[PATCH 4 of 4] changelog: lazy decode user (API)

[Augie Fackler]

> On Mar 1, 2016, at 11:22, Matt Mackall <mpm at selenic.com> wrote:
> 
> On Sat, 2016-02-27 at 23:27 -0800, Gregory Szorc wrote:
>> # HG changeset patch
>> # User Gregory Szorc <gregory.szorc at gmail.com>
>> # Date 1456641258 28800
>> #      Sat Feb 27 22:34:18 2016 -0800
>> # Node ID ee98b780730118e8a8948396507633a0460c154e
>> # Parent  8427442ba08dd8dc324ea9e1fd30f65c89b2b753
>> changelog: lazy decode user (API)
>> 
>> This appears to show a similar speedup as the previous patch.
> 
> These two scare me (and are against our encoding conventions).
> 
> I like the idea of being lazy here and I've definitely seen the hit for this in
> profiles, but I worry that this will leak utf-8 data to users that are expecting
> local strings and we won't discover the problem until some end user runs it on a
> non-utf-8 system months down the road.
> 
> Because these sorts of encoding confusions are very hard to keep track of in a
> weakly-typed system, our rule has always been: limit the exposure of system to
> the secondary types as far as possible. Which is why ALL changelog
> encoding/decoding is handled today in just a couple functions in changelog.py
> and we mostly don't have to think about it.
> 
> If we want to go further down the lazy road, I can imagine shimming in a
> lightweight class (still in changelog.py) to replace the return tuple. It can be
> initialized with the raw changeset data and have accessor methods or members to
> unpack/decode the pieces. Then we'll still be lazy without subtly changing the
> types of the legacy API that we're still using all over the place.
> 
> Or, we can leave changeset.read() alone and add another entrypoint that contexts
> can use for lazy parsing.

I like this option (readraw() or something).

> 
> -- 
> Mathematics is the supreme nostalgia of our time.
> 
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