broken references in jar manifests

[Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>]

Hi Guix,

we have a problem with jar manifests.  When we use the Class-Path
property to ensure that an executable can find its dependencies on the
class path at run-time, we end up with a manifest like this:

--8<---------------cut here---------------start------------->8---
Manifest-Version: 1.0
Class-Path: /gnu/store/i28vi94r8z9f0x02zgkrv87w16ibmqkw-java-htsjdk-2.
 10.1/share/java/htsjdk.jar
Created-By: 1.8.0_151 (Oracle Corporation)
Main-Class: picard.cmdline.PicardCommandLine
--8<---------------cut here---------------end--------------->8---

Note that the Class-Path property is broken into two lines.  This means
that the reference scanner will miss it and grafting will fail.

Breaking up lines longer than 70 characters is according to the manifest
specification, see
https://docs.oracle.com/javase/7/docs/technotes/guides/jar/jar.html

We cannot just use a wrapper that sets CLASSPATH, because it appears to
be ignored when using a manifest with “Main-Class”.  “Main-Class” is
required for executables that can be run like this:

    java -jar whatever.jar

A possible work-around might be to patch the class loader of our JDKs to
look at a custom Guix-specific file, which we will include in each jar.
That file would be allowed to have longer lines.

There are two disadvantages:

1) we need to patch the JDK
2) the jars would not do the right thing when executed with a different
JDK (e.g. on a foreign distro).

What do you think?

--
Ricardo