incomplete fs-update may leave laptop bootable

[Quozl]

We have already fixed zhashfs to write the zero block last, which works well with laptops that have had nothing on their internal storage.

However, if the internal storage already contains an identical partition map, it is not cleared, and so a partial fs-update can leave the laptop bootable to some extent.

Proposed fix, olpc-os-builder:bin/zhashfs to write a zeroed zero block first.

[Quozl]

Fix committed.

[Quozl]

Fix was not in the olpc-os-builder instance used to produce 21007o2.zd.

Test case: "head -2 file.zd | grep zblock: | cut -f2 -d' '" should yield 0.

[dsd]

Test in 12.1.0 build 8.

[greenfeld]

I interrupted an fs-update on a XO-1.75 with 12.1.0 os8 and the system defaulted to trying to boot off of /wlan only.

[Quozl]

And is that what we want?

[greenfeld]

That's a reasonable question (perhaps for another ticket):

Do we want to change OFW's boot path so it doesn't continue searching after the internal drive fails in this case?

We actually search for what appears to be four items if internal booting fails - "ext:\zimage /prober /usb/ethernet /wlan"