[Dovecot] flat file mailbox formats

[Timo Sirainen]

I've tried to write the new mbox code in a way that it could be moved
into generic flat-file library with mbox-specific code being implemented
on top of that.

The code would be useful for all formats that require modifying message
headers to store the metadata. How many such are there?

I suppose all such formats could be implemented by separating mails into
3 logical parts:
 - header ("From ...\n")
 - mail data (headers and body)
 - footer ("\n")

Except keeping a separate header and footer is kind of annoying, my
current code uses:
 - mail separator ("\nFrom ...\n")
 - first message header's skip counter: how many characters to remove
from mail separator for the first message (1, for \n before the "From
..")
 - last message's footer

Somewhat uglier, but I think all needed formats could be implemented
with it? It's also simper and more efficient to implement..

Besides mbox, I know at least two variations which are used:

AAAA-box:
 - I remember seeing this somewhere years ago, but I'm not sure if it's
still used or if it has a real name..
 - separator is four ^A characters (ascii 1) and LF
 - header skip counter is 5
 - last message footer would be the same ^A^A^A^A\n

dotbox:
 - mails are stored in SMTP format, ie. lines beginning with '.' are
prefixed with another '.'. the '.' prefixes will have to be removed for
IMAP, so this isn't as simple as others to implement.
 - separator is ".\n"
 - header skip counter is 2
 - last message footer is ".\n"

Any thoughts?

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