Shared folders

This document describes how shared folders are implemented by Courier-IMAP, and SqWebMail.

Courier-IMAP and SqWebMail actually have two different shared folder implementations, for two situations: A) Filesystem permissions-based shared folders, for systems with traditional shell login accounts, and mailboxes; and B) virtual shared folders, for closed systems that provide mail access only, with all mailboxes using the same system userid/groupid, and no end-user shell login access.

This documentation is also applicable to the Courier mail server, which includes Courier-IMAP and SqWebMail as its components. There are some minor implementation differences between the standalone Courier-IMAP and SqWebMail packages, and Courier; namely the different locations of several key configuration files. Aside from that, both implementation are equivalent.

Virtual shared folders

Terminology

Virtual shared folders are implemented in a completely different manner. In a virtual setup, all mailboxes use the same system userid and groupid; there is no shell login access to the mail server, and all access is managed by setting up access control lists. Each individual user may voluntarily grant access to a folder to another user or group of user. Access control lists permit fine-grained control. It is possible to specify what different users are permitted to do to the folder or its contents. Since there is no shell login access, all access to mail folders occurs through an IMAP connection to the server, and the IMAP server observes the defined access control lists on each folder.

NOTE:

The references to IMAP in this documentation equally apply to SqWebMail as well. This documentation file is included in Courier-IMAP's and SqWebMail's tarballs. When reading this documentation file in the SqWebMail tarball, mentally replace all references to IMAP with webmail. SqWebMail does not use IMAP, the server reads the maildirs directory; but it uses the same shared library as Courier-IMAP, which explains the similar implementation.

For more information on how to set up access control lists, see the maildiracl(1) manual page.

Technical Overview

After logging in, the server process runs in the logged in account's maildir. As the INSTALL file describes in great detail, virtual mail accounts may be maintained via a wide variety of back-end databases, including MySQL, PostgresSQL, or an LDAP directory. If now the server needs to check the access controls on another account's mail folder, to determine if the user has access to it, the server needs to now locate where the other account is located.

However, there are several reasons why the server cannot directly go back to the authentication database. The first reason is that after logging in the server process no longer has root privileges; and database configuration files normally require root privileges (since they may contain administrative passwords). Although there might be ways to hack around that, the second reason is the show-stopper: IMAP's design permits clients to enumerate all available shared mail accounts, or arbitrary subsets of them. And, in fact, many of them do just that; mainly because IMAP's design leaves no other alternative, in some situations.

Clearly, it is not feasible to have hundreds of clients hammering at the database server, repeatedly downloading huge lists of available shared folders (which may number in tens of thousands, on a busy server). Therefore, the following approach is used.

One or more flat text files are set up, which contain an index of all virtual mail folders. The list is, essentially, downloaded from the authentication database. The text file contain a subset of the data in the authentication database; only the minimum amount of fields necessary to specify the account's name, and where the account lives. The server process quickly reads the text file(s) in order to locate another account's folder, and to check its access control list.

The system administrator will need to set up a regularly-scheduled process that rebuilds the shared folder index. Yes, that means that a new account's shared folders will not be accessible, by other accounts, until the shared folder index gets rebuilt (but the account still has immediate access to its own folders, after creation). The shared folder index will be updated in a manner that allows the update to occur live, without requiring any system downtime.

On moderately-sized systems it might be feasible to invoke the shared index update script as part of the process that adds or removes an account; thereby the shared folder index files will always be kept up to date.

In its most simple form, the shared index is a single file, called SYSCONFDIR/shared/index, where "SYSCONFDIR" is the server's configuration directory. The following table provides the default locations of the configuration file:

NOTE:

If the "shared" directory doesn't exist, just create it.

Blank lines in this file are ignored, as well as lines that begin with a single "#" character, which are arbitrary comments. Each remaining line specifies the location of an account's maildir. The line contains the following fields, separated by a single tab character:

1. name
2. system userid
3. system groupid
4. virtual home directory
5. maildir path, relative to the virtual home directory

"name" SHOULD be the account's login name (which in some rare configurations may not actually match the IMAP login used by the client; this is the name that's logged to syslog when the account successfully logs in, which is usually the same as the IMAP login id). In most situations all accounts will have the same system userid and groupid. The server doesn't actually do anything with these fields at this time, but may do so in the future.

The virtual account's home directory is taken verbatim from the authentication database. The maildir path field is optional. If missing, it defaults to "./Maildir". Combining the home directory, and the maildir path, results in the location where the specified account's maildir folders may be found.

