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diff --git a/src/mailman/workflows/common.py b/src/mailman/workflows/common.py
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+# Copyright (C) 2015-2017 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Common support between subscription and unsubscription."""
+
+import uuid
+import logging
+
+from datetime import timedelta
+from email.utils import formataddr
+from enum import Enum
+from itertools import chain
+
+from mailman.app.membership import delete_member
+from mailman.config import config
+from mailman.core.i18n import _
+from mailman.email.message import UserNotification
+from mailman.interfaces.address import IAddress
+from mailman.interfaces.bans import IBanManager
+from mailman.interfaces.member import (AlreadySubscribedError, MemberRole,
+                                       MembershipIsBannedError,
+                                       NotAMemberError)
+from mailman.interfaces.pending import IPendable, IPendings
+from mailman.interfaces.subscriptions import (SubscriptionPendingError,
+                                              TokenOwner)
+from mailman.interfaces.template import ITemplateLoader
+from mailman.interfaces.user import IUser
+from mailman.interfaces.usermanager import IUserManager
+from mailman.interfaces.workflows import ISubscriptionWorkflow
+from mailman.utilities.datetime import now
+from mailman.utilities.string import expand, wrap
+from mailman.workflows.base import Workflow
+from zope.component import getUtility
+from zope.interface import implementer
+
+
+log = logging.getLogger('mailman.subscribe')
+
+
+class WhichSubscriber(Enum):
+    address = 1
+    user = 2
+
+
+class WhichWorkflow(Enum):
+    subscription = 1
+    unsubscription = 2
+
+
+@implementer(IPendable)
+class PendableSubscription(dict):
+    PEND_TYPE = 'subscription'
+
+
+@implementer(IPendable)
+class PendableUnsubscription(dict):
+    PEND_TYPE = 'unsubscription'
+
+
+class SubscriptionWorkflowCommon(Workflow):
+    """Common support between subscription and unsubscription."""
+
+    def __init__(self, mlist, subscriber):
+        super().__init__()
+        self.mlist = mlist
+        self.address = None
+        self.user = None
+        self.which = None
+        self.member = None
+        self._set_token(TokenOwner.no_one)
+        # The subscriber must be either an IUser or IAddress.
+        if IAddress.providedBy(subscriber):
+            self.address = subscriber
+            self.user = self.address.user
+            self.which = WhichSubscriber.address
+        elif IUser.providedBy(subscriber):
+            self.address = subscriber.preferred_address
+            self.user = subscriber
+            self.which = WhichSubscriber.user
+        self.subscriber = subscriber
+
+    @property
+    def user_key(self):
+        # For save.
+        return self.user.user_id.hex
+
+    @user_key.setter
+    def user_key(self, hex_key):
+        # For restore.
+        uid = uuid.UUID(hex_key)
+        self.user = getUtility(IUserManager).get_user_by_id(uid)
+        if self.user is None:
+            self.user = self.address.user
+
+    @property
+    def address_key(self):
+        # For save.
+        return self.address.email
+
+    @address_key.setter
+    def address_key(self, email):
+        # For restore.
+        self.address = getUtility(IUserManager).get_address(email)
+        assert self.address is not None
+
+    @property
+    def subscriber_key(self):
+        return self.which.value
+
+    @subscriber_key.setter
+    def subscriber_key(self, key):
+        self.which = WhichSubscriber(key)
+
+    @property
+    def token_owner_key(self):
+        return self.token_owner.value
+
+    @token_owner_key.setter
+    def token_owner_key(self, value):
+        self.token_owner = TokenOwner(value)
+
+    def _restore_subscriber(self):
+        # Restore a little extra state that can't be stored in the database
+        # (because the order of setattr() on restore is indeterminate), then
+        # continue with the confirmation/verification step.
+        if self.which is WhichSubscriber.address:
+            self.subscriber = self.address
+        else:
+            assert self.which is WhichSubscriber.user
+            self.subscriber = self.user
+
+    def _set_token(self, token_owner):
+        assert isinstance(token_owner, TokenOwner)
+        pendings = getUtility(IPendings)
+        # Clear out the previous pending token if there is one.
+        if self.token is not None:
+            pendings.confirm(self.token)
+        # Create a new token to prevent replay attacks.  It seems like this
+        # would produce the same token, but it won't because the pending adds a
+        # bit of randomization.
