[RFC/PATCH] commit-graph: verify swapped zero/non-zero generation cases

In verify_one_commit_graph(), we have code that complains when a commit is found with a generation number of zero, and then later with a non-zero number. It works like this:

  1. When we see an entry with generation zero, we set the
     generation_zero flag to GENERATION_ZERO_EXISTS.

  2. When we later see an entry with a non-zero generation, we complain
     if the flag is GENERATION_ZERO_EXISTS.

There's a matching GENERATION_NUMBER_EXISTS value, which in theory would be used to find the case that we see the entries in the opposite order:

  1. When we see an entry with a non-zero generation, we set the
     generation_zero flag to GENERATION_NUMBER_EXISTS.

  2. When we later see an entry with a zero generation, we complain if
     the flag is GENERATION_NUMBER_EXISTS.

But that doesn't work; step 2 is implemented, but there is no step 1. We never use NUMBER_EXISTS at all, and Coverity rightly complains that step 2 is dead code.

We can fix that by implementing that step 1.

Signed-off-by: Jeff King <peff@peff.net>
---
This is marked as RFC because I'm still confused about a lot of things.
For one, my explanation above about what the code is doing is mostly a
guess. It _looks_ to me like that's what the existing check is trying to
do. But if so, then why is the generation_zero flag defined outside the
loop over each object? I'd think it would be a per-object thing.

Likewise, just below this code, we check:

                if (generation_zero == GENERATION_ZERO_EXISTS)
                        continue;

Is the intent here "if this is the zero-th generation, we can skip the rest of the loop because there are no more parents to look at"? If so, then would it make more sense to check commit_graph_generation() directly? I took care to preserve the existing behavior by pushing the set of NUMBER_EXISTS into an "else", but it seems like a weird use of the flag to me.

So I kind of wonder if there's something I'm not getting here. Coverity is definitely right that our "step 2" is dead code (because we never set NUMBER_EXISTS). But I'm not sure if we should be deleting it, or trying to fix an underlying bug.

 commit-graph.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)