Shadow Database Usage in the Bugzilla::DB world

[Gervase Markham]

David Miller wrote:
> It seems that our usage of shadow databases go completely screwed over 
> by the switch to using DBI instead of the old SendSQL and friends. (This 
> is a 2.20 regression)
> 
> See https://bugzilla.mozilla.org/show_bug.cgi?id=313695
> 
> The short version is Bugzilla->switch_to_shadow_db() can't change a $dbh 
> variable that you already have in your possession.

Well, maybe I'm wrong but surely it _could_ - if we actually issued 
proxy objects which our DBI class kept track of, rather than real 
objects, and switch_to_shadow_db() walked the list changing them over. 
The proxy objects would be identical in interface to real $dbh objects, 
and just pass the calls through to their current $real_dbh.

That would rely on people not calling it when they were in the middle of 
a DB operation of some sort, which I guess is reasonable.

Gerv