Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

From: Christian Couder <chriscool@tuxfamily.org>
Date: Aug 30, 2013, 05:41 UTC
Message-ID: <20130830.074124.1031903859178992049.chriscool@tuxfamily.org>
In-Reply-To: <xmqqli3k7abw.fsf@gitster.dls.corp.google.com>

From: Junio C Hamano <gitster@pobox.com>

Show 29 quoted lines

>
> Christian Couder <chriscool@tuxfamily.org> writes:
> 
>> But if all the objects that point to an object, called O, are to be
>> replaced, then in most cases object O probably doesn't need to be
>> replaced. It's probably sufficient to create the new object, called
>> O2, that would replace object O and to replace all the objects
>> pointing to object O with objects pointing to O2.
> 
> Hmmmm.
> 
> What the above says, with "probably" and "most cases", can easily
> inferred by anybody remotely intelligent, and the only reason to
> have something like the above paragraph would be if it assures that
> the statement holds without these qualifications to weaken it, which
> it doesn't.  I am not sure this paragraph adds much value.
> 
>> The only case where someone might really want to replace object 0,
>> with an object O2 of a different type, and all the objects pointing to
>> it, is if it's really important, perhaps for external reasons, to have
>> object O's SHA1 point to O2.
> 
> The same comment applies here.
> 
>> And anyway, if one really wants to do that, it can still be done using
>> "git update-ref".
> 
> And I really do not think this sentence is right---you can justify
> with the same sentence to remove "git replace" wrapper.

Ok, so the commit message (excluding the Acked-by and Signed-off-by) will be only:

------------------------------------------

Users replacing an object with one of a different type were not prevented to do so, even if it was obvious, and stated in the doc, that bad things would result from doing that.

To avoid mistakes, it is better to just forbid that though.

If one object is replaced with one of a different type, the only way to keep the history valid is to also replace all the other objects that point to the replaced object. That's because:

* Annotated tags contain the type of the tagged object.

* The tree/parent lines in commits must be a tree and commits, resp.

* The object types referred to by trees are specified in the 'mode'
  field:
    100644 and 100755    blob
    160000               commit
    040000               tree
  (these are the only valid modes)

* Blobs don't point at anything.

The doc will be updated in a later patch.

------------------------------------------

> The earlier suggestion to bypass the new hand-holding with "--force"
> is more sensible, I think.

There is already a --force option, but I can add a --force-type in a another patch.

Thanks, Christian.