Re: [PATCH] commit-slab: clarify slabname##_peek()'s return value

[Jeff King <peff@peff.net>]

On Tue, Mar 10, 2020 at 04:30:49PM +0100, SZEDER Gábor wrote:

> Ever since 862e730ec1 (commit-slab: introduce slabname##_peek()
> function, 2015-05-14) the slabname##_peek() function is documented as:
> 
>   This function is similar to indegree_at(), but it will return NULL
>   until a call to indegree_at() was made for the commit.
> 
> This, however, is usually not the case.  If indegree_at() allocates
> memory, then it will do so not only for the single commit it got as
> parameter, but it will allocate a whole new, ~512kB slab.  Later on,
> if any other commit's 'index' field happens to point into an already
> allocated slab, then indegree_peek() for such a commit will return a
> valid non-NULL pointer, pointing to a zero-initialized location in the
> slab, even if no indegree_at() call has been made for that commit yet.
> 
> Update slabname##_peek()'s documentation to clarify this.

Yeah, I agree the existing documentation is misleading. Your update
looks good to me.

I thought at first we might simply be able to say:

  This function is similar to indegree_at(), but it will avoid
  allocating new slab memory (so its result is suitable only for
  reading, not writing).

But I think it's worth mentioning that the caller needs to handle both
NULL or a possible zero-initialized value, as your patch does.

I also wondered if we could make life easier for the caller by
collapsing these cases. I.e., always returning a zero-initialized value,
and never NULL. All of the callers do something like:

  struct blame_origin *get_blame_suspects(struct commit *commit)
  {                               
          struct blame_origin **result;
                  
          result = blame_suspects_peek(&blame_suspects, commit);
  
          return result ? *result : NULL;
  }

all of which could be turned into a single blame_suspects_peek() call if
it just consistently returned a zero-initialized value (it's a little
confusing in this example because we're storing pointers, so the
zero-initialized value is _also_ NULL, but it's a different type).

But that would get a bit awkward, because peek() returns a pointer, not
a value (as it should, because the type we're storing may be a compound
type, which we generally avoid passing or returning by value).  So we'd
actually need to return a pointer to a zero-initialized dummy value. Not
impossible, but getting a bit odd.

-Peff