+        self.token_owner = token_owner
+        if token_owner is TokenOwner.no_one:
+            self.token = None
+            return
+
+        pendable = self.pendable_class()(
+            list_id=self.mlist.list_id,
+            email=self.address.email,
+            display_name=self.address.display_name,
+            when=now().replace(microsecond=0).isoformat(),
+            token_owner=token_owner.name,
+            )
+        self.token = pendings.add(pendable, timedelta(days=3650))
+
+    @classmethod
+    def pendable_class(cls):
+        @implementer(IPendable)
+        class Pendable(dict):
+            PEND_TYPE = cls.name
+        return Pendable
+
+
+class SubscriptionBase(SubscriptionWorkflowCommon):
+
+    def __init__(self, mlist, subscriber):
+        super().__init__(mlist, subscriber)
+        self._workflow = WhichWorkflow.subscription
+
+    def _step_sanity_checks(self):
+        # Ensure that we have both an address and a user, even if the address
+        # is not verified.  We can't set the preferred address until it is
+        # verified.
+        if self.user is None:
+            # The address has no linked user so create one, link it, and set
+            # the user's preferred address.
+            assert self.address is not None, 'No address or user'
+            self.user = getUtility(IUserManager).make_user(self.address.email)
+        if self.address is None:
+            assert self.user.preferred_address is None, (
+                "Preferred address exists, but wasn't used in constructor")
+            addresses = list(self.user.addresses)
+            if len(addresses) == 0:
+                raise AssertionError('User has no addresses: {}'.format(
+                    self.user))
+            # This is rather arbitrary, but we have no choice.
+            self.address = addresses[0]
+        assert self.user is not None and self.address is not None, (
+            'Insane sanity check results')
+        # Is this subscriber already a member?
+        if (self.which is WhichSubscriber.user and
+                self.user.preferred_address is not None):
+            subscriber = self.user
+        else:
+            subscriber = self.address
+        if self.mlist.is_subscribed(subscriber):
+            # 2017-04-22 BAW: This branch actually *does* get covered, as I've
+            # verified by a full coverage run, but diffcov for some reason
+            # claims that the test added in the branch that added this code
+            # does not cover the change.  That seems like a bug in diffcov.
+            raise AlreadySubscribedError(           # pragma: nocover
+                self.mlist.fqdn_listname,
+                self.address.email,
+                MemberRole.member)
+        # Is this email address banned?
+        if IBanManager(self.mlist).is_banned(self.address.email):
+            raise MembershipIsBannedError(self.mlist, self.address.email)
+        # Check if there is already a subscription request for this email.
+        # Look at all known subscription workflows, because any pending
+        # subscription workflow is exclusive.
+        sub_workflows = [workflow_class
+                         for workflow_class in config.workflows.values()
+                         if ISubscriptionWorkflow.implementedBy(workflow_class)
+                         ]
+        generators = [getUtility(IPendings).find(mlist=self.mlist,
+                                                 pend_type=sub_workflow.name)
+                      for
+                      sub_workflow in sub_workflows]
+        pendings = chain.from_iterable(generators)
+        for token, pendable in pendings:
+            if pendable['email'] == self.address.email:
+                raise SubscriptionPendingError(self.mlist, self.address.email)
+        # Start out with the subscriber being the token owner.
+
+    def _step_do_subscription(self):
+        # We can immediately subscribe the user to the mailing list.
+        self.member = self.mlist.subscribe(self.subscriber)
+        assert self.token is None and self.token_owner is TokenOwner.no_one, (
+            'Unexpected active token at end of subscription workflow')
+
+
+class UnsubscriptionBase(SubscriptionWorkflowCommon):
+
+    def __init__(self, mlist, subscriber):
+        super().__init__(mlist, subscriber)
+        if IAddress.providedBy(subscriber) or IUser.providedBy(subscriber):
+            self.member = self.mlist.regular_members.get_member(
+                self.address.email)
+        self._workflow = WhichWorkflow.unsubscription
+
+    def _step_subscription_checks(self):
+        assert self.mlist.is_subscribed(self.subscriber)
+
+    def _step_do_unsubscription(self):
+        try:
+            delete_member(self.mlist, self.address.email)
+        except NotAMemberError:
+            # The member has already been unsubscribed.
+            pass
+        self.member = None
+        assert self.token is None and self.token_owner is TokenOwner.no_one, (
+            'Unexpected active token at end of subscription workflow')
+
+
+class RequestMixin:
+
+    def _step_send_confirmation(self):
+        self._set_token(TokenOwner.subscriber)
+        self.push('do_confirm_verify')
+        self.save()
+        if self._workflow is WhichWorkflow.subscription:
+            template_name = 'list:user:action:subscribe'
+        else:
+            template_name = 'list:user:action:unsubscribe'
+
+        subject = 'confirm {}'.format(self.token)
+        confirm_address = self.mlist.confirm_address(self.token)
+        email_address = self.address.email
+        # Send a verification email to the address.
