Fix long-standing issue with user updates not dispatching properly.

This fix is long coming. For a long time due to the addition of a
global user cache, the on_member_update event would only have the
updated user in the very first dispatch due to a quirk in the reference
only being updated once.

In order to fix this issue two things had to change:

1. There had to be a new event, `on_user_update` to complement
   the equivalent member event.
2. Unnecessary copies of User had to be removed to compensate for the
   performance hit from the diffing.

While doing these two fixes I also re-evaluated some more unnecessary
copies done during the PRESENCE_UPDATE to add member case while
fetch_offline_members=False is set or due to chunking issues. The
number of copies was brought down from 2 to 1, discounting the original
Member creation. Unsure on the benefits of this one, however.

N.B: this doesn't change the pre-existing behaviour of on_member_update

3 files changed, 43 insertions, 8 deletions

diff --git a/discord/member.py b/discord/member.py
index b3961cf2..6107038d 100644
--- a/discord/member.py
+++ b/discord/member.py
@@ -190,6 +190,17 @@ class Member(discord.abc.Messageable, _BaseUser):
         return cls(data=data, guild=message.guild, state=message._state)
 
     @classmethod
+    def _from_presence_update(cls, *, data, guild, state):
+        clone = cls(data=data, guild=guild, state=state)
+        to_return = cls(data=data, guild=guild, state=state)
+        to_return._client_status = {
+            key: value
+            for key, value in data.get('client_status', {}).items()
+        }
+        to_return._client_status[None] = data['status']
+        return to_return, clone
+
+    @classmethod
     def _copy(cls, member):
         self = cls.__new__(cls) # to bypass __init__
 
@@ -200,7 +211,10 @@ class Member(discord.abc.Messageable, _BaseUser):
         self.nick = member.nick
         self.activities = member.activities
         self._state = member._state
-        self._user = User._copy(member._user)
+
+        # Reference will not be copied unless necessary by PRESENCE_UPDATE
+        # See below
+        self._user = member._user
         return self
 
     async def _get_channel(self):
@@ -230,9 +244,15 @@ class Member(discord.abc.Messageable, _BaseUser):
 
         if len(user) > 1:
             u = self._user
-            u.name = user.get('username', u.name)
-            u.avatar = user.get('avatar', u.avatar)
-            u.discriminator = user.get('discriminator', u.discriminator)
+            original = (u.name, u.avatar, u.discriminator)
+            # These keys seem to always be available
+            modified = (user['username'], user['avatar'], user['discriminator'])
+            if original != modified:
+                to_return = User._copy(self._user)
+                u.name, u.avatar, u.discriminator = modified
+                # Signal to dispatch on_user_update
+                return to_return, u
+        return False
 
     @property
     def status(self):
diff --git a/discord/state.py b/discord/state.py
index ab3f2ac7..3d420a0a 100644
--- a/discord/state.py
+++ b/discord/state.py
@@ -461,11 +461,14 @@ class ConnectionState:
                 # skip these useless cases.
                 return
 
-            member = Member(guild=guild, data=data, state=self)
+            member, old_member = Member._from_presence_update(guild=guild, data=data, state=self)
             guild._add_member(member)
+        else:
+            old_member = Member._copy(member)
+            user_update = member._presence_update(data=data, user=user)
+            if user_update:
+                self.dispatch('user_update', user_update[0], user_update[1])
 
-        old_member = Member._copy(member)
-        member._presence_update(data=data, user=user)
         self.dispatch('member_update', old_member, member)
 
     def parse_user_update(self, data):
diff --git a/docs/api.rst b/docs/api.rst
index 7371537d..5f1466e5 100644
--- a/docs/api.rst
+++ b/docs/api.rst
@@ -439,13 +439,25 @@ to handle it, which defaults to print a traceback and ignoring the exception.
 
     - status
     - game playing
-    - avatar
     - nickname
     - roles
 
     :param before: The :class:`Member` that updated their profile with the old info.
     :param after: The :class:`Member` that updated their profile with the updated info.
 
+.. function:: on_user_update(before, after)
+
+    Called when a :class:`User` updates their profile.
+
+    This is called when one or more of the following things change:
+
+    - avatar
+    - username
+    - discriminator
+
+    :param before: The :class:`User` that updated their profile with the old info.
+    :param after: The :class:`User` that updated their profile with the updated info.
+
 .. function:: on_guild_join(guild)
 
     Called when a :class:`Guild` is either created by the :class:`Client` or when the