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Man Pages


Manual Reference Pages  -  MAIL::FOLDER (3)

.ds Aq ’

NAME

Mail::Folder - A folder-independant interface to email folders.

CONTENTS

SYNOPSIS

use Mail::Folder;

DESCRIPTION

This base class, and companion subclasses provide an object-oriented interface to email folders independant of the underlying folder implementation.

<B>WARNING: This code is in alpha release. Expect the interface to change.B>

The following folder interfaces are provided with this package:
Mail::Folder::Mbox Ye olde standard mailbox format.
Mail::Folder::Maildir An interface to maildir (a la qmail) folders. This is a very interesting folder format. It is ’missing’ some of the nicer features that some other folder interfaces have (like the message sequences in MH), but is probably one of the more resilient folder formats around.
Mail::Folder::Emaul Emaul is a folder interfaces of my own design (in the loosest sense of the word :-\)). It is vaguely similar to MH. I wrote it to flesh out earlier versions of the Mail::Folder package.
Mail::Folder::NNTP This is the beginnings of an interface to NNTP. Some of the Mail::Folder methods are not implemented yet, and no regression tests have been written.
Here is a snippet of code that retrieves the third message from a mythical emaul folder and outputs it to stdout:



  use Mail::Folder::Emaul;

  $folder = new Mail::Folder(emaul, "mythicalfolder");
  $message = $folder->get_message(3);
  $message->print(\*STDOUT);
  $folder->close;



METHODS

new($foldertype [, CW%options])

new($foldertype, CW$folder_name [, CW%options])

Create a new, empty Mail::Folder object of the specified folder type. If $folder_name is specified, then the open method is automatically called with that argument.

If $foldertype is AUTODETECT then the foldertype is deduced by querying each registered foldertype for a match.

Options are specified as hash items using key and value pairs.

The following options are currently built-in:
o Create

If set, open creates the folder if it does not already exist.

o Content-Length

If set, the Content-Length header field is automatically created or updated by the append_message and update_message methods.

o DotLock

If set and appropriate for the folder interface, the folder interface uses the ’.lock’ style of folder locking. Currently, this is only used by the mbox interface - please refer to the documentation for the mbox interface for more information. This mechanism will probably be replaced with something more generalized in the future.

o Flock

If set and appropriate for the folder interface, the folder interface uses the flock style of folder locking. Currently this is only used by the mbox interface - please refer to the documentation for the mbox interface for more information. This mechanism will probably be replaced with something more generalized in the future.

o NFSLock

If set and appropriate for the folder interface, the folder interface takes extra measures necessary to deal with folder locking across NFS. These special measure typically consist of constructing lock files in a special manner that is more immune to the atomicity problems that NFS has when creating a lock file. Use of this option generally requires the ability to use long filenames on the NFS server in question.

o NotMUA

If the option is set, the folder interface still makes updates like deletes and appends, and the like, but does not save the message labels or the current message indicator.

If the option is not set (the default), the folder interface saves the persistant labels and the current message indicator as appropriate for the folder interface.

The default setting is designed for the types of updates to the state of mail mssages that a mail user agent typically makes. Programmatic updates to folders might be better served to turn the option off so labels like ’seen’ aren’t inadvertantly set and saved when they really shouldn’t be.

o Timeout

If this options is set, the folder interface uses it to override any default value for Timeout. For folder interfaces doing network communications it is used to specify the maximum amount of time, in seconds, to wait for a response from the server. For folder interfaces doing local file locking it is used to specify the maximum amount of time, in seconds, to wait for a lock to be acquired. For the maildir interface it is, of course, meaningless :-).

o DefaultFolderType

If the Create option is set and AUTODETECT is being used to determine the folder type, this option is used to determine what type of folder to create.

    open($folder_name)

Open the given folder and populate internal data structures with information about the messages in the folder. If the Create option is set, then the folder will be created if it does not already exist.

The read-only attribute is set if the underlying folder interface determines that the folder is read-only.

Please note that I have not done any testing for using this module against system folders. I am a strong advocate of using a filter package or mail delivery agent that migrates the incoming email to the home directory of the user. If you try to use MailFolder against a system folder, you deserve what you get. Consider yourself warned. I have no intention, at this point in time, to deal with system folders and the related issues. If you work on it, and get it working in a portable manner, let me know.

Folder interfaces are expected to perform the following tasks:
o Call the superclass new method.
o Call set_readonly if folder is not writable.
o Call remember_message for each message in the folder.
o Initialize current_message.
o Initialize any message labels from the persistant storage that the folder has.

    close

Performs any housecleaning to affect a ’closing’ of the folder. It does not perform an implicit sync. Make sure you do a sync before the close if you want the pending deletes, appends, updates, and the like to be performed on the folder.

