Move ESMPT LIMITS extension from experimental to mainline

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                                   Maildir++

   In this document:
     * HOWTO.maildirquota
     * Mission statement
     * Definitions and goals
     * Contents of a maildirsize
     * Calculating maildirsize
     * Calculating the quota for a Maildir++
     * Delivering to a Maildir++
     * Reading from a Maildir++
     * Bugs

HOWTO.maildirquota

   The remaining portion of this document is a technical description of
   the maildir quota extension. This section is a brief overview of this
   extension.

  What is a maildirquota?

   If you would like to have a quota on your maildir mailboxes, the best
   solution is to always use filesystem-based quotas: per-user usage
   quotas that is enforced by the operating system.

   This is the best solution when the default Maildir is located in each
   account's home directory. This solution will NOT work if Maildirs are
   stored elsewhere, or if you have a large virtual domain setup where a
   single userid is used to hold many individual Maildirs, one for each
   virtual user.

   This extension to the maildir format allows a "voluntary" maildir
   quota implementation that does not rely on filesystem-based quotas.

  When maildirquota will not work.

   For this quota mechanism to work, all software that accesses a maildir
   must observe this quota protocol. It follows that this quota mechanism
   can be easily circumvented if users have direct (shell) access to the
   filesystem containing the users' maildirs.

   Furthermore, this quota mechanism is not 100% effective. It is
   possible to have a situation where someone may go over quota. This
   quota implementation uses a deliberate trade-off. It is necessary to
   use some form of locking in order to have a complete bulletproof quota
   enforcement, but maildirs mail stores were explicitly designed to
   avoid any kind of locking. This quota approach does not use locking,
   and the tradeoff is that sometimes it is possible for a few extra
   messages to be delivered to the maildir, before the door is
   permanently shot.

   For best performance, all maildir clients should support this quota
   extension, however there's a wide degree of tolerance here. As long as
   the mail delivery agent that puts new messages into a Maildir uses
   this extension, the quota will be enforced without excessive
   degradation.

   In the worst case scenario, quotas are automatically recalculated
   every fifteen minutes. If a maildir goes over quota, and a mail client
   that does not support this quota extension removes enough mail from
   the maildir, the mail delivery agent will not be immediately informed
   that the maildir is now under quota. However, eventually the correct
   quota will be recalculated and mail delivery will resume.

   Mail user agents sometimes put messages into the maildir themselves.
   Messages added to a maildir by a mail user agent that does not
   understand the quota extension will not be immediately counted towards
   the overall quota, and may not be counted for an extensive period of
   time. Additionally, if there are a lot of messages that have been
   added to a maildir from these mail user agents, quota recalculation
   may impose non-trivial load on the system, as the quota recalculator
   will have to issue the stat system call for each message.

  How to implement the quota

   The best way to do that is to modify your mail server to implement the
   protocol defined by this document. Not everyone, of course, has this
   ability. Therefore, an alternate approach is available.

   This package creates a very short utility called "deliverquota". It
   will NOT be installed anywhere by default, unless this maildir quota
   implementation is a part of a larger package, in which case the parent
   package may install this utility somewhere. If you obtained the
   maildir package separately, you will need to compile it by running the
   configure script, then by running make.

   deliverquota takes two arguments. deliverquota reads the message from
   standard input, then delivers it to the maildir specified by the first
   argument to deliverquota. The second argument specifies the actual
   quota for this maildir, as defined elsewhere in this document.
   deliverquota will deliver the message to the maildir, making a best
   effort not to exceed the stated quota. If the maildir is over quota,
   deliverquota terminates with exit code 77. Otherwise, it delivers the
   message, updates the quota, and terminates with exit code 0.

   Therefore, proceed as follows:
     * Copy deliverquota to some convenient location, say /usr/local/bin.
     * Configure your mail server to use deliverquota. For example, if
       you use Qmail and your maildirs are all located in $HOME/Maildir,
       replace the './Maildir/' argument to qmail-start with the
       following:
'| /usr/local/bin/deliverquota ./Maildir 1000000S'




       This sets a one million byte limit on all Maildirs. As I
       mentioned, this is meaningless if login access is available,
       because the individual account owner can create his own
       $HOME/.qmail file, and ignore deliverquota. Note that in this
       case, you MUST use apostrophes on the qmail-start command line, in
       order to quote this as one argument.

