[Xen-devel] pygrub fails to read single partition with grub bootsector in 4.1.0-rc

[M A Young <m.a.young@xxxxxxxxxxxx>]

I have a single partition (actually under lvm) which starts with a grub 
boot sector. pygrub in 4.0.1 coped with this successfully, but under 
4.1.0-rc2 I get the error
Traceback (most recent call last):
  File "pygrub.orig", line 773, in <module>
    if not fs:
NameError: name 'fs' is not defined

The problem is when you install grub on a partition (it can be on the MBR 
or on the boot sector of a partition), it installs an MBR-like boot 
sector, in particular ending with 0xaa55 in bytes 510 and 511.
In 4.1.0 pygrub sees this and decides in get_partition_offsets() that it 
is looking at an MBR, however when it checks the offsets it finds they are 
all zero so returns an empty list of offsets to try, resulting in the 
error above (in 4.0.1 the default is to return the offset in the first 
partition, which is 0 so it worked). The attached patch aims to detect 
this situation an return an offset of zero in this case, though perhaps it 
makes sense to default to an offset of 0 rather than a blank list if no 
appropriate offsets are detected.
        Micahel Young

pygrubfix.patch
Description: Text document

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