Folder interfaces are expected to perform the following tasks:
o Appropriate cleanup specific to the folder interface.
o Return the result of calling the superclass close method.

    sync

Synchronize the folder with the internal data structures. The folder interface processes deletes, updates, appends, refiles, and dups. It also reads in any new messages that have arrived in the folder since the last time it was either opened or synced.

Folder interface are expected to perform the following tasks:
o Call the superclass sync method.
o Lock the folder.
o Absorb any new messages
o Perform any pending deletes and updates.
o Update the folder persistant storage of current message.
o Update the folder persistant storage of message labels.
o Unlock the folder.

    pack

For folder formats that can have holes in the message number sequence (like mh) this will rename the files in the folder so that there are no gaps in the message number sequence.

Please remember that because this method might renumber the messages in a folder. Any code that remembers message numbers outside of the object could get out of sync after a pack.

Folder interfaces are expected to perform the following tasks:
o Call the superclass pack method.
o Perform the guts of the pack
o Renumber the Messages member of $self.
o Do not forget to update current_message based on the renumbering.

    get_message($msg_number)

Retrieve a Mail::Internet reference to the specified $msg_number. A fatal error is generated if no folder is currently open or if $msg_number isn\’t in the folder.

If present, it removes the Content-Length field from the message reference that it returns.

It also caches the header just as get_header does.

Folder interfaces are expected to perform the following tasks:
o Call the superclass get_message method.
o Extract the message into a Mail::Internet object.

get_mime_message($msg_number [, parserobject] [, CW%options])

Retrieves a MIME::Entity reference for the specified $msg_number. Returns undef on failure.

It essentially calls get_message_file to get a file to parse, creates a MIME::Parser object, configures it a little, and then calls the read method of MIME::Parser to create the MIME::Entity object.

If parserobject is specified it will be used instead of an internally created parser object. The parser object is expected to a class instance and a subcless (however far removed) of MIME::ParserBase.

Options are specified as hash items using key and value pairs.

Here is the list of known options. They essentially map into the MIME::Parser methods of the same name. For documentation regarding these options, refer to the documentation for MIME::Parser.
o output_dir
o output_prefix
o output_to_core

    get_message_file($msg_number)

Acts like get_message() except that a filename is returned instead of a Mail::Internet object reference.

A fatal error is generated if no folder is currently open or if $msg_number isn\’t in the folder.

Please note that get_message_file does not perform any ’From’ escaping or unescaping regardless of the underlying folder architecture. I am working on a mechanism that will resolve any resulting issues with this malfeature.

Folder interfaces are expected to perform the following tasks:
o Call the superclass get_message_file method.
o Extract the message into a temp file (if not already in one) and return the name of the file.

    get_header($msg_number)

Retrieves a message header. Returns a reference to a Mail::Header object. It caches the result for later use.

A fatal error is generated if no folder is currently open or if $msg_number isn\’t in the folder.

Folder interfaces are expected to perform the following tasks:
o Call the superclass get_header method.
o Return the cached entry if it exists.
o Extract the header into a Mail::Internet object.
o Cache it.

    get_mime_header($msg_number)

Retrieves the message header for the given message and returns a reference to MIME::Head object. It actually calls get_header, creates a MIME::Head object, then stuffs the contents of the Mail::Header object into the MIME::Head object.

A fatal error is generated if no folder is currently open or if $msg_number isn\’t in the folder.

get_fields($msg_number, CW@fieldnames)

Retrieves the fields, named in @fieldnames, from message $msg_number.

At first glance, this method might seem redundant. After all, Mail::Header provides the equivalent functionality. This method is provided to allow Mail::Folder interfaces for caching folder formats to take advantage of the caching. Those interfaces can override this method as they see fit.

The result is a list of field values in the same order as specified by the method arguments. If called in a list content, the resulting list is returned. If called in a scalar context, a reference to the list is returned.

    append_message($mref)

Add a message to a folder. Given a reference to a Mail::Internet object, it appends it to the end of the folder. The result is not committed to the original folder until a sync is performed.

The Content-Length field is added to the written file if the Content-Length option is enabled.

This method will, under certain circumstances, alter the message reference that was passed to it. If you are writing a folder interface, make sure you pass a dup of the message reference when calling the SUPER of the method. For examples, see the code for the stock folder interfaces provided with Mail::Folder.

update_message($msg_number, CW$mref)

Replaces the message identified by $msg_number with the contents of the message in reference to a <B>Mail::InternetB> object $mref. The result is not committed to the original folder until a sync is performed.