   If you would like to use different quotas for different users, you
   will have to put together a separate process or a script that looks up
   the appropriate quota for the recipient, and runs deliverquota
   specifying the quota. If no login access to the mail server is
   available, you can simply create a separate $HOME/.qmail for every
   recipient.

   That's pretty much it. If you handle a moderate amount of mail, I have
   one more suggestion. For the first couple of weeks, run deliverquota
   setting the second argument to an empty string. This disables quota
   enforcement, however it still activates certain optimizations that
   permit very fast quota recalculation. Messages delivered by
   deliverquota have their message size encoded in their filename; this
   makes it possible to avoid stat-ing the message in the Maildir, when
   recalculating the quota. Then, after most messages in your maildirs
   have been delivered by deliverquota, activate the quotas!!!

  maildirquota-enhanced applications

   This is a list of applications that have been enhanced to support the
   maildirquota extension:
     * maildrop - mail delivery agent/mail filter.
     * SqWebmail - webmail CGI binary.

   These applications fall into two classes:
     * Mail delivery agents. These applications read some externally
       defined table of mail recipients and their maildir quota.
     * Mail clients. These applications read maildir quota information
       that has been defined by the mail delivery agent.

   Mail clients generally do not need any additional setup in order to
   use the maildirquota extension. They will automatically read and
   implement any quota specification set by the mail delivery agent.

   On the other hand, mail delivery agents will require some kind of
   configuration in order to activate the maildirquota extension for some
   or all recipients. The instructions for doing that depends upon the
   mail delivery agent. The documentation for the mail delivery agent
   should be consulted for additional information.
     _________________________________________________________________

Mission statement

   Maildir++ is a mail storage structure that's based on the Maildir
   structure, first used in the Qmail mail server. Actually, Maildir++ is
   just a minor extension to the standard Maildir structure.

   For more information, see http://www.qmail.org/man/man5/maildir.html.
   I am not going to include the definition of a Maildir in this
   document. Consider it included right here. This document only
   describes the differences.

   Maildir++ adds a couple of things to a standard Maildir: folders and
   quotas.

   Quotas enforce a maximum allowable size of a Maildir. In many
   situations, using the quota mechanism of the underlying filesystem
   won't work very well. If a filesystem quota mechanism is used, then
   when a Maildir goes over quota, Qmail does not bounce additional mail,
   but keeps it queued, changing one bad situation into another bad
   situation. Not only know you have an account that's backed up, but now
   your queue starts to back up too.

Definitions, and goals

   Maildir++ and Maildir shall be completely interchangeable. A Maildir++
   client will be able to use a standard Maildir, automatically
   "upgrading" it in the process. A Maildir client will be able to use a
   Maildir++ just like a regular Maildir. Of course, a plain Maildir
   client won't be able to enforce a quota, and won't be able to access
   messages stored in folders.

   Folders are created as subdirectories under the main Maildir. The name
   of the subdirectory always starts with a period. For example, a folder
   named "Important" will be a subdirectory called ".Important". You
   can't have subdirectories that start with two periods.

   A Maildir++ client ignores anything in the main Maildir that starts
   with a period, but is not a subdirectory.

   Each subdirectory is a fully-fledged Maildir of its own, that is you
   have .Important/tmp, .Important/new, and .Important/cur. Everything
   that applies to the main Maildir applies equally well to the
   subdirectory, including automatically cleaning up old files in tmp. A
   Maildir++ enhancement is that a message can be moved between folders
   and/or the main Maildir simply by moving/renaming the file (into the
   cur subdirectory of the destination folder). Therefore, the entire
   Maildir++ must reside on the same filesystem.

   Within each subdirectory there's an empty file, maildirfolder. Its
   existence tells the mail delivery agent that this Maildir is a really
   a folder underneath a parent Maildir++.

   Only one special folder is reserved: Trash (subdirectory .Trash).
   Instead of marking deleted messages with the D flag, Maildir++ clients
   move the message into the Trash folder. Maildir++ readers are
   responsible for expunging messages from Trash after a system-defined
   retention interval.

