[PATCH 2 of 2 V2] revset: skip old style lookup if external whitespace are detected

[Feld Boris]

On 13/02/2018 12:47, Yuya Nishihara wrote:
> On Mon, 12 Feb 2018 18:00:52 +0100, Boris Feld wrote:
>> # HG changeset patch
>> # User Boris Feld <boris.feld at octobus.net>
>> # Date 1518448909 -3600
>> #      Mon Feb 12 16:21:49 2018 +0100
>> # Node ID b0f45e1376e2d0f32023e197c51802bc21c60490
>> # Parent  f02fd7ca256d044c4a51c3f3fc0ecaf95d23e03d
>> # EXP-Topic noname
>> # Available At https://bitbucket.org/octobus/mercurial-devel/
>> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r b0f45e1376e2
>> revset: skip old style lookup if external whitespace are detected
>>
>> Since label cannot contains leading or trailing whitespace we can skip looking
>> for them. This is useful in repository with slow labels (eg: special type of
>> tags). Short command running on a specific revision can benefit from such
>> shortcut.
>>
>> eg on a repository where loading tags take 0.4s:
>>
>> 1: hg log --template '{node}\n' --rev 'rev(0)'
>>     0.560 seconds
>>
>> 2: hg log --template '{node}\n' --rev ' rev(0)'
>>     0.109 seconds
> Seems okay, but isn't it too obscure that prefixing with ' ' is the fast
> way of querying?

Yes, it is a bit obscure but it's the best solution we came up with 
existing code. I sent a new patch that implements the fast path at the 
individual name-space level in a way that seems cleaner and more useful.

Another solution is we could force revset evaluation using a `set:` (or 
`revset:`) prefix. This way we could have both a clean and explicit way 
of implementing the fast path.
> Instead, maybe we can make lookup() to not search slow labels assuming these
> labeling schemes didn't exist in pre-revset era.
We are afraid it is a bit more complex than that. Because `rev(0)` is a 
valid tag and a valid bookmark, we don't think we can ever skip this 
lookup call (yes, in our case, both tags and bookmarks are expensive to 
load).
> Alternatively, we could add
> a config knob to switch off the old-style range support.
Having a config knob for this seems weird. We don't expect users to find 
it and really understand what the config is about. It would be useful 
for large corporate users with centralized config, but they would need 
to set the flag on every one of their scripts/servers involving Mercurial.

Cheers,