This method will, under certain circumstances, alter the message reference that was passed to it. If you are writing a folder interface, make sure you pass a dup of the message reference when calling the SUPER of the method. For examples, see the code for the stock folder interfaces provided with Mail::Folder.

Folder interfaces are expected to perform the following tasks:
o Call the superclass update_message method.
o Replace the specified message in the working copy of the folder.

refile($msg_number, CW$folder_ref)

Moves a message from one folder to another. Note that this method uses delete_message and append_message so the changes will show up in the folder objects, but will need a syncs performed in order for the changes to show up in the actual folders.

dup($msg_number, CW$folder_ref)

Copies a message to a folder. Works like refile, but does not delete the original message. Note that this method uses append_message so the change will show up in the folder object, but will need a sync performed in order for the change to show up in the actual folder.

A fatal error is generated if no folder is currently open or if $msg_number isn\’t in the folder.

    delete_message(@msg_numbers)

Mark a list of messages for deletion. The actual delete in the original folder is not performed until a sync is performed. This is merely a convenience wrapper around add_label. It returns 1.

If any of the items in @msg_numbers are array references, delete_message will expand out the array reference(s) and call add_label for each of the items in the reference(s).

    undelete_message(@msg_numbers)

Unmarks a list of messages marked for deletion. This is merely a convenience wrapper around delete_label. It returns 1.

If any of the items in @msg_numbers are array references, undelete_message will expand out the array reference(s) and call delete_label for each of the items in the reference(s).

    message_list

Returns a list of the message numbers in the folder. The list is not guaranteed to be in any specific order.

    qty

Returns the quantity of messages in the folder.

    first_message

Returns the message number of the first message in the folder.

    last_message

Returns the message number of the last message in the folder.

    next_message

    next_message($msg_number)

Returns the message number of the next message in the folder relative to $msg_number. If $msg_number is not specified then the message number of the next message relative to the current message is returned. It returns 0 if there is no next message (ie. at the end of the folder).

    prev_message

    prev_message($msg_number)

Returns the message number of the previous message in the folder relative to $msg_number. If $msg_number is not specified then the message number of the next message relative to the current message is returned. It returns 0 is there is no previous message (ie. at the beginning of the folder).

    first_labeled_message($label)

Returns the message number of the first message in the folder that has the label $label associated with it. Returns 0 is there are no messages with the given label.

    last_labeled_message($label)

Returns the message number of the last message in the folder that has the label $label associated with it. Returns 0 if there are no messages with the given label.

next_labeled_message($msg_number, CW$label)

Returns the message number of the next message (relative to $msg_number) in the folder that has the label $label associated with it. It returns 0 is there is no next message with the given label.

prev_labeled_message($msg_number, CW$label)

Returns the message number of the previous message (relative to $msg_number) in the folder that has the label $label associated with it. It returns 0 is there is no previous message with the given label.

    current_message

    current_message($msg_number)

When called with no arguments returns the message number of the current message in the folder. When called with an argument set the current message number for the folder to the value of the argument.

For folder mechanisms that provide persistant storage of the current message, the underlying folder interface will update that storage. For those that do not, changes to current_message will be affect while the folder is open.

    sort($func_ref)

Returns a sorted list of messages. It works conceptually similar to the regular perl sort. The $func_ref that is passed to sort must be a reference to a function. The function will be passed two <B>Mail::HeaderB> message references and it must return an integer less than, equal to, or greater than 0, depending on how the list is to be ordered.

    select($func_ref)

Returns a list of message numbers that match a set of criteria. The method is passed a reference to a function that is used to determine the match criteria. The function will be passed a reference to a <B>Mail::InternetB> message object containing only a header.

The list of message numbers returned is not guaranteed to be in any specific order.

    inverse_select($func_ref)

Returns a list of message numbers that do not match a set of criteria. The method is passed a reference to a function that is used to determine the match criteria. The function will be passed a reference to a <B>Mail::InternetB> message object containing only a header.

The list of message numbers returned is not guarenteed to be in any specific order.

add_label($msg_number, CW$label)

Associates $label with $msg_number. The label must have a length > 0 and should be a printable string, although there are currently no requirements for this.

add_label will return 0 if $label is of zero length, otherwise it returns 1.

The persistant storage of labels is dependant on the underlying folder interface. Some folder interfaces may not support arbitrary labels. In this case, the labels will not exist when the folder is reopened.