Inaccessible accounts are silently ignored. This allows the shared folder index to be itself shared between multiple servers even though that accounts themselves are not shared. Each server will only see the accounts it can access.

As mentioned previously, IMAP clients will often want to obtain a complete list of shared folders. If the mail server has more than a a couple of hundred accounts, a single index file may be inefficient, and the mail accounts should be segregated into different groups. An account group entry in the index file looks like this:

1. group name
2. *
3. filename

When the second tab-delimited field is a single asterisk, the first field is taken to be a name of an account group; and "filename" is the name of another, separate, index file that lists all accounts that belong in this group. The second file should also be installed in SYSCONFDIR/shared.

When it's time to use account groups, SYSCONFDIR/shared/index will usually contain only group entries, with the accounts themselves dispersed in other files in the same directory.

Account names and group names use the UTF-8 character set. This is not usually an issue for account names, which are almost always limited to the Latin character set. Group names may be arbitrary, though, and include non-Latin characters, using UTF-8.

Courier does not allow IMAP folder names to contain the "." or the "/" characters. The shared index file may include those characters in account or group names, and they will be automatically replaced by spaces (which are allowed).

This is not normally an issue, unless there exists two separate accounts or groups whose names are the same, except that one name contains spaces, and the other name contains the forbidden characters. This is obviously not something that is likely to occur, but if it did the end result would be that one of the names will have its punctuation characters replaced by spaces, and now two virtual folders will exist with the same name.

If this rather unlikely scenario somehow materializes the solution is to enable the IMAP_SHAREDMUNGENAMES setting in Courier-IMAP's configuration file. When enabled, this setting replaces the "." and "/" with a two character sequence: "\:" and "\;". Any the existing backslashes are doubled, thus preserving folder name uniqueness. This is not something you'd want to do unless you have no other choice.

The equivalent setting for SqWebMail (or Courier's webmail server) is the SQWEBMAIL_SHAREDMUNGENAMES. When using SqWebMail at the same time as Courier-IMAP, and if IMAP_SHAREDMUNGENAMES is set, the SQWEBMAIL_SHAREDMUNGENAMES variable must also be set in order for everyone to be in sync. This can be done in one of two different ways:

1. Set this variable before running the sqwebmaild start command:

   SQWEBMAIL_SHAREDMUNGENAMES=1
   export SQWEBMAIL_SHAREDMUNGENAMES
   sqwebmaild start
   

2. Set this environment variable in the web server's configuration files. With Apache, for example:

   SetEnv SQWEGBMAIL_SHAREDMUNGENAMES 1
   

Many Shared Accounts

When the number of mail accounts begins to exceed a couple of thousand (or even less, depending on the mail server), even account groups will not be enough. If a single mail server has tens of thousands of accounts, an individual IMAP client may potentially access hundreds of thousands of folders.

It is a sad fact of life that there are poorly-designed IMAP clients that insist on searching for every accessible folder, every time. They will stubornly scan the entire IMAP folder namespace, recursively reading each folder hierarchy, to compile a list of available folders. Even though there may only be a few folders shared by their owners, the server still has to check whether the IMAP client has access to each folder, in order to compile the list of accessible folders. No mail server will like an IMAP client that insists on reading the access control lists of hundreds of thousand of folders.

This problem is solved by setting the "sharedgroup" option for each account. The INSTALL file contains a more specific description of how to initialize account options. Normally, the shared folder index file is "SYSCONFDIR/shared/index"; however, if the "sharedgroup" option is set, the value of "sharedgroup" is appended to the shared folder index filename. So, if user1's account has the "sharedgroup=12" option, as far as this account is concerned the shared folder index file is "SYSCONFDIR/shared/index12". Note that SYSCONFDIR/shared/index12 may still contain account group entries that point to other shared index files.

This enables the ability to partition all accounts into smaller "universes". The list of accounts is broken up into individual universe. Accounts within a universe can only see other accounts in the same universe. Even if a given folder's access control lists permit global access, only accounts in the same universe will be able to access it.

Also note that trans-universal wormholes are possible. Two, or more, top-level index files may list the same set of account groups, and those accounts will be visible from both (or more) universes.

IMPORTANT: Under no circumstances should you install circular wormholes (index file A lists index file B as a group, and index file B lists index file A as a group). The consequences would be disastrous for both universes.

