UBUNTU: [Debian] postinst -- improve relative symlink detection with missing files

When the symlinks are made we attempt to use relative links if that
would work.  However this relies on the file we are making the link to to
actually exist.  When it does not we fall back to absolute.  This impacts
the initrd links which are made before we make the initrd itself.

When the caller has asked us to use a specific handle file and that file
does not yet exist, see if there are any other files we can use in that
directory.  In the common case this will be a version specific file and
highly unique.

BugLink: http://bugs.launchpad.net/bugs/1248053Signed-off-by: Andy Whitcroft <apw@canonical.com>

debian/control-scripts/postinst

@@ -217,6 +217,21 @@ sub test_relative {
   die "Internal Error: No such dir $params{'New Dir'} "
     unless -d $params{'New Dir'};
 
+  # If the caller specified a test file, and it does not exist try and find
+  # another file to use in its stead.  As we are installing kernels we can
+  # use any versioned file from this kernel which should be in the same place.
+  if (defined $params{'Test File'} and
+      ! -f $params{'Old Dir'} . '/' . $params{'Test File'}) {
+
+      my @possible = glob($params{'Old Dir'} . "/*$version*");
+      if ($#possible >= 0) {
+        $params{'Test File'} = `basename "$possible[0]"`;
+        chomp($params{'Test File'});
+        warn "Test relative: selected $params{'Test File'}"
+          if $DEBUG;
+      }
+  }
+
   warn "Test relative: testing $params{'Old Dir'} -> $params{'New Dir'}"
     if $DEBUG;
   chomp($cwd = `pwd`);