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From Julian Foad <julianf...@gmail.com>
Subject Re: svnsync crash on empty repo
Date Sun, 06 Sep 2015 08:48:45 GMT
Bert Huijben wrote:
> Julian Foad wrote:
>> Bert wrote:
>>> I don't think we should fix this with a 'revision+1' to explicitly allow
>>> many bad ranges,
>>
>> Why?
>>
>> An empty range isn't inherently "bad". Allowing an empty range isn't
>> bad. Being able to specify an empty range in many different ways isn't
>> bad. Changing a released API to make a previously no-op case now throw
>> an assertion failure breaks our compatibility guarantees. That is bad.
>
> An empty range isn't bad, but your patch appears to also explicitly allow undefined ranges,
if I read it correctly.
>
> I would call A range of r1 upto r0 defined (empty), but I would call a range or r4 upto
r3 with current HEAD r3 undefined.
>
> We use r1 and r0 as special in quite a few places where we really just want to point
at the initial revision,

I am totally against the idea of treating r1 as a special value.

I didn't think we used r1 as special anywhere. Can you point to any of
those places, please?

(There was a case added to the mergeinfo parsing a few years ago that
treated r1 as special -- treating "change r1" as an invalid merge
source -- and I removed that special case because it was illogical.
That is the only case I recall seeing.)

> I see no problem with that but I don't want to introduce a definition of 'HEAD+1' as
there is no we can declare stable behavior on that... It may even change partway through an
operation, the moment that revision is committed.

I am not proposing that "HEAD+1" is a special value. I propose that
"r4 up to r3" specifies an empty range because there are no integers R
where (R >= 4 and R <= 3). It has nothing to do with the value of
HEAD, so doesn't matter if HEAD changes.

Logically a range of revisions (in the sense of *changes*) is defined
by a starting state (aka snapshot) and an ending state. When we want
the 'replay_range' API to replay the single change between (state) r2
and (state) r3, the API is defined in such a way that we have to pass
(starting_state + 1, ending_state), that is (r3, r3).

Now consider if we request to replay an empty range. To replay the
empty range between (state) r3 and (state) r3, we have to pass in
parameters (r3+1, r3). That doesn't mean we're actually going to
dereference the state (r3+1). If HEAD is r3, dereferencing r4 would be
an error. But the API only takes this input (r4) in order to refer to
a starting state r3, because that's how the API was defined. It
doesn't dereference r4, so no problem. Like how a pointer to the end
of a memory block in C is defined as valid even if the byte it points
to is outside the allocated block, as long as we don't dereference it.

The only place where the value of HEAD matters is when you want to
check that the revision range we specified is no higher than HEAD.
Think of it logically. If we specified an empty range using (R+1, R)
then we have not specified a range that goes higher than HEAD even if
(R+1) == (HEAD+1). Therefore, HEAD+1 is acceptable as the "start"
parameter *iff* the "end" parameter is is less than "start".


> (It would be nicer if we used SVN_INVALID_REVNUM in more cases as the magic MIN value,
but at least ra_svn will assert if we would try to do that, as it doesn't allow transferring
r-1 as a normal serialized revision. So fixing that requires a timemachine)
>
> Svnrdump had a bug where it actually passed HEAD+1 as base revision of a commit in a
specific case... It is better to catch these errors on the client than to rely on not having
concurrent commits to define behavior of our tools.

I'm not suggesting to rely on not having concurrent commits!

BTW what's your opinion on maintaining backward compatibility of this
API? One option, if you want to disallow specifying an empty range, is
to create a new API and keep the old one as it was.

- Julian

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