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 (2018-06-23). 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 agent. There are currently three different methods for sending outbound email messages from Fossil to the external mail agent:

1. "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.

2. "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.

3. "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 /msgadmin page

Web pages for administrators only:

• The /setup_email page
• The /subscribers page

Test command:

• The test-alert command
• The test-add-alerts command