There are a few standard labels that have implied meaning. Unless stated, these labels are not actually acted on my the module interface, rather they represent a standard set of labels for MUAs to use.
o deleted

This is used by the delete_message and sync to process the deletion of messages. These will not be reflected in any persistant storage of message labels.

o edited

This tag is added by update_message to reflect that the message has been altered. This behaviour may go away.

o seen

This means that the message has been viewed by the user. The concept of seen is nebulous at best. The get_message method sets this label for any message it is asked to retrieve.

o filed
o replied
o forwarded
o printed

delete_label($msg_number, CW$label)

Deletes the association of $label with $msg_number.

Returns 0 if the label $label was not associated with $msg_number, otherwise returns a 1.

    clear_label($label)

Deletes the association of $label for all of the messages in the folder.

Returns the quantity of messages that were associated with the label before they were cleared.

label_exists($msg_number, CW$label)

Returns 1 if the label $label is associated with $msg_number otherwise returns 0.

    list_labels($msg_number)

Returns a list of the labels that are associated with $msg_number.

If list_labels is called in a scalar context, it returns the quantity of labels that are associated with $msg_number.

The returned list is not guaranteed to be in any specific order.

    list_all_labels

Returns a list of all the labels that are associated with the messages in the folder. The items in the returned list are not guaranteed to be in any particular order.

If list_all_labels is called in a scalar context, it returns the quantity of labels that are associated with the messages.

    select_label($label)

Returns a list of message numbers that have the given label $label associated with them.

If select_label is called in a scalar context, it will return the quantity of messages that have the given label.

    foldername

Returns the name of the folder that the object has open.

    message_exists($msg_number)

Returns 1 if the folder object contains a reference for $msg_number, otherwise returns 0.

    set_readonly

Sets the readonly attribute for the folder. This will cause the sync command to not perform any updates to the actual folder.

    is_readonly

Returns 1 if the readonly attribute for the folder is set, otherwise returns 0.

    get_option($option)

Returns the setting for the given option. Returns undef if the option does not exist.

set_option($option, CW$value)

Set $option to $value.

    debug($value)

Set the level of debug information for the object. If $value is not given then the current debug level is returned.

    debug_print($text)

Outputs $text, along with some other information to STDERR. The format of the outputted line is as follows:

-<gt $subroutine $self $text>

WRITING A FOLDER INTERFACE

    General Concepts

In general, writing a folder interface consists of writing a set of methods that overload some of the native ones in Mail::Folder. Below is a list of the methods that will typically need to overridden. See the code of the folder interfaces provided with the package for specific examples.

Basically, the goal of an interface writer is to map the mechanics of interfacing to the folder format into the methods provided by the base class. If there are any obvious additions to be made, let me know. If it looks like I can fit them in and they make sense in the larger picture, I will add them.

If you set about writing a folder interface and find that something is missing from this documentation, please let me know.

    Initialization

The beginning of a new folder interface module should start with something like the following chunk of code:



    package Mail::Folder::YOUR_FOLDER_TYPE;
    @ISA = qw(Mail::Folder);
    use Mail::Folder;

    Mail::Folder::register_folder_type(Mail::Folder::YOUR_FOLDER_TYPE,
                                       your_folder_type_name);



    Envelopes

Please take note that inter-folder envelope issues are not complete ironed out yet. Some folder types (maildir via qmail) actually store all of the envelope information, some (mbox) only store a portion of it, and others do not store any. Electronic has a rich history of various issues related this issue (anyone out there remember the days when many elm programs were compiled to use the ’From_’ field for replies instead of the fields in the actual header - and then everyone started do non-uucp email? :-).

Depending on the expectations, the scale of the problem is relative. Here is what I have done so far to deal with the problem.

In the stock folder interfaces, the underlying Mail::Internet object is created with the ’MailFrom’ option set to ’COERCE’. This will cause it to rename a ’From_’ field to a ’Mail-From’ field. All interface writers should do the same. This will prevent the interface writer from needing to deal with it themselves.

For folder interfaces that require part or all of the envelope to be present as part of the stored message, then coercion is sometimes necessary. As an example, the maildir folder format uses a ’Return-Path’ field as the first line in the file to signify the sender portion of the envelope. If that field is not present, then the interfaces tries to synthesize it by way of the ’Reply-To’, ’From’, and ’Senderfields (in that order). Currently, it croaks if it fails that sequence of fields (this will probably change in the future - feedback please). At some time in the future, I am going to try to provide some generalized routines to perform these processes in a consistant manner across all of the interfaces; in the mean time, keep an eye out for issues related to this whole mess.

Every folder interface should take to prevent some of the more common problems like being passed in a message with a ’From_’ field. If all other fields that carry similar information are present, then delete the field. If the interface can benefit from coercing it into another field that would otherwise be missing, go for it. Even if all of the other interfaces do the right thing, a user might hand it a mail message that contains a ’From_’ field, so one cannot be to careful.

