[p2-dev] Pack200 and P2 rules headaches

[Thomas Hallgren <thomas@xxxxxxx>]

Pack200 has one fairly serious quirk that might have an effect on how we 
want to set up rules in P2. Here's an excerpt from the docs:

"In order to maintain backward compatibility, if the input JAR-files are 
solely comprised of 1.5 (or  lesser) classfiles, a 1.5 compatible pack 
file is  produced.  Otherwise a 1.6 compatible pack200 file is produced."

This perhaps looks simple and clear, but there is one implication that 
isn't mentioned and that is what happens when there are zero classfiles 
in the jar. Zero classfiles means that Pack200 see no reason to be 
backward compatible with 1.5. No big deal right? Just refrain from 
packing such files. But no, we need to look deeper. Most jars that are 
nested, contains zero class files in the top jar. And that's where it 
has implications on P2.

Consider the following:
A.jar contains B.jar and C.jar. Both B.jar and C.jar contain class files 
but A.jar does not. With our current approach, this leads to an A.jar 
that, if packed by a Java 6 JRE, only can be unpacked by a Java 6 JRE. 
The fact that all .class files are Java 1.5 compatible doesn't help. So 
what can we do?

Let's assume that we only pack jars that contain .class files. This 
would mean that we get A.jar that now contains B.jar.pack.gz and 
C.jar.pack.gz. This doesn't work very well, since P2 now only sees the 
A.jar. It doesn't know that it contains nested jars that needs to be 
unpacked. We cannot call the jar A.jar.pack.gz either since P2 then 
assumes that it's packed. What we end up with is a container with packed 
material that cannot itself be packed. How do we solve that?

A workaround for the problem would be to add a dummy 1.5 compliant 
.class file when this situation occurs. The unpacker could remove it so 
it would never become visible.

Thoughts?

- thomas