Sieve implementation for Dovecot v1.2

(NOTE: This is the UNSTABLE development branch)

Introduction
------------

Sieve is a machine language specifically tailored for internet message 
filtering. This package compiles into a Sieve plugin for the Dovecot local 
delivery agent called Deliver. The plugin adds Sieve filtering support to the 
delivery process.  

Previously, the same functionality was provided by the cmusieve plugin for 
Dovecot. This old plugin is based on the CMU Sieve implementation included with
the Cyrus project. This new package provides a complete rewrite of the Sieve 
engine integrating it tightly with Dovecot. The actual execution of the Sieve 
actions is based on the original cmusieve plugin, but only on the code added to 
interface the CMU Sieve implementation with Dovocot. 

The main reason for rewriting the Sieve engine is to provide more reliable 
script execution and to provide better error messages to users and system 
administrators. Also, since the Sieve language evolves quickly, with new 
language extensions published every year, the aim is to provide support for 
quickly extending the engine with new features. 

Features
--------

* Well-structured 3-stage compiler:
 
  Uses dovecot framework and avoids using lex/yacc. Compiler doesn't bail on 
  first error, but tries to find more. Produced errors are aimed to be useful 
  and generally user-comprehensible. Things like 'Generic error' are a nuisance 
  of the past. 

* Highly extendable with new Sieve capabilities: 

  This keeps the possibility of plugins in mind. It should eventually provide 
  the necessary infrastructure for at least all currently known (proposed) 
  extensions. The goal is to keep the extension interface provided by sieve 
  engine as generic as possible, i.e. without explicit support for specific 
  extensions. New similar extensions can then use the same interface methods 
  without changes to the sieve engine code. If an extension is not loaded using 
  the require command, the compiler truly does not know of its existance. 

* Supports all extensions provided by the original CMUSieve plugin:
 
  In addition, it has support for the new and very useful variables extension
  (see next section). 

  NOTE: The original CMUSieve plugin is based on old specifications of the 
  imap4flags and enotify extension. Among other subtle differences, these 
  extensions were known as 'imapflags' and 'notify' for the CMU Sieve plugin.
  Support for the old imapflags extension is provided for backwards compatibility.

* Supports executing multiple scrips sequentially.
  
  Using this feature it is possible to execute administrator-controlled Sieve
  scripts before and after the user's Sieve script is executed. As long as the
  verdict is at least (implicit) keep, the execution will continue with the next
  script. Multiple scripts can be executed before or after the user's script by 
  specifying directories containing sieve files.

* Supported by ManageSieve service:

  This Sieve implementation is supported by the ManageSieve implementation for 
  Dovecot v1.2. Therefore, ManageSieve support can be added to Dovecot for the
  new Sieve plugin just as for the cmusieve plugin.

* Test suite included:
	
  This package includes a test suite to automatically asses whether the compiled 
  sieve engine works correctly. The test suite is an extension to the Sieve 
  language and is therefore easily extended with new tests. Currently, the 
  test suite is mostly limited to testing script processing. The performed actions 
  are not tested fully yet. 

Implementation Status
---------------------

The the core of the language (as specified in RFC 5228) is fully supported. In 
addition to that, this Sieve implementation features various extensions. The 
following list outlines the implementation status of each supported extension:
	
  Base specification (RFC5228):
    fileinto: full
    reject: full (without Dovecot LMTP currently no refuse support)
    envelope: full
    encoded-character: full

  Other RFCs/drafts:
    subaddress: full (limited configurability)
    comparator-i;ascii-numeric: full
    relational: full 
    copy: full
    regex: mostly full; but suboptimal and no UTF-8
    body: mostly full, but text body-transform implementation is simple
        and some issues make it still not completely RFC incompliant. 
    include: almost full; global namespace missing 
    vacation: mostly full; handling of utf-8 in headers is non-existant
    imap4flags: full (old imapflags supported for backwards compatibility)
    variables: mostly full; currently no support for future namespaces 
    notify: full, mailto support only; needs to be tested more

All implemented extensions are like the engine itself currently experimental. 
A status of 'full' does not necessarily mean that the extension is bug-free or 
even fully RFC-compliant. Check the TODO file for open issues.
 
Many more extensions to the language exist. Not all of these extensions are 
useful for Dovecot in particular, but many of them are. Currently, the author 
has taken notice of the following extensions:

    environment: planned
    date,index: planned
    editheader: planned, needs additional support from Dovecot though.
    mimeloop: planned

These extensions will be added as soon as the necessary infrastructure is 
available. 

Compiling and Configuring
-------------------------

Refer to INSTALL file.

Using
-----

The main purpose of this package is to replace the existing cmusieve plugin that 
is currently available for Dovecot's deliver. With this respect it is currently 
not very different from the cmusieve plugin implementation.

Unlike cmusieve, this sieve module logs runtime errors to <scriptfile>.log if 
it can and not <scriptfile>.err. It appends new timestamped log entries to the 
end of the logfile. If the log grows too large (currently > 10kB), the logfile 
is rotated to <scriptfile>.log.0 and <scriptfile>.log starts out empty again. 

The cmusieve plugin compiled the script into a file with an appended 'c', e.g. 
'test.sievec'. This new implementation recognizes scripts to have the .sieve 
extension. The binary is (by default) written to a file with extension .svbin. 
This means that the default .dovecot.sieve is compiled into .dovecot.svbin. 
Included scripts are currently always compiled into the main binary, meaning 
that no other files are written and no permission to do so is necessary for the 
global script directories. 

To test the sieve engine outside deliver, it is useful to try the commands that 
exist in the src/sieve-tools/ directory of this package. After installation, 
these are available at your $prefix/bin directory. The following commands are 
installed:

sievec     - Compiles sieve scripts into a binary representation for later 
             execution.

sieve-test - This is a universal Sieve test tool for testing the effect of a
             Sieve script on a particular message. It allows compiling, running 
             and testing Sieve scripts. It can either be used to display the
             actions that would be performed on the provided test message or it
             can be used to test the actual delivery of the message and show the
             messages that would normally be sent through SMTP.

sieved     - Dumps the content of a Sieve binary file for (development) 
             debugging purposes.

When installed, man pages are also available for these commands. In this package
the man pages are present in doc/man and can be viewed before install using e.g.: 

man -l doc/man/sieve-test.1

Various example scripts are bundled in the directory 'examples'. These scripts
were downloaded from various locations. View the top comment in the scripts for 
url and author information.

Known issues
------------

Most open issues are outlined in the TODO file. The more generic ones are (re-)
listed here:

- Compile errors are sometimes a bit obscure and long. This needs work. 
  Suggestions for improvement are welcome. 
- The documentation needs work. The wiki needs to be updated with the newly 
  supported features and extensions.

Authors
-------

Refer to AUTHORS file.

Contact Info
------------

Stephan Bosch <stephan at rename-it dot nl>
IRC: Freenode, #dovecot, S[r]us

Please use the Dovecot mailing list <dovecot at dovecot.org> for questions about 
this package. You can post to the list without subscribing, the mail then waits 
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