Re: [PATCH v4 4/6] patch-id: make it stable against hunk reordering

On Wed, Apr 23, 2014 at 03:05:42PM -0700, Junio C Hamano wrote:

"Michael S. Tsirkin" [off-list ref] writes:

quoted

On Wed, Apr 23, 2014 at 10:39:23AM -0700, Junio C Hamano wrote:

quoted

Are these three patches the same as what has been queued on
mt/patch-id-stable topic and cooking in 'next' for a few weeks?

Not exactly - at your request I implemented git config
options to control patch id behaviour.
Documentation and tests updated to match.

After comparing the patches 4-6 and the one that has been in 'next'
for a few weeks, I tried to like the new one, but I couldn't.

I'm fine with the one in next too.
I was under the impression that you wanted me to change
the behaviour so I worked on this, but previous version was sufficient
for my purposes (which is really all about putting diff.orderfile
upstream).

The new one, if I am reading the patch correctly, makes the stable
variant the default if

 - the configuration explicitly asks to use it; or

 - diff.orderfile, which is a new configuration that did not exist,
   is used.

At the first glance, the latter makes it look as if that will not
hurt any existing users/repositories, but the thing is, the producer
of the patches is different from the consumer of the patches.  There
needs to be a way for a consumer to say "I need stable variant" on
the patches when computing "patch-id" on a patch that arrived.  As
long as two different producers use two different orders, the
consumer of the patches from these two sources is forced to use the
stable variant if some sort of cache is kept keyed with the
patch-ids.

But "diff.orderfile" configuration is all and only about the
producer, and does not help the case at all.

OK, we can just drop that, that's easy.

Compared to it, the previous version forced people who do not have a
particular reason to choose between the variants to use the new
"stable" variant, which was a lot simpler solution.

The reason why I merged the previous one to 'next' was because I
wanted to see if the behaviour change is as grave as I thought, or I
was just being unnecessary cautious.  If there is no third-party
script that precomputes something and stores them under these hashes
that breaks with this change, I do not see any reason not to make
the new "stable" one the default.

Ah okay, so we just wait a bit and see and if all is quiet the
existing patch will graduate to master with time?
Totally cool.
I thought you wanted me to add the config option, but if everyone
is happy as is, I don't need it personally at all.

I however suspect that the ideal "stable" implementation may be
different from what you implemented.  What if we compute a patch-id
on a reordered patch as if the files came in the "usual" order?

ATM patch id does not have any concept of the usual order,
so that's one problem - how does one figure out what the order would be?
I have no idea - is this documented anywhere?
Also I'm guessing this would depend on the state of the git tree which
would be another behaviour change: previously patch-id worked
fine outside any git tree.

That would be another way to achieve the stable-ness for sane cases
(i.e. no funny "you could split one patch with two hunks into two
patches with one hunk each twice mentioning the same path" which is
totally an uninteresting use case---diff.orderfile would not even
produce such a patch)

Yes I'm thinking we should drop this hunk in the patch:
let's support reordering files but not splitting them.
This makes the change even smaller, so I now think we should
just go for it.

and a huge difference would be that it would
produce the same patch-id as existing versions of Git, if the patch
is not reordered.  Is that asking for a moon (I admit that I haven't
looked at the code involved too deeply)?

Yes this would be a bunch of code to write - certainly much more complex than
the existing small patch which just tweaks the checksum slightly to make
it stable.

-- 
MST