Functional Overview

As mentioned in the previous section, it is necessary to set up a regularly-scheduled system process that updates the shared folder index files. The procedure for doing so is site-specific. Individual sites will need to put together a set of custom scripts that create the shared folder index files in SYSCONFDIR/shared. This is likely to be a highly site-specific code; however Courier installs several generic shell scripts that can be used as a working starting point.

The most important step in the process is actually the final step of installing new shared folder index files in SYSCONFDIR/shared. They must be installed in a way that can be done live, without shutting down the system, and without affecting any existing processes. This can be done using the “sharedindexinstall” script, which may be found in the sbin directory. To use sharedindexinstall, first create the shared index files in a temporary directory called SYSCONFDIR/shared.tmp, then run the script to move all the files in this directory to SYSCONFDIR/shared.

So a typical script that updates shared index files will generally look like this (this example uses Courier-IMAP, for SqWebMail the directory locations will be different):

---

#!/bin/sh
sysconfdir="/usr/lib/courier-imap/etc"  # Or /etc/courier, or whatever...
sbindir="/usr/lib/courier-imap/sbin"    # Or, wherever it actually is...

rm -rf $sysconfdir/shared.tmp
mkdir $sysconfdir/shared.tmp || exit 1

#
# A magical process creates updated shared index files right about now...
#

$sbindir/sharedindexinstall

---

NOTE

Existing IMAP server processes may continue to use cached shared folder index data for some time, after sharedindexinstall. This will not cause any problems.

Using authenumerate

In most cases, systems that use a single shared index file are likely to need to only run the “authenumerate” program in order to build the shared folder index. As long as Courier's authentication modules are properly configured (and authdaemond is running) authenumerate will download the list of accounts from the configured authentication module, and generate a suitably-formatted list on standard output. So the complete shared folder index update script will look like this:

---

#!/bin/sh
sysconfdir="/usr/lib/courier-imap/etc"
sbindir="/usr/lib/courier-imap/sbin"

rm -rf $sysconfdir/shared.tmp
mkdir $sysconfdir/shared.tmp || exit 1

$sbindir/authenumerate -s >$sysconfdir/shared.tmp/index || exit 1

$sbindir/sharedindexinstall

---

The functionality to enumerate accounts is new to Courier-IMAP 3.0. When upgrading from an earlier versions, systems that are configured to use a custom MySQL, PostgreSQL, or LDAP queries will need to enter a new configuration setting in the appropriate configuration file. The update process will add a new variable to the configuration file, which must be initialized to contain the custom query that reads the account list accordingly. In most cases the query only needs to be a slight variation on the existing query that checks the password of a specific account that's requesting authentication.

Systems that do not use the custom query option should not need to make any additional setting, as the standard query authentication variables contain all the information that's needed to obtain a complete list of accounts.

If only a small proportion of your users are entitled to use shared mailboxes, then it helps scalability enormously if you restrict the shared index file to contain just those accounts. The -s flag to authenumerate implements this, by discarding all accounts which have the option disableshared set to a non-zero value. Further efficiency can be gained by customising your database query so that the database itself returns only the relevant accounts. Use option MYSQL_ENUMERATE_CLAUSE, PGSQL_ENUMERATE_CLAUSE or LDAP_ENUMERATE_FILTER as appropriate.

Note also that you can set DEFAULTOPTIONS="disableshared=1" in authdaemonrc to make sharing disabled for everyone, and then set option disableshared=0 only on permitted accounts.

NOTE

authenumerate tries not to download the complete results of each query into a memory buffer, but it may be that this still happens due to circumstances out of its control (e.g. older versions of MySQL or PostgreSQL client libraries may force this to happen). If so, it's possible that authenumerate's memory requirements may be large enough to affect the running system. In this case you will need to come up with an alternative mechanism to obtain the list of accounts, in some other way.

NOTE

The PAM library does not have a function that obtains a list of available accounts. Therefore, on all systems that use the authpwd, authshadow, or authpam modules, authenumerate works in exactly the same way: by using the getpwent() function. Systems that use certain PAM modules, such as ones that authenticate against a MySQL, a PostgreSQL, or an LDAP database, will not be able to use authenumerate, and must come up with a suitable replacement.

Using sharedindexsplit

As mentioned in the technical overview, a single index file may not be feasible if the number of accounts is more than a thousand, or so. At that point it becomes necessary to split the shared folder index into multiple files.