   When a Maildir++ reader sees a message marked with a D flag it may at
   its option: remove the message immediately, move it into Trash, or
   ignore it.

   Can folders have subfolders, defined in a recursive fashion? The
   answer is no. If you want to have a client with a hierarchy of
   folders, emulate it. Pick a hierarchy separator character, say ":".
   Then, folder foo/bar is subdirectory .foo:bar.

   This is all that there's to say about folders. The rest of this
   document deals with quotas.

   The purpose of quotas is to temporarily disable a Maildir, if it goes
   over the quota. There is one and only major goal that this quota
   implementation tries to achieve:
     * Place as little overhead as possible on the mail system that's
       delivering to the Maildir++

   That's it. To achieve that goal, certain compromises are made:
     * Mail delivery will stop as soon as possible after Maildir++'s size
       goes over quota. Certain race conditions may happen with Maildir++
       going a lot over quota, in rare circumstances. That is taken into
       account, and the situation will eventually resolve itself, but you
       should not simply take your systemwide quota, multiply it by the
       number of mail accounts, and allocate that much disk space. Always
       leave room to spare.
     * How well the quota mechanism will work will depend on whether or
       not everything that accesses the Maildir++ is a Maildir++ client.
       You can have a transition period where some of your mail clients
       are just Maildir clients, and things should run more or less well.
       There will be some additional load because the size of the Maildir
       will be recalculated more often, but the additional load shouldn't
       be noticeable.

   This won't be a perfect solution, but it will hopefully be good
   enough. Maildirs are simply designed to rely on the filesystem to
   enforce individual quotas. If a filesystem-based quota works for you,
   use it.

   A Maildir++ may contain the following additional file: maildirsize.

Contents of maildirsize

   maildirsize contains two or more lines terminated by newline
   characters.

   The first line contains a copy of the quota definition as used by the
   system's mail server. Each application that uses the maildir must know
   what it's quota is. Instead of configuring each application with the
   quota logic, and making sure that every application's quota definition
   for the same maildir is exactly the same, the quota specification used
   by the system mail server is saved as the first line of the
   maildirsize file. All other application that enforce the maildir quota
   simply read the first line of maildirsize.

   The quota definition is a list, separate by commas. Each member of the
   list consists of an integer followed by a letter, specifying the
   nature of the quota. Currently defined quota types are 'S' - total
   size of all messages, and 'C' - the maximum count of messages in the
   maildir. For example, 10000000S,1000C specifies a quota of 10,000,000
   bytes or 1,000 messages, whichever comes first.

   All remaining lines all contain two integers separated by a single
   space. The first integer is interpreted as a byte count. The second
   integer is interpreted as a file count. A Maildir++ writer can add up
   all byte counts and file counts from maildirsize and enforce a quota
   based either on number of messages or the total size of all the
   messages.

Calculating maildirsize

   In most cases, changes to maildirsize are recorded by appending an
   additional line. Under some conditions maildirsize has to be
   recalculated from scratch. These conditions are defined later. This is
   the procedure that's used to recalculate maildirsize:
    1. If we find a maildirfolder within the directory, we're delivering
       to a folder, so back up to the parent directory, and start again.
    2. Read the contents of the new and cur subdirectories. Also, read
       the contents of the new and cur subdirectories in each Maildir++
       folder, except Trash. Before reading each subdirectory, stat() the
       subdirectory itself, and keep track of the latest timestamp you
       get.
    3. If the filename of each message is of the form xxxxx,S=nnnnn or
       xxxxx,S=nnnnn:xxxxx where "xxxxx" represents arbitrary text, then
       use nnnnn as the size of the file (which will be conveniently
       recorded in the filename by a Maildir++ writer, within the
       conventions of filename naming in a Maildir). If the message was
       not written by a Maildir++ writer, stat() it to obtain the message
       size. If stat() fails, a race condition removed the file, so just
       ignore it and move on to the next one.
    4. When done, you have the grand total of the number of messages and
       their total size. Create a new maildirsize by: creating the file
       in the tmp subdirectory, observing the conventions for writing to
       a Maildir. Then rename the file as maildirsize.Afterwards, stat
       all new and cur subdirectories again. If you find a timestamp
       later than the saved timestamp, REMOVE maildirsize.
    5. Before running this calculation procedure, the Maildir++ user
       wanted to know the size of the Maildir++, so return the calculated
       values. This is done even if maildirsize was removed.

