exceptions
 #
 # Copyright 2005-2008 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """Mercurial exceptions.
 
 This allows us to catch exceptions at higher levels without forcing
 imports.
 """
 
 from __future__ import absolute_import
 
 # Do not import anything here, please
 
-class HintException(Exception):
+class Hint(object):
+    """Mix-in to provide a hint of an error
+
+    This should come first in the inheritance list to consume **kw and pass
+    only *args to the exception class.
+    """
     def __init__(self, *args, **kw):
-        Exception.__init__(self, *args)
+        super(Hint, self).__init__(*args)
         self.hint = kw.get('hint')
 
-class RevlogError(HintException):
+class RevlogError(Hint, Exception):
     pass
 
 class FilteredIndexError(IndexError):
     pass
 
 class LookupError(RevlogError, KeyError):
     def __init__(self, name, index, message):
         self.name = name
         self.index = index
         # this can't be called 'message' because at least some installs of
         # Python 2.6+ complain about the 'message' property being deprecated
         self.lookupmessage = message
         if isinstance(name, str) and len(name) == 20:
             from .node import short
             name = short(name)
         RevlogError.__init__(self, '%s@%s: %s' % (index, name, message))
 
     def __str__(self):
         return RevlogError.__str__(self)
 
 class FilteredLookupError(LookupError):
     pass
 
 class ManifestLookupError(LookupError):
     pass
 
 class CommandError(Exception):
     """Exception raised on errors in parsing the command line."""
 
-class InterventionRequired(HintException):
+class InterventionRequired(Hint, Exception):
     """Exception raised when a command requires human intervention."""
 
-class Abort(HintException):
+class Abort(Hint, Exception):
     """Raised if a command needs to print an error and exit."""
 
 class HookLoadError(Abort):
     """raised when loading a hook fails, aborting an operation
 
     Exists to allow more specialized catching."""
 
 class HookAbort(Abort):
     """raised when a validation hook fails, aborting an operation
 
     Exists to allow more specialized catching."""
 
 class ConfigError(Abort):
     """Exception raised when parsing config files"""
 
 class UpdateAbort(Abort):
     """Raised when an update is aborted for destination issue"""
 
 class MergeDestAbort(Abort):
     """Raised when an update is aborted for destination issues"""
 
 class NoMergeDestAbort(MergeDestAbort):
     """Raised when an update is aborted because there is nothing to merge"""
 
 class ManyMergeDestAbort(MergeDestAbort):
     """Raised when an update is aborted because destination is ambigious"""
 
 class ResponseExpected(Abort):
     """Raised when an EOF is received for a prompt"""
     def __init__(self):
         from .i18n import _
         Abort.__init__(self, _('response expected'))
 
-class OutOfBandError(HintException):
+class OutOfBandError(Hint, Exception):
     """Exception raised when a remote repo reports failure"""
 
-class ParseError(HintException):
+class ParseError(Hint, Exception):
     """Raised when parsing config files and {rev,file}sets (msg[, pos])"""
 
 class UnknownIdentifier(ParseError):
     """Exception raised when a {rev,file}set references an unknown identifier"""
 
     def __init__(self, function, symbols):
         from .i18n import _
         ParseError.__init__(self, _("unknown identifier: %s") % function)
         self.function = function
         self.symbols = symbols
 
-class RepoError(HintException):
+class RepoError(Hint, Exception):
     pass
 
 class RepoLookupError(RepoError):
     pass
 
 class FilteredRepoLookupError(RepoLookupError):
     pass
 
 class CapabilityError(RepoError):
     pass
 
 class RequirementError(RepoError):
     """Exception raised if .hg/requires has an unknown entry."""
 
 class UnsupportedMergeRecords(Abort):
     def __init__(self, recordtypes):
         from .i18n import _
         self.recordtypes = sorted(recordtypes)
         s = ' '.join(self.recordtypes)
         Abort.__init__(
             self, _('unsupported merge state records: %s') % s,
             hint=_('see https://mercurial-scm.org/wiki/MergeStateRecords for '
                    'more information'))
 
 class LockError(IOError):
     def __init__(self, errno, strerror, filename, desc):
         IOError.__init__(self, errno, strerror, filename)
         self.desc = desc
 
 class LockHeld(LockError):
     def __init__(self, errno, filename, desc, locker):
         LockError.__init__(self, errno, 'Lock held', filename, desc)
         self.locker = locker
 
 class LockUnavailable(LockError):
     pass
 
 # LockError is for errors while acquiring the lock -- this is unrelated
 class LockInheritanceContractViolation(RuntimeError):
     pass
 
 class ResponseError(Exception):
     """Raised to print an error with part of output and exit."""
 
 class UnknownCommand(Exception):
     """Exception raised if command is not in the command table."""
 
 class AmbiguousCommand(Exception):
     """Exception raised if command shortcut matches more than one command."""
 
 # derived from KeyboardInterrupt to simplify some breakout code
 class SignalInterrupt(KeyboardInterrupt):
     """Exception raised on SIGTERM and SIGHUP."""
 
 class SignatureError(Exception):
     pass
 
 class PushRaced(RuntimeError):
     """An exception raised during unbundling that indicate a push race"""
 
 # bundle2 related errors
 class BundleValueError(ValueError):
     """error raised when bundle2 cannot be processed"""
 
 class BundleUnknownFeatureError(BundleValueError):
     def __init__(self, parttype=None, params=(), values=()):
         self.parttype = parttype
         self.params = params
         self.values = values
         if self.parttype is None:
             msg = 'Stream Parameter'
         else:
             msg = parttype
         entries = self.params
         if self.params and self.values:
             assert len(self.params) == len(self.values)
             entries = []
             for idx, par in enumerate(self.params):
                 val = self.values[idx]
                 if val is None:
                     entries.append(val)
                 else:
                     entries.append("%s=%r" % (par, val))
         if entries:
             msg = '%s - %s' % (msg, ', '.join(entries))
         ValueError.__init__(self, msg)
 
 class ReadOnlyPartError(RuntimeError):
     """error raised when code tries to alter a part being generated"""
 
 class PushkeyFailed(Abort):
     """error raised when a pushkey part failed to update a value"""
 
     def __init__(self, partid, namespace=None, key=None, new=None, old=None,
                  ret=None):
         self.partid = partid
         self.namespace = namespace
         self.key = key
         self.new = new
         self.old = old
         self.ret = ret
         # no i18n expected to be processed into a better message
         Abort.__init__(self, 'failed to update value for "%s/%s"'
                        % (namespace, key))
 
 class CensoredNodeError(RevlogError):
     """error raised when content verification fails on a censored node
 
     Also contains the tombstone data substituted for the uncensored data.
     """
 
     def __init__(self, filename, node, tombstone):
         from .node import short
         RevlogError.__init__(self, '%s:%s' % (filename, short(node)))
         self.tombstone = tombstone
 
 class CensoredBaseError(RevlogError):
     """error raised when a delta is rejected because its base is censored
 
     A delta based on a censored revision must be formed as single patch
     operation which replaces the entire base with new content. This ensures
     the delta may be applied by clones which have not censored the base.
     """
 
 class InvalidBundleSpecification(Exception):
     """error raised when a bundle specification is invalid.
 
     This is used for syntax errors as opposed to support errors.
     """
 
 class UnsupportedBundleSpecification(Exception):
     """error raised when a bundle specification is not supported."""