Holger Parplies wrote at about 03:29:00 +0200 on Wednesday, July 20, 2011:
 > Hi,
 > 
 > sorry for not replying earlier. In case you're still wondering (otherwise for
 > the archives) ...
 > 
 > Jeffrey J. Kosowsky wrote on 2011-02-07 14:15:05 -0500 [[BackupPC-users] 
 > *BUMP* *BUMP* Re: BackupPC perl code hacking question... (Craig any chance 
 > you might have a suggestion?)]:
 > > Let me rewrite my earlier posting to be more clear so maybe someone
 > > can help me.
 > 
 > Well, let's rearrange your message so it makes sense (did I ever mention that
 > I don't like top posting? ;-).
 > 
 > >  > Jeffrey J. Kosowsky wrote at about 12:53:28 -0500 on Monday, December 
 > > 13, 2010:
 > >  >  > For reasons I can explain later, I am trying to set
 > >  >  > $Conf{RsyncdPasswd} in the main routine of BackupPC_dump (I am
 > >  >  > actually trying to do something a bit more complex but this is easier
 > >  >  > to understand).
 > >  >  > 
 > >  >  > Now since %Conf = $bpc->Conf(),
 > 
 > You are aware that this is a hash copy operation, right?
 > 
 > >  >  > I would have thought that for example
 > >  >  > setting $Conf{RsyncPasswd} = "mypasswd" would then be pushed down to
 > >  >  > all the routines called directly or indirectly from BackupPC_dump.
 > 
 > Well, it is as long as they use the "my %Conf" from BackupPC_dump and not
 > $bpc->Conf(). You modified the copy, not the original hash in the
 > BackupPC::Lib object.
 > 
 > >  >  > However, in Rsync.pm where the value of $Conf{RsyncPasswd} is 
 > > actually
 > >  >  > used, the value remains at ''.
 > 
 > Yes, because Rsync.pm gets a reference to the unmodified BackupPC::Lib 
 > object's
 > hash (BackupPC::Xfer::Protocol, line 59, sub new, "conf => $bpc->{Conf}").
 > 
 > >  >  > (Of course setting the paramter the "normal" way within a config file
 > >  >  > works and shows up as set in Rsync.pm)
 > 
 > That is because the code in BackupPC::Lib that reads the config file saves 
 > the
 > values.
 > 
 > >  >  > I'm sure I must be missing something about how perl inherits and/or
 > >  >  > overwrites variables... but I am stumped here...
 > 
 > It's really simple. If you get a reference to a hash ($conf = \%conf), then
 > you modify the original, if you get a copy (%conf = %conf_orig), you don't.
 > What makes things complicated here is that you need to follow the code around
 > through various modules and subs, and that each copy of %conf is named the
 > same :-).
 > 
 > > Here is a simplified version of my actual command
 > > 
 > > $Conf{DumpPreUserCmd} = "&{sub {\$args[1]{RsyncdPasswd} = `ssh
 > > -x mybackuypclient get_rsyncd_secret`}}";
 > > 
 > > This uses the fact that $args[1]=$Conf
 > 
 > Actually, it's \%Conf (a reference, not a copy), so modifying 
 > *BackupPC_dump's
 > copy* works. $Conf would be a scalar, which might coincidentally contain a
 > reference to a hash. Using an element of a hash is "$Conf{Foo}", using an
 > element of a hash a scalar is pointing to is "$Conf->{Foo}". You might even
 > have both visible at the same time, but only if you are either a bad
 > programmer or enjoy confusing people.
 > 
 >      my $Conf = $bpc->Conf();
 >      my %Conf = (XferMethod => "snailmail");
 >      print $Conf{XferMethod}, "\n";      # probably prints "rsync"
 >      print $Conf->{XferMethod}, "\n";    # prints "snailmail"
 > 
 > > So, that \$args[1]{RsyncdPasswd} is equivalent to
 > 
 > ... a syntax error? ;-)
 > 
 > $args [1] should be a reference to BackupPC_dump's %Conf, and 
 > $args [1] -> {RsyncdPasswd} should reference the corresponding entry (as an
 > lvalue, so you can assign to it). "->" between braces "[]"/"{}" is implied 
 > and
 > may be left out. I'm not sure how the reference operator binds in your 
 > example.
 > As it seems to work, let's ignore it for now.
 > 
 > > $Conf{RsyncdPasswd}.
 > 
 > ... in BackupPC_dump (because that is what was passed in).
 > 
 > > [...]
 > > So, my question is is there any way to dynamically set Conf parameters
 > > along the lines I am trying to do?
 > 
 > Well, you'd have to modify the original hash.
 > 
 >      $bpc->{Conf}->{RsyncdPasswd} = `ssh ...`;
 > 
 > That is not strictly legal, but it should work. Note that this change would
 > *not* propagate to any copies previously made (like the "my %Conf" in
 > BackupPC_dump). Using references to $bpc->{Conf} everywhere instead of copies
 > would probably make things much easier, but on the downside, it would mean 
 > you
 > can't make local modifications to $Conf->{...} that are not meant to 
 > propagate
 > to the rest of the code (or, put differently, you could more easily
 > accidentally clobber BackupPC::Lib's state). You'll notice that
 > BackupPC::Xfer::Protocol (as quoted above) actually *does* use a reference.
 > 
 > Hope that helps.

Hmmmm... very helpful...
Got it to work after correcting a pair of bugs in Lib.pm

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