[Dovecot] A view quirks

[Andreas Jaekel]

Aloha Timo.


Thanks for the other fixes.


> > If I copy from the current folder to the current folder I get
> > a scrambled mailbox and an error.  This might be selfmade, though.
>
>Yeah. It's in my TODO list too. Problem is that it uses the same file
>descriptor for both reading and writing and the seek offsets get mixed
>up. This could be fixed by always seeking to correct position, but
>that's a bit annoying since either you have to do it always (unnecessary
>nearly always) or add some extra kludgy checks. You could also be just
>reading with mmap() or pread() but I'm not sure if it's a good idea to
>rely on them either.


Well, always seeking on input and output works, (I tried)
except it breaks APPEND, which reads from a socket, which
fails seeking, which I have no clean way of knowing since
it's all hidden behind the stream wrappers.

I think a clean way may be to change the stream wrappers so
they remember which streams are dup()s of other streams.  Then,
on each read or write operation they could consider the 'original
stream' a kind of master record in order to store the current
cursor position there on every operation.  And check -before-
every operation wether the master record's pos is equal to
their own wanted position.  If not, a seek is needed.

Somewhat like:

if (stream->master_copy->current_pos != stream->current_pos)
         stream->seek(stream->current_pos);

After that the only problem would be to find the master record
when creating new streams, but that should be possible.

Another possible solution is to change output to always use
pwrite(), avoiding the need for seeks completly.



>Yes, the stream stuff is kind of ugly and difficult to follow. Maybe
>still has some bugs too.. Cleaning them up would be a good idea, but I'm
>not sure if I know how. I don't think the class system is too bad
>though, you can't do it much better with C.

Well, it's quite confusing sometimes, but it's also quite handy.  It took
me a while to figure out that mstreams make a difference between
mail header read() and mail body read(), though :)



> > BTW: what exactly is "max_pos"?
>
>You mean high_pos? It's .. um.. some kludge.. that I added to fix some
>problem.. :) I think it went:
>
>Normally "pos" contains the amount of data that has been read from the
>underlying stream. But when you temporarily set a read limit, it has to
>shrink "pos" as well or things break. So "high_pos" is there just to
>remember what the "pos" was before any read limits.
>
>It should work so that istream.c does every stream's buffering. The
>istream-*.c then just provide a way to fill the buffer and seek around
>in the stream.

Hmm... when I made my copy of the mbox backend I had to modify
my version of i_stream_create_mbox() to add a call to i_stream_seek()
after  i_stream_set_read_limit(),  or else mail_storage_save() would
break with unexpected EOF.  The reason was that high_pos was != 0, but
I don't remember the details anymore. :)