The sharedindexsplit script reads a list of all accounts, formatted as a single shared folder index (which, incidentally, matches authenumerate's output format as well) and splits it into multiple files. The first argument to sharedindexsplit is the name of the directory where the output files are going to be created. The directory must be empty.

sharedindexsplit splits the index into multiple files, based on either the 'sharedgroup' account option, or the first character or characters of the account's name. Use the optional second argument to sharedindexsplit to specify a number, if splitting the account list based on initial characters is desired. If splitting based on the 'sharedgroup' account option then use the -o flag to authenumerate to get it to include the account options in its output.

Perl 5.8.0, or greater, must be installed if account names include non-Latin characters. For best results, the shared folder index input to sharedindexsplit should already be sorted, but it doesn't have to be.

Therefore, the following scripts should produce good results on a system with a large, but a moderate number of accounts. Split on 'sharedgroup' if you have a number of separate 'universes' where sharing is only permitted within each universe; otherwise split on the account name if all users can potentially access all shared mailboxes.

---

#!/bin/sh
sysconfdir="/usr/lib/courier-imap/etc"
sbindir="/usr/lib/courier-imap/sbin"

rm -rf $sysconfdir/shared.tmp
mkdir $sysconfdir/shared.tmp || exit 1

# split on the 'sharedgroup' option
$sbindir/authenumerate -s -o >$sysconfdir/shared.tmp/.tmplist || exit 1
$sbindir/sharedindexsplit $sysconfdir/shared.tmp <$sysconfdir/shared.tmp/.tmplist  || exit 1
rm -f $sysconfdir/shared.tmp/.tmplist
$sbindir/sharedindexinstall

---

#!/bin/sh
sysconfdir="/usr/lib/courier-imap/etc"
sbindir="/usr/lib/courier-imap/sbin"

rm -rf $sysconfdir/shared.tmp
mkdir $sysconfdir/shared.tmp || exit 1

# split on the first 1 character of the username
$sbindir/authenumerate -s >$sysconfdir/shared.tmp/.tmplist || exit 1
$sbindir/sharedindexsplit $sysconfdir/shared.tmp 1 <$sysconfdir/shared.tmp/.tmplist  || exit 1
rm -f $sysconfdir/shared.tmp/.tmplist
$sbindir/sharedindexinstall

---

authenumerate is saved to a temporary file, instead of being piped directly, is so that its exit code can be checked. We want to abort the entire process if authenumerate terminates with a non-zero exit code. If authenumerate pipes its output directly to sharedindexsplit, and aborts with an error, then sharedindexsplit will read an empty shared folder index, consequently the output directory will be empty, and then sharedindexinstall will obligingly install an empty list of shared folders.

As mentioned previously, at some point authenumerate's memory requirements may become too much to handle, and an alternative mechanism will need to be improvised. sharedindexsplit's memory requirements are not dependent on the number of accounts, so this script can still be used even with very many accounts.

When it's time to use account groups, SYSCONFDIR/shared/index will usually contain only group entries, with the accounts themselves dispersed in other files in the same directory.

Combining Courier-IMAP's and SqWebMail index files

The information in this section is applicable only when running both Courier-IMAP and SqWebMail packages. This is not applicable when running Courier, which includes both the IMAP server and the webmail server. In the Courier build, both servers are set up to use the same shared index file.

But, if both standalone builds are installed, Courier-IMAP will want to use /usr/lib/courier-imap/etc/shared/index as its shared index file, while SqWebMail will read /usr/local/share/sqwebmail/shared/index.

To make both servers use the same shared folder index, create a soft link:

ln -s /usr/lib/courier-imap/etc/shared /usr/local/share/sqwebmail/shared

It's important to create a soft link to the entire "shared" directory, otherwise index group files will not work.

IMAP Access Control List and account groups

Even after setting up shared folders indexes correctly, the mail client will show an empty list of shared folders. This is because even though each account's folders may be shared, they will not be visible to other accounts until the mail account's owner explicitly grants public access to a folder. This can be done using an IMAP mail client that understands access control lists, using Courier's SqWebMail, or the maildiracl command. The maildiracl manual page includes an overview of access control lists and how to control who gets what kind of rights on a folder. See the maildiracl manual page for more information.

Filesystem permissions-based shared folders

Terminology

• Sharable Maildir - a maildir that contains folders that can be shared.
• Sharable folder - one or more folders in a sharable Maildir that can be shared.

