On 06/08/2015 06:43 PM, Stefan Beller wrote:
> On Mon, Jun 8, 2015 at 4:45 AM, Michael Haggerty <mhagger@xxxxxxxxxxxx> wrote:
>> [...]
>> +/*
>> + * Delete the specified reference. If old_sha1 is non-NULL and not
>> + * NULL_SHA1, then verify that the current value of the reference is
>> + * old_sha1 before deleting it.
>
> And here I wondered what the distinction between NULL and non-NULL,
> but NULL_SHA1
> is and digging into the code, there is none. (As you can also see in
> this patch above with
> (old_sha1 && !is_null_sha1(old_sha1)) ? old_sha1 : NULL,
> but when digging deeper, the !is_null_sha1(old_sha1) is an arbitrary
> limitation (i.e.
> ref_transaction_delete and ref_transaction_update don't differ between
> those two cases.)
>
> The distinction comes in at lock_ref_sha1_basic, where I think we may
> want to get rid of
> the is_null_sha1 check and depend on NULL/non-NULL as the difference
> for valid and invalid
> sha1's?
I'm having a little trouble understanding your comment.
The convention for ref_transaction_update() and friends is that
* old_sha1 == NULL
We don't care whether the reference existed prior to the
update, nor what its value was.
* *old_sha1 is NULL_SHA1
(by which I mean that old_sha1 points at 20 zero bytes; I hope
that's clear even though NULL_SHA1 is not actually defined
anywhere): The reference must *not* have existed prior to the
update.
* old_sha1 has some other value
The reference must have had that value prior to the update.
lock_ref_sha1_basic() distinguishes between { NULL vs. NULL_SHA1 vs.
other values } in the same way that ref_transaction_update() does.
The delete_ref() function doesn't follow the same convention. It treats
NULL and NULL_SHA1 identically, as "don't care".
It probably makes sense to change delete_ref() use the same convention
as ref_transaction_update(), but there are quite a few callers and I
didn't have the energy to review them all as part of this patch series.
So I left it unchanged and just documented the status quo better.
> That said, do we want to add another sentence to the doc here saying
> non-NULL and not
> NULL_SHA1 are treated the same or is it clear enough?
> With or without this question addressed:
> Reviewed-by: Stefan Beller <sbeller@xxxxxxxxxx>
In set notation,
"non-NULL" =
"non-NULL and not NULL_SHA1" ∪
"non-NULL and equal to NULL_SHA1"
The latter two are *not* treated the same, so I don't see how we can
claim that "non-NULL" and "not NULL_SHA1" are treated the same. I must
be misunderstanding you.
Would it help if I changed the comment to
Delete the specified reference. If old_sha1 is non-NULL and not
NULL_SHA1, then verify that the current value of the reference is
old_sha1 before deleting it. If old_sha1 is NULL or NULL_SHA1,
delete the reference it it exists, regardless of its old value.
?
Michael
--
Michael Haggerty
mhagger@xxxxxxxxxxxx
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