Calculating the quota for a Maildir++

   This is the procedure for reading the contents of maildirsize for the
   purpose of determine if the Maildir++ is over quota.
    1. If maildirsize does not exist, or if its size is at least 5120
       bytes, recalculate it using the procedure defined above, and use
       the recalculated numbers. Otherwise, read the contents of
       maildirsize, and add up the totals.
    2. The most efficient way of doing this is to: open maildirsize, then
       start reading it into a 5120 byte buffer (some broken NFS
       implementations may return less than 5120 bytes read even before
       reaching the end of the file). If we fill it, which, in most
       cases, will happen with one read, close it, and run the
       recalculation procedure.
    3. In many cases the quota calculation is for the purpose of adding
       or removing messages from a Maildir++, so keep the file descriptor
       to maildirsize open. A file descriptor will not be available if
       quota recalculation ended up removing maildirsize due to a race
       condition, so the caller may or may not get a file descriptor
       together with the Maildir++ size.
    4. If the numbers we got indicated that the Maildir++ is over quota,
       some additional logic is in order: if we did not recalculate
       maildirsize, if the numbers in maildirsize indicated that we are
       over quota, then if maildirsize was more than one line long, or if
       the timestamp on maildirsize indicated that it's at least 15
       minutes old, throw out the totals, and recalculate maildirsize
       from scratch.

   Eventually the 5120 byte limitation will always cause maildirsize to
   be recalculated, which will compensate for any race conditions which
   previously threw off the totals. Each time a message is delivered or
   removed from a Maildir++, one line is added to maildirsize (this is
   described below in greater detail). Most messages are less than 10K
   long, so each line appended to maildirsize will be either between
   seven and nine bytes long (four bytes for message count, space, digit
   1, newline, optional minus sign in front of both counts if the message
   was removed). This results in about 640 Maildir++ operations before a
   recalculation is forced. Since most messages are added once and
   removed once from a Maildir, expect recalculation to happen
   approximately every 320 messages, keeping the overhead of a
   recalculation to a minimum. Even if most messages include large
   attachments, most attachments are less than 100K long, which brings
   down the average recalculation frequency to about 150 messages.

   Also, the effect of having non-Maildir++ clients accessing the
   Maildir++ is reduced by forcing a recalculation when we're potentially
   over quota. Even if non-Maildir++ clients are used to remove messages
   from the Maildir, the fact that the Maildir++ is still over quota will
   be verified every 15 minutes.

Delivering to a Maildir++

   Delivering to a Maildir++ is like delivering to a Maildir, with the
   following exceptions:
    1. Follow the usual Maildir conventions for naming the filename used
       to store the message, except that append ,S=nnnnn to the name of
       the file, where nnnnn is the size of the file. This eliminates the
       need to stat() most messages when calculating the quota. If the
       size of the message is not known at the beginning, append ,S=nnnnn
       when renaming the message from tmp to new.
    2. As soon as the size of the message is known (hopefully before it
       is written into tmp), calculate Maildir++'s quota, using the
       procedure defined previously. If the message is over quota, back
       out, cleaning up anything that was created in tmp.
    3. If a file descriptor to maildirsize was opened for us, after
       moving the file from tmp to new append a line to the file
       containing the message size, and "1".

Reading from a Maildir++

   Maildir++ readers should mind the following additional tasks:
    1. Make sure to create the maildirfolder file in any new folders
       created within the Maildir++.
    2. When moving a message to the Trash folder, append a line to
       maildirsize, containing a negative message size and a '-1'.
    3. When moving a message from the Trash folder, follow the steps
       described in "Delivering to Maildir++", as far as quota logic
       goes. That is, refuse to move messages out of Trash if the
       Maildir++ is over quota.
    4. Moving a message between other folders carries no additional
       requirements.