[PATCH 4 of 4 NEW-CONCEPT] track-tags: write all tag changes to a file

[Jun Wu]

Excerpts from Pierre-Yves David's message of 2017-04-06 01:09:47 +0200:
> 
> On 04/05/2017 10:54 PM, Jun Wu wrote:
> > I don't think "writing things that hook *may* need to filesystem" is a good
> > approach. It introduces unnecessary overhead if the hook does not need that
> > information.
> 
> I do not find the overhead concerning.

This is from your commit message:

  This code is disabled by default as there is still various performance
  concerns.  Some require a smarter use of our existing tag caches and some
  other require rework around the transaction logic to skip execution when
  unneeded.  These performance improvement have been delayed, I would like
  to be able to experiment and stabilize the feature behavior first.

> The thing to keep in mind is that if they are many things to write down, 
> this also means many thing changed during the transaction. So the 
> overhead is likely minimal. In the tags cases, just updating the various 
> tags related cache is going to be much more expensive that writing this 
> to disk.
> 
> > I think a better way is to have per-hook config to let hooks explicitly
> > declare what they need, before we run them.
> 
> I afraid it is too fragile. Forgetting to configure the hook properly 
> will be easy and the error will likely pass silently as hook will run 
> normally with the same data as if the transaction touched no tags.

Do you mean that a global config option is less fragile?

> 
> Cheers,
>