Re: Optimizing writes to unchanged files during merges?

From: Elijah Newren <newren@gmail.com>
Date: Apr 13, 2018, 20:04 UTC
Message-ID: <CABPp-BFZZJ=rgOYqgsxD1A-Q5AU91e-HQ6WbAsGSqNppADXySQ@mail.gmail.com>
In-Reply-To: <CA+55aFwi9pTAJT_qtv=vHLgu=B1fdXBoD96i8Y5xnbS=zrfSzg@mail.gmail.com>

On Fri, Apr 13, 2018 at 10:14 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, Apr 13, 2018 at 12:02 AM, Elijah Newren <newren@gmail.com> wrote:

Show 9 quoted lines

>> However, it turns out we have this awesome function called
>> "was_tracked(const char *path)" that was intended for answering this
>> exact question.  So, assuming was_tracked() isn't buggy, the correct
>> patch for this problem would look like:
>
> Apparently that causes problems, for some odd reason.
>
> I like the notion of checking the index, but it's not clear that the
> index is reliable in the presence of renames either.

Yes, precisely. Checking the *current* index is not reliable in the presence of renames.

Trying to use the current index as a proxy for what was in the index before the merge started is a problem. But we had a copy of the index before the merge started; we just discarded it at the end of unpack_trees(). We could keep it around instead. That would also have the benefits of making the was_dirty() checks more accurate too, as using the mtime's in the current index as a proxy for what was in the original index has the potential for the same kinds of problems.

Show 13 quoted lines

>>   A big series
>> including that patch was merged to master two days ago, but
>> unfortunately that exact patch was the one that caused some
>> impressively awful fireworks[1].
>
> Yeah, so this code is fragile.
>
> How about we take a completely different approach? Instead of relying
> on fragile (but clever) tests, why not rely on stupid brute force?
>
> Yeah, yeah, it's bad to be stupid, but sometimes simple and stupid
> really does work.
>

<snip>

> Comments? Because considering the problems this code has had, maybe
> "stupid" really is the right approach...

It's certainly tempting as an interim solution. I have an alternative interim solution that I think explains well why the code here had been fragile, and how to just implement what we want to know rather than making approximations to it, which I just posted at [2]. But I can see the draw of just gutting the code and replacing with simple brute force. Long term, I think the correct solution is still Junio's suggested rewrite[1]. My alternative is slightly closer to that end-state, so I favor it over simple brute-force, but if others have strong preferences here, I can be happy with either.

Elijah

[1] https://public-inbox.org/git/xmqqd147kpdm.fsf@gitster.mtv.corp.google.com/ [2] https://public-inbox.org/git/20180413195607.18091-1-newren@gmail.com/