+        template = getUtility(ITemplateLoader).get(template_name, self.mlist)
+        text = expand(template, self.mlist, dict(
+                token=self.token,
+                subject=subject,
+                confirm_email=confirm_address,
+                user_email=email_address,
+                # For backward compatibility.
+                confirm_address=confirm_address,
+                email_address=email_address,
+                domain_name=self.mlist.domain.mail_host,
+                contact_address=self.mlist.owner_address,
+                ))
+        msg = UserNotification(email_address, confirm_address, subject, text)
+        msg.send(self.mlist, add_precedence=False)
+        # Now we wait for the confirmation.
+        raise StopIteration
+
+    def _step_do_confirm_verify(self):
+        # Restore a little extra state that can't be stored in the database.
+        self._restore_subscriber()
+        # Reset the token so it can't be used in a replay attack.
+        self._set_token(TokenOwner.no_one)
+        # The user has confirmed their subscription request, and also verified
+        # their email address if necessary.  This latter needs to be set on the
+        # IAddress, but there's nothing more to do about the confirmation step.
+        # We just continue along with the workflow.
+        if self.address.verified_on is None:
+            self.address.verified_on = now()
+        self.verified = True
+        self.confirmed = True
+
+        if self._workflow is WhichWorkflow.unsubscription:
+            self.member = self.mlist.regular_members.get_member(
+                self.address.email)
+
+
+class VerificationMixin(RequestMixin):
+
+    def __init__(self, pre_verified=False):
+        self.verified = pre_verified
+
+    def _step_verification_checks(self):
+        # Is the address already verified, or is the pre-verified flag set?
+        if self.address.verified_on is None:
+            if self.verified:
+                self.address.verified_on = now()
+            else:
+                # The address being subscribed is not yet verified, so we need
+                # to send a validation email that will also confirm that the
+                # user wants to be subscribed to this mailing list.
+                self.push('send_confirmation')
+
+
+class ConfirmationMixin(RequestMixin):
+
+    def __init__(self, pre_confirmed=False):
+        self.confirmed = pre_confirmed
+
+    def _step_confirmation_checks(self):
+        # If the subscription has been pre-confirmed, then we can skip the
+        # confirmation check.
+        if not self.confirmed:
+            # The user must confirm their subscription.
+            self.push('send_confirmation')
+
+
+class ModerationMixin:
+
+    def __init__(self, pre_approved=False):
+        self.approved = pre_approved
+
+    def _step_moderation_checks(self):
+        # Does the moderator need to approve the request?
+        if not self.approved:
+            self.push('get_moderator_approval')
+
+    def _step_get_moderator_approval(self):
+        # Here's the next step in the workflow, assuming the moderator
+        # approves of the request.  If they don't, the workflow and
+        # request will just be thrown away.
+        self._set_token(TokenOwner.moderator)
+        self.push('restore')
+        self.save()
+
+        if self._workflow is WhichWorkflow.subscription:
+            workflow_name = 'subscription'
+            template_name = 'list:admin:action:subscribe'
+        else:
+            workflow_name = 'unsubscription'
+            template_name = 'list:admin:action:unsubscribe'
+
+        log.info('{}: held {} request from {}'.format(
+            self.mlist.fqdn_listname, workflow_name, self.address.email))
+        # Possibly send a notification to the list moderators.
+        if self.mlist.admin_immed_notify:
+            subject = _(
+                'New $workflow_name request to $self.mlist.display_name '
+                'from $self.address.email')
+            username = formataddr(
+                (self.subscriber.display_name, self.address.email))
+            template = getUtility(ITemplateLoader).get(
+                template_name, self.mlist)
+            text = wrap(expand(template, self.mlist, dict(
+                member=username,
+                )))
+            # This message should appear to come from the <list>-owner so as
+            # to avoid any useless bounce processing.
+            msg = UserNotification(
+                self.mlist.owner_address, self.mlist.owner_address,
+                subject, text, self.mlist.preferred_language)
+            msg.send(self.mlist)
+        # The workflow must stop running here.
+        raise StopIteration
+
+    def _step_restore(self):
+        # Prevent replay attacks.
+        self._set_token(TokenOwner.no_one)
+        # Restore a little extra state that can't be stored in the database.
+        self._restore_subscriber()