About the Inter Process Communication Protocol
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JWebmail needs to read a users mail box in maildir format.
JWebmail runs under its own user commonly called jwebmail.
The mailbox has rights of its own user or there is a special user for all
mailboxes under a domain. To gain access there is a setuid program called
qmail-authuser that checks acces rights and starts another program.
This program is called `extractor`.

Version 1
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In this version all arguments where passed by command line arguments and
json was returned.
Binary data was sent after a json header and a newline.

Version 2
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Here protobuf is used for message interchange.

A process in spawned for every command.
The method is given as a process argument and the arguments are a single
message on stdin which is close afterwards. The response is a single protobuf
message on stdout after which the process exits.

Version 3
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In this version one process is spawned for every jwebmail request.
This is done by switching to varlink a json based rpc protocol.
A small disadvantage is that all binary data needs to be base64 encoded
and passed inside of a json string.