repair: AGFL rebuild fails if btree split required

repair: AGFL rebuild fails if btree split required

In phase 5 we rebuild the freelist btrees, the AGF and the AGFL from
all the free space information we have gathered and resolved during
pahses 3 and 4. If the freespace information is laid out just right,
we end up having to allocate free space for the AGFL blocks.

If the size of the free space we allocate from is larger than the
space we need, then we have to insert the remainder back into the
freespace btree. For the by-size tree, this means we are likely to
be removing a record from one leaf, and then inserting the remainder
- a smaller size - into another leaf.

The issue is that the leaf blocks to the left of the original leaf
block we removed the extent record from are full and hence require a
split to insert the new record. That, of course, requires a free
block in the AGFL to allocate from, and now we have a chicken and
egg situation: there are no free blocks in the AGFL because we are
setting it up.

As a result, setting up the free list silently fails, leaving the
freespace btrees in an inconsistent state and the AGFL in question
empty. When the filesystem is next mounted, the first allocation
from that AGF results in attempting to fix the AGFL, and it then
does exactly the same thing as the repair code, fails to allocate a
block during the split and fails. This results in an immediate
shutdown because the transaction doing the allocation is dirty by
this stage.

The fix for the problem is to make repair handle rebulding the btree
differently. If we leave ispace for a couple of records in each
btree leaf and node, there is never a need for a split to occur when
initially setting up the AGFL. This results in repair doing the
right thing, and hence the runtime problems after mount don't occur.
Further, add error checking the the AGFL setup code and abort repair
if we have a failure to correctly set up the AGFL so we catch this
problem at repair time, not mount time...

Reported-by: Barkley Vowk <bvowk@box.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>