How image installs work [was Re: The image question]

[Jordan K. Hubbard]

On Mar 12, 2007, at 1:47 PM, Ryan Schmidt wrote:

> Seems to me that it would be ideal if I could have version A of a  
> library installed, then install a whole bunch of software that  
> depends on that library. Then install a new version B of the  
> library which breaks backward compatibility. Keep both versions of  
> the library installed at the same time. Attempting to uninstall  
> version A would show you which other packages still depend on  
> version A of the library, so that you can rebuild those packages  
> against version B. Once you've done all that, version A can be  
> uninstalled.

As a number of people have pointed out, this does not absolutely  
require image mode in order to work since you can always just do name- 
munging on all the different B, C, D, ... versions of that library  
(the dylibs already version, but let's assume that this is a complex  
library and also has headers/helpers that need to be munged as well  
in order to achieve peaceful multi-version co-existence).  It simply  
makes it a bit more elegant since name munging comes for free as a  
result of partitioning the namespace by version string in the depot.

The hard part comes when you decide to upgrade all the things which  
depend on A.  Sure, it's easy enough to programmatically determine  
how many things depend on A and how many depend on B, but rolling  
back all the dependencies on A and rebuilding them in hopes that  
they'll still build AND will properly latch on to B, there's just no  
guarantee that this is going to work when so many parts of the system  
are moving independently.   In the case where you have several dozen  
dependents, chances are excellent that the automation is going to  
succeed in rebuilding about half of them (remember, at any given  
time, at least 50% of MacPorts actually fail to build right now) and  
the other half will be in some hosed, uninstalled but not  
successfully reinstalled, state.

If you further assume that people are relying on this software, it's  
pretty depressing.   It's sort of like saying, as an OS vendor, that  
any given software update MIGHT render your system completely  
inoperative, but no more than 50% of the time.  Go ahead, apply it!   
You have a 50% chance of coming through it completely unharmed, after  
all, so why are you being such a wuss!  :-)

- Jordan
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