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.

SUBSCRIBER → The subscriber table records the email address for people who want to receive email notifications. Each subscriber has a subscriberCode which is a random 32-byte blob that uniquely identifies the subscriber. There are also fields to indicate what kinds of notifications the subscriber wishes to receive, whether or not the email address of the subscriber has been verified, etc.
PENDING_ALERT → The PENDING_ALERT table contains records that define events about which notification emails might need to be sent. A pending_alert always refers to an entry in the EVENT table. The EVENT table is part of the standard schema and records timeline entries. In other words, there is one row in the EVENT table for each possible timeline entry. The PENDING_ALERT table refers to EVENT table entries for which we might need to send notification emails.
EMAIL_BOUNCE → This table is intended to record email bounce history so that subscribers with excessive bounces can be turned off. That logic has not yet been implemented so the EMAIL_BOUNCE table is currently unused.

Note that "subscribers" are distinct from "users" in the USER table. A "user" is someone who has a login and password. A "subscriber" is an email address that receives notification events. Users can be subscribers, and there is a SUBSCRIBER.SUNAME field that records the linkage between users and subscribers. But it is also possible to be a user without being a subscriber, or to be a subscriber without being a user.

Sending Email Messages

Fossil expects to interact with an external mail transfer agent. There are currently three different methods for sending outbound email messages from Fossil to the external mail agent:

"pipe" → Invoke an external command that accepts the email message on standard input. This is useful if the host computer has a command like /usr/sbin/sendmail that will accept well-formed email messages from standard input and forward them to the appropriate destination.
"db" → Write outgoing email messages into an SQLite database file. The self-hosting Fossil website uses this technique because Fossil runs inside a reduced-privilege chroot jail and cannot invoke commands like /usr/sbin/sendmail. A separate TCL script running outside of the jail monitors the email queue database and forwards email messages to the Postfix mail transfer agent. There is an example TCL script in the tools/email-sender.tcl file of the source tree that shows how this is done.
"dir" → Write outgoing email messages as individual files in a designated directory. This might be useful for testing and debugging.

Internally, there is a fourth email sending method named "stdout" which simply writes the text of the email message on standard output. The "stdout" method is used for testing and debugging.

Perhaps we will add an "smtp" sending method in the future. The main problem with an "smtp" delivery method is that front-line Fossil running inside the privilege jail would need to deal with all kinds of errors from SMTP, such as unable to connect, or connection resets, etc. SMTP expects the sender to have the ability to retry, does it not?

The emails transmitted have a well-formed header. The downstream processing is expected to extract the "To:", "From:", "Subject:" and whatever other attributes it needs from the email header text.

All emails are text/plain and use a transfer-encoding of base64.

There is a utility command-line program named "tools/decode-email.c" in the Fossil source tree. If you compile this program, you can use it to convert the base64 transfer-encoding into human-readable output for testing and debugging.

Receiving Email Messages

Inbound email messages (for example bounces from failed notification emails) should be relayed to the "fossil email inbound" command. That command is currently a no-op place-holder. At some point, we will need to design and write a bounce-message processing system for Fossil.

Controlling The Setup

Commands:

The email command

Web pages:

The /subscribe page
The /alerts page
The /unsubscribe page
The /msgtoadmin page

Web pages for administrators only:

The /setup_email page
The /subscribers page

Test command:

The test-alert command
The 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 fossil rebuild, the EVENT table is rebuilt from scratch; since we do not want users to get notifications for every historical check-in, 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 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.