Questions about delta encoding implementation and virtual files

Hi!, 

After digging in plugin building documentation and checking the provided
examples, I have some doubts I have not been able to clarify by my own.
I describe them below in case you could give me a hand :). I would be
very thankful if you could help me a little clarifying this doubts :) 

- I'm working in trying to create an open source version of the delta
encoding plugin by using the bacula-fd plugin api. When working on it I
have seen Bacula's source is aware of delta and delta file
sequentiation. I have seen for instance, even a .bvfs command exists for
showing deltas of a file id. But, what  I have not found is that Bacula
works on that Delta files generation (patch generation, signatures,
etc...). I assume that Bacula in the non-fd part, acts just as a just
delta file holder keeping the files and stores the patch sequentiation
just that. Bacula keeps records of deltas in database (and file
storages) but only fd works with them (with probably a library like
librsync in the delta plugin) in the sense of applying patches over an
original file or even generating deltas when backup. Am I wrong?. Was
just for understanding the nice work done and what's already written and
free in Bacula's source for this purpose. 

- By the way, I have one question about virtual files. I have not seen
very clear (perhaps my problem as don't understand it) how to work with
them. I understand the concept, but have not seen a clear example of how
for instance in the backup you create a virtual file, how do you see it
in bvfs and finally... what you get after restoring. In page 36/146 of
Bacula 11 for developers pdf, you say "This will create a virtual file."
but really you are entering in the structure : 

sp->type = FT_REG;
sp->statp.st_mode = 0700 | S_IFREG; 

FT_REG and S_IFREG both are for regular files.... what exactly causes a
virtual file to be created?. Perhaps st_size -1?. 

Are they relevant for what I'm trying to do?. It seems Bacula handles
delta sequentiation so... perhaps for this purpose I shouldn't need
"virtual files"?. 

- I'm planning to implement delta encoding by checking the previous day
file signature done by librsync. Instead of looking at the filesystem it
would be nice if I could take a look at that signature in the last
backup done (yesterday backup). Could it be possible in some manner,
that if I see a file passed in EventHandleBackupFile() to check if
yesterdays signature exists in the backup of yesterday, and then read
the yesterday signature from the own backup?. I mean, instead of having
to leave the signature in the being backed server's filesystem. 

- The last one :) . For restoring, and for the code seen (for instance
in insert_missing_delta()) I assume Bacula detects we are restoring a
delta compressed file. Then I assume Bacula restores apart from the own
initial file, patches to arrive to the day we want to restore to. Am I
wrong?. Perhaps later in a post-restore job I could run a shell script
that tries to find patches pending to be applied to a parent file. I
suppose then I could apply and the backup would become finally restored.
Does some other more elegant way you could advise me?. 

Thank you so much really, 

Best regards,

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