Technical Overview

• They are a somewhat logical extension of the base Maildir format.
• Older versions of Courier-IMAP and SqWebMail will completely ignore shared folders.
• Messages in shared folders do not count towards any individual maildir quotas (see README.maildirquota.html).
• Shared folders are implemented as additional, Maildir-based folders. The messages will be soft links to the messages in the sharable folder. Soft links will be used instead of hard links in order to be able to have a folder containing large messages, and then be able to reclaim space immediately when the messages are purged, instead of waiting for everyone to sync up and delete their hard link. The flip side is that you can expect the usual sorts of errors and increased confusion if someone attempts to read a message that has just been removed, but their client (Courier-IMAP or SqWebMail) doesn't know that yet. That's unavoidable. You'll probably have to set some kind of a policy in order to keep the expected insanity to a minimum.
• Sharable folders are subscribed to by creating a separate maildir-type directory within the main Maildir. It's created in a separate subdirectory that is ignored by other Maildir clients.
• The new directory will contains soft links to the messages in the sharable folder. "Synchronization" code will be used to synchronize the contents of the sharable folder, and the copy of it in the regular Maildir. Once links to the messages are created, they can be manipulated like regular messages in a Maildir. This procedure will be transparently performed by Courier-IMAP or SqWebMail behind the scenes.
• Access to shared folders is controlled by a combination of an explicit configuration, plus regular filesystem permissions. If account owners have shell access to the server, they can create shared folders, and link their mailbox to other accounts' shared folders. Then, the actual access is controlled by regular filesystem permissions. The owner of the shared folder will use the regular filesystem permission to give read and/or write access to either everyone else, or everyone else who uses the same system group ID. Read access allows people to read messages from a shared folder. Write access allows people to add and delete messages in the shared. The owner of the shared folder can remove any message, everyone else will be able to remove only their own messages.

Functional Overview

Generally speaking, shared folders are configured using a feature-enhanced maildirmake command as follows:

• make install will install a maildirmake command that has a bunch of funny options. The modified maildirmake command is installed into Courier-IMAP's or SqWebMail's installation directory.
• Somebody, maybe you, will use some of these options to create a maildir with modified permissions. The same somebody will run maildirmake again, with a few other options, to create folders in that maildir, also with modified permissions. Those permissions allows global read (and optionally write) access to those folders.
• Do NOT use this maildir as the primary mailbox, INBOX, for an account. Instead, you must create this maildir separately, perhaps as $HOME/Maildir-shared, then set it up as one of your sharable maildirs (see below), and access it in shared mode. Because you own it, you have unlimited read/write access to it. The previously mentioned options will select whether or not access permissions are given to everyone else, and they do not apply to you.
• Everyone else will run maildirmake with even more funny options. Those options install a link from their own maildir to a maildir that contains sharable folders. A given maildir may contain links to more than one sharable maildir. As long as their system user/group permissions allow them to access messages, they can SUBSCRIBE via their IMAP client to the folder, or use the SUBSCRIBE function in SqWebmail.
• If people do not have shell access, the system administrator will have to run maildirmake on their behalf, in order to create the links to the sharable maildirs.
• People with write access can add messages to a sharable folder. Messages from a sharable folder can be removed only by the owner of the shared folder, or by the owner of each message.
• This sharable maildir implementation cannot use maildir soft-quotas. There cannot be a soft-quota installed in a sharable maildir. If you need quota support, you will have to use filesystem-based quotas. There are some sticky issues with updating quotas on a sharable maildir.

Also, note that anyone, not just the superuser, can create a sharable maildir in their account, and invite anyone else to access it, as long as their system user/group permissions allow them access.

To summarize:

• Use the -S option to maildirmake to create a maildir that can contain sharable folders.
• Use the -s and -f options to maildirmake to create sharable folders. The same sharable maildir may contain multiple sharable folders with different access permissions, as selected by the -s option.
• Use the --add option to install a link from a regular maildir to a sharable maildir. Afterwards, IMAP clients will be able to subscribe to folders in the sharable maildir.
• Use the --del option to remove a link.

For more information on these options, see the maildirmake manual page that will be installed.

More on additional options for the maildirmake command

The "new" and "cur" subdirectories will not be used or shared, although they will still be created. Both SqWebMail and Courier-IMAP create their auxiliary files in the main Maildir with the group and world permissions turned off, so this maildir can, theoretically, still be used as the primary INBOX, but I don't recommend that.

