Re: [PATCH v2] sha1_name: fix uninitialized memory errors

On Tue, Feb 27, 2018 at 06:47:04AM -0500, Derrick Stolee wrote:

Peff made an excellent point about the nested if statements. This
goes back to Christian's original recommendation.

-- >8 --

During abbreviation checks, we navigate to the position within a
pack-index that an OID would be inserted and check surrounding OIDs
for the maximum matching prefix. This position may be beyond the
last position, because the given OID is lexicographically larger
than every OID in the pack. Then nth_packed_object_oid() does not
initialize "oid".

Use the return value of nth_packed_object_oid() to prevent these
errors.

Reported-by: Christian Couder <redacted>
Signed-off-by: Derrick Stolee <redacted>

Thanks, this looks good to me.

Semi-related, I wondered also at the weird asymmetry in the if-else,
which is:

  if ...
  else if ...
  if ...

but the comment directly above says: "we consider a maximum of three
objects nearby". I think it's actually two, because you can only trigger
one of the first two conditionals.

Is that right?

Let's imagine we're looking for object 1234abcd.  If we didn't find a
match, then we might have:

  1234abcc 
  1234abce <-- first points here

in which case we need to check both first-1 and first. And we do.

If we do have a match, then we might see:

  1234abcc
  1234abcd <-- first points here
  1234abce

and we must check first-1 and first+1, but _not_ first.

So I think the code is right, but the comment is wrong.

-Peff