[commands] Copy HelpCommand instances to prevent race conditions.

Fixes #2123

Slight breaking change if someone had an expectation that no copies
were made behind the scene (which is sensible), however writing code
that relies on this expectation is probably buggy anyway.

discord/ext/commands/help.py

@@ -25,6 +25,7 @@ DEALINGS IN THE SOFTWARE.
 """
 
 import itertools
+import copy
 import functools
 import inspect
 import re
@@ -166,15 +167,23 @@ def _not_overriden(f):
 
 class _HelpCommandImpl(Command):
     def __init__(self, inject, *args, **kwargs):
-        super().__init__(*args, **kwargs)
+        super().__init__(inject.command_callback, *args, **kwargs)
+        self._original = inject
         self._injected = inject
 
-        on_error = inject.on_help_command_error
-        try:
-            on_error.__help_command_not_overriden__
-        except AttributeError:
-            # overridden
-            self.on_error = on_error
+    async def prepare(self, ctx):
+        self._injected = injected = self._original.copy()
+        injected.context = ctx
+        self.callback = injected.command_callback
+
+        on_error = injected.on_help_command_error
+        if not hasattr(on_error, '__help_command_not_overriden__'):
+            if self.cog is not None:
+                self.on_error = self._on_error_cog_implementation
+            else:
+                self.on_error = on_error
+
+        await super().prepare(ctx)
 
     async def _parse_arguments(self, ctx):
         # Make the parser think we don't have a cog so it doesn't
@@ -221,13 +230,6 @@ class _HelpCommandImpl(Command):
         cog.walk_commands = wrapped_walk_commands
         self.cog = cog
 
-        on_error = self._injected.on_help_command_error
-        try:
-            on_error.__help_command_not_overriden__
-        except AttributeError:
-            # overridden so let's swap it with our cog specific implementation
-            self.on_error = self._on_error_cog_implementation
-
     def _eject_cog(self):
         if self.cog is None:
             return
@@ -238,16 +240,18 @@ class _HelpCommandImpl(Command):
         cog.walk_commands = cog.walk_commands.__wrapped__
         self.cog = None
 
-        on_error = self._injected.on_help_command_error
-        try:
-            on_error.__help_command_not_overriden__
-        except AttributeError:
-            # overridden so let's swap it with our cog specific implementation
-            self.on_error = self._on_error_cog_implementation
-
 class HelpCommand:
     r"""The base implementation for help command formatting.
 
+    .. note::
+
+        Internally instances of this class are deep copied every time
+        the command itself is invoked to prevent a race condition
+        mentioned in :issue:`2123`.
+
+        This means that relying on the state of this class to be
+        the same between command invocations would not as expected.
+
     Attributes
     ------------
     context: Optional[:class:`Context`]
@@ -275,6 +279,25 @@ class HelpCommand:
 
     MENTION_PATTERN = re.compile('|'.join(MENTION_TRANSFORMS.keys()))
 
+    def __new__(cls, *args, **kwargs):
+        # To prevent race conditions of a single instance while also allowing
+        # for settings to be passed the original arguments passed must be assigned
+        # to allow for easier copies (which will be made when the help command is actually called)
+        # see issue 2123
+        self = super().__new__(cls)
+
+        # Shallow copies cannot be used in this case since it is not unusual to pass
+        # instances that need state, e.g. Paginator or what have you into the function
+        # The keys can be safely copied as-is since they're 99.99% certain of being
+        # string keys
+        deepcopy = copy.deepcopy
+        self.__original_kwargs__ = {
+            k: deepcopy(v)
+            for k, v in kwargs.items()
+        }
+        self.__original_args__ = deepcopy(args)
+        return self
+
     def __init__(self, **options):
         self.show_hidden = options.pop('show_hidden', False)
         self.verify_checks = options.pop('verify_checks', True)
@@ -284,8 +307,13 @@ class HelpCommand:
         self.context = None
         self._command_impl = None
 
+    def copy(self):
+        obj = self.__class__(*self.__original_args__, **self.__original_kwargs__)
+        obj._command_impl = self._command_impl
+        return obj
+
     def _add_to_bot(self, bot):
-        command = _HelpCommandImpl(self, self.command_callback, **self.command_attrs)
+        command = _HelpCommandImpl(self, **self.command_attrs)
         bot.add_command(command)
         self._command_impl = command
 
@@ -707,12 +735,7 @@ class HelpCommand:
         some state in your subclass before the command does its processing
         then this would be the place to do it.
 
-        The default implementation sets :attr:`context`.
-
-        .. warning::
-
-            If you override this method, be sure to call ``super()``
-            so the help command can be set up.
+        The default implementation does nothing.
 
         .. note::
 
@@ -726,7 +749,7 @@ class HelpCommand:
         command: Optional[:class:`str`]
             The argument passed to the help command.
         """
-        self.context = ctx
+        pass
 
     async def command_callback(self, ctx, *, command=None):
         """|coro|