The recipient portion of the envelope is pretty much not dealt with at all. If it presents any major issues, describe them to me and I will try to work something out.

    Methods to override

The following methods will typically need to be overridden in the folder interface.
o open
o close
o sync
o pack
o get_header
o get_message
o get_message_file
o update_message

FOLDER INTERFACE METHODS

This section describes the methods that for use by interface writers. Refer to the stock folder interfaces for examples of their use.

    register_type($type)

Registers a folder interface with Mail::Folder.

    is_valid_folder_format($foldername)

In a folder interface, this method should return 1 if it thinks the folder is valid format and return 0 otherwise. It is used by the Mail::Folder open method when AUTODETECT is used as the folder type. The open method iterates through the list of known folder interfaces until it finds one that answer yes to the question.

This method is always overrided by the folder interface. A fatal occurs if it isn\’t.

    init

This is a stub entry called by new. The primary purpose is to provide a method for subclasses to override for initialization to be performed at constructor time. It is called after the object members variables have been initialized and before the optional call to open. The new method will return undef if the init method returns 0. Only interface writers need to worry about this one.

    create($foldername)

In a folder interface, this method should return 1 after it successfully creates a folder with the given name and return 0 otherwise.

This method is always overrided by the folder interface. The base class method returns a 0 so that if create is not defined in the folder interface, the call to create will return failure.

cache_header($msg_number, CW$header_ref)

Associates $header_ref with $msg_number in the internal header cache of the object.

    invalidate_header($msg_number)

Clobbers the header cache entry for $msg_number.

    remember_message($msg_number)

Add an entry for $msg_number to the internal data structure of the folder object.

    forget_message($msg_number)

Removes the entry for $msg_number from the internal data structure of the folder object.

MISC

    detect_folder_type($foldername)

Returns the folder type of the given $foldername. Returns undef if it cannot deduce the folder type.

CAVEATS

    Forking children

If a script forks while having any folders open, only the parent should make any changes to the folder. In addition, when the parent closes the folder, related temporary files will be reaped. This temporary file cleanup will not occur for the child. I am contemplating a more general solution to this problem, but until then <B>ONLY PARENTS SHOULD MANIPULATE MAILB>.

    Folder locking

Ugh... Folder locking...

Please note that I am not pleased with the folder locking as it is currently implemented in Mail::Folder for some of the folder interfaces. It will improve.

Folder locking is problematic in certain circumstances. For those not familier with some of these issues, I will elaborate.

An interface like maildir has no locking issues. This is because the design of the folder format inherently eliminates the need for locking (cheers from the crowd).

An interface like nntp has no locking issues, because it merely implements an interface to a network protocol. Locking issues are left as an exercise to the server on the other end of a socket.

Interfaces like mbox, on the other hand, are another story.

Taken as a whole, the locking mechanism(s) used for mbox folders are not inherently *rock-solid* in all circumstances. To further complicate things, there are a several variations that have been implemented as attempts to work around the fundemental issues of the design of mbox folder locking.

In the simplest implementation, an mbox folder merely uses a dotlock file as a semaphore to prevent simultaneous updates to the folder. All processes are supposed to cooperate and honor the lock file.

In a non-NFS environment, the only typical issue with a dotlock is that the code implementing the lock file needs to be written in such a way as to prevent race conditions be testing for the locking and creating the lockfile. This is typically done with a O_EXCL flag passed to the call to open(2). This allows for an atomic creation of the lock file if and only if the file does not already exist, assuming the operating system implements the O_EXCL feature. Some operating systems implementations have also resorted to using lockf, fcntl, or flock as way to atomically test and set the folder lock. The major issue for Mail::Folder in this type of environment is to merely detect what flavor(s) is necessary and implement it.

In an NFS environment, the story is somewhat different and a lot more complicated. The O_EXCL is not atomic across NFS and some implementations of flock do not work across NFS, and not all operating systems use flock to lock mbox folders. To further complicate matters, all processes that lock mbox folder need to do it un such a way that all clients mounting the data can also cooperate in the locking mechanism.

Here are a few of the outstanding folder locking issues in Mail::Folder for folder interfaces that do not provide a native way to solve locking issues.
o only DotLock is supported

There are snippets of code related to flock, but I have disabled it for a time.

o not NFS safe

Sorry...

We now return you to your regularly scheduled program...

AUTHOR

Kevin Johnson <kjj@pobox.com>

COPYRIGHT

Copyright (c) 1996-1998 Kevin Johnson <kjj@pobox.com>.

All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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