The -S option is not limited to the system administrator. In fact, anyone can use the -S option, to create shared maildirs that they maintain.

Shared folders are created like any other folder, using the -f option to maildirmake. However, that normally creates a folder that is not sharable, because it will not have any group or world permissions. Therefore, maildirmake will take the following options to create a sharable folder:

• -s read will create a "moderated" folder. The folder will have group and world read permissions, letting anyone read messages, but only the owner of the sharable Maildir can add messages to the sharable folder.
• -s write will create an "unmoderated" folder. The folder will have group and world read/write permissions, letting anyone read and add messages to the folder. The folder will be created with the sticky bit set, like the /tmp directory, preventing people from removing someone else's messages.
• -s read,group/-s write,group append a comma and the word "group" to modify the semantics of moderated and unmoderated folders only on the group level, not the group and world level, as far as permissions go. This restricts sharing the folder only to members of the same system group as the owner of the sharable maildir.

It's worth noting that it is perfectly permissible for folders in the same sharable maildir to have different access levels.

Also, this is driven entirely by filesystem permissions, so theoretically it's possible to create a folder that has write permissions for the group, and read permissions for everyone else. However, I'm too lazy to actually do it. Feel free to patch maildirmake to add this functionality, then send me the patch.

Accessing shared folders

The rest of the document consists of technical implementation notes.

Accessing a collection of shared folders is implemented by a new file that is installed in the primary maildir (usually $HOME/Maildir), and a new subdirectory hierarchy underneath the primary maildir, which are hereby declared.

shared-maildirs

This file must be created by the administrator or by the maildir owner, manually. This file lists all available sharable maildirs that this maildir can access in shared mode (confused yet?). This file contains one or more lines of the following format:

name /path/to/shared/maildir

"name" is an arbitrary name that's given to this collection of shared folders. The name may not contain slashes, periods, or spaces. This is followed by a pathname to the maildir containing shared folders. Note that this is NOT the sharable folder itself, but the maildir that contains one or more sharable folders. The maildir client will be able to selectively subscribe to any sharable folder in that maildir.

shared-folders

This subdirectory forms the root of all the shared folders that are subscribed to. Let's say that shared-maildirs has the following contents:

firmwide /home/bigcheese/Maildir-shared tech /home/gearhead/Maildir-shared

Subscribing to folders 'golf' and 'boat' in bigcheese's shared Maildir, and to the folder 'maintenance' in gearhead's shared Maildir involves creating the following subdirectories: shared-folders/firmwide/.golf, shared-folders/firmwide/.boat, and shared-folders/tech/.maintenance.

shared

Subscribing to a shared folder

• Read shared-maildirs.
• Read the shared-folders hierarchy to determine which folders are already subscribed to.
• Read the folders in each maildir listed in shared-folders, and ignore the ones that are already subscribed to. Make sure to stat() each folder to make sure that you can read it.
• If necessary, create shared-folders/name, and create the shared-folders/name/shared soft link.
• Create shared-folders/name/.foldername, then create the standard tmp, new, and cur subdirectories. All the directories are created without any group or world permissions.

Unsubscribing

• Delete everything in shared-folders/name/foldername.
• If this is there are no more subscribed folders in this sharable maildir, delete shared-folders/name for good measure as well.

Opening a shared folder

The process of "opening" a shared folder involves basically "syncing" the contents of shared-folders/name/.foldername with the contents of the sharable folder in the original sharable maildir.

• Do the usual processing on the tmp subdirectory.
• Read the contents of shared-folders/name/foldername/shared/new. If you find anything in there, rename it to ../cur, ignoring any errors from the rename. The sharable maildir owner can arrange for mail to be delivered directly to this folder, and the first one to open it will put the message into cur.
• Read the contents of shared-folder/name/foldername/shared/cur. Call this directory "A", for short.
• Read the contents of shared-folder/name/foldername/cur. Call this directory "B", for short.
• stat() A and B to see if they live on different devices.
• Remove all messages in B that do not exist in A. You want to compare the filenames up to the : character.
• Create soft links from messages that exist in A and do not exist in B. The name in B should not have any flags after the :, so it shows up as a new message.
• You can now do whatever you normally do with a maildir. Note that new is completely ignored, though. Moving messages IN and OUT of the shared folder involves copying the message as if it were a new message. Copying the message INTO the shared folder means copying the message into the underlying sharable folder's tmp/cur directory, and it will show up after the next sync.