Fossil

Design of Email Notification
============================

This document contains high-level design notes for the email
notification system in Fossil.  Use this document to get a better
understanding of how Fossil handles email notification, to help
with doing custom configurations, or to help contribute features.

This document assumes expert-level systems knowledge.  A separate
tutorial for setting up email notification by non-experts will be
generated once the email notification system stabilizes.

Email notification is under active development as of this writing
(2018-06-25).  Check back frequently for updates.

Data Design
-----------

There are three new tables in the repository database.  These tables
are not created in new repositories by default.  The tables only
come into existance if email notification is configured and used.

   *  The [/setup_email](/help?cmd=/setup_email) page
   *  The [/subscribers](/help?cmd=/subscribers) page

Test command:

   *  The [test-alert](/help?cmd=test-alert) command
   *  The [test-add-alerts](/help?cmd=test-add-alerts) command

Email Address Verification
--------------------------

When anonymous passers-by on the internet sign up for email notifications,
their email address must first be verified.  An email message is sent to
the address supplied inviting the user to click on a link.  The link includes
the random 32-byte subscriberCode in hex.  If anyone visits the link, the
email address is verified.

There is no password.  Knowledge of the subscriberCode is sufficient to
control the subscription.  This is not a secure as a separate password,
but on the other hand it is easier for the average subscriber to deal
with in that they don't have to come up with yet another password.  Also,
even if the subscriberCode is stolen, the worst that can happens is that
the thief can change your subscription settings.  No PII (other than
the subscriber's email address) is available to an attacker with the
subscriberCode.  Nor can knowledge of the subscriberCode lead to a
email flood or other annoyance attack, as far as I can see.

If subscriberCodes are ever compromised, new ones can be generated
as follows:

>   UPDATE subscriber SET subscriberCode=randomblob(32);

Perhaps the system be enhanced to randomize the
subscriberCodes periodically - say just before each daily digest
is sent out?

User Control Of Their Subscription
----------------------------------

If a user has a separate account with a login and password for
the repository, then their subscription is linked to their account.
On the /login page is a link to a page to control their subscription.

For users without logins, they can request a link to a page for
controling their subscription on the /alerts or /unsubscribe page.
The link is sent via email, and includes the subscriberCode.

Internal Processing Flow
------------------------

Almost all of the email notification code is found in the "src/email.c"
source file.

When email notifications are enabled, a trigger is created in the schema
(the "email_trigger1" trigger) that adds a new entry to the
PENDING_ALERT table every time a row is added to the EVENT table.
During a "rebuild", the EVENT table is rebuilt from scratch, and we
do not want users to get notifications for every historical check-in,
so the trigger is disabled during "rebuild".

Email notifications are sent out by the email_send_alerts() function.
This function is can be called by having a cron job invoke the
"fossil email exec" command.  Or, if the email-autoexec setting is
enabled, then email_send_alerts() is invoked automatically after each
successful webpage is generated.  The latter approach is used on the
Fossil self-hosting repository.  The email_send_alerts() function is
a no-op (obviously) if there are no pending events to be sent.

Digests are handled by recording the time of the last digest in the
"email-last-digest" setting, and only sending a new digest if the
current time is one day or later after the last digest.

Individual emails are sent to each subscriber.  I ran tests and found
that I could send about 1200 emails/second, which is fast enough so that
I do not need to resort to trying to notify multiple subscribers with
a single email.  Because each subscriber gets a separate email, the
system can include information in the email that is unique to the
subscriber, such as a link to the page to edit their subscription.  That
link includes the subscriberCode.

Other Notes
-----------

The "fossil configuration pull subscriber" command pulls down the content
of the SUBSCRIBER table.  This is intended to as a backup-only.  It
is not desirable to have two or more systems sending emails to the
same people for the same repository, as that would mean users would
receive duplicate emails.  Hence the settings that control email 
notifications are not transmitted with the pull.  The "push", "export",
and "import" commands all work similarly