[krbdev.mit.edu #7695] krb5-1.11.3/1.10.6 - full resync may fail and still result in ulog being updated

[Richard Basch via RT]

Agreed... I'll fix my 1.11 branch.

But also fix the conditional dump, which is seriously broken (this was one
of the 3 GIT commits I supplied). I had a QA environment where I restored a
dump, but the old from_master had a later serial number than what was
present in the new ulog. It doesn't do a proper sanity check and thought the
from_master was ok to use.  I fixed the logic to actually search through the
ulog for the corresponding entry to determine if the dump header really does
match a ulog entry. I first perform a quick check if the latest dump matches
the latest ulog header (which dovetails with my hierarchical tree
propagation patches which I submitted earlier, but the check is agnostic as
to whether that patch is applied or not) -- this is the only optimization
which is "safe" and guaranteed. For instance, if a ulog is reinitialized,
the new ulog may not have any entries to match against, but the slaves
should not defer getting a database resync from the master until an update
is performed solely because of this condition (that was the theoretical
driver behind retaining the optimization). I did not confirm whether it
might potentially result in a second full resync after the first update, but
with or without conditional dumps, the issue already would have existed (so
I wasn't worried, and for this section of code, it seemed to be the logical
action and if the theoretical double-sync issue exists, the problem should
be fixed elsewhere). The only other optimization would possibly have been to
match against the first sno entry in the ulog header, since that might be
the only other condition where a dump is not required. The only reason I did
not do that initially is for busy sites, by the time you made the dump and
sent it, the other side may already need another one (it is a fringe case,
where accepting the first entry without a ulog history vs. not accepting it
both have potential consequences of requiring another full resync anyway; I
opted to simply let it do another dump if it wasn't in the actual ulog
history).