[PATCH STABLE] changegroup: store old heads as a set

[Augie Fackler]

> On Mar 23, 2017, at 10:59 PM, Gregory Szorc <gregory.szorc at gmail.com> wrote:
> 
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1490324099 25200
> #      Thu Mar 23 19:54:59 2017 -0700
> # Branch stable
> # Node ID ba2c20c45b3e0202e52bbd3252f9a8214d8a2e06
> # Parent  2915cc1d3429f9b11c9cf48d45990f982fff8e7a
> changegroup: store old heads as a set

Queued for stable. Thanks.


> Previously, the "oldheads" variable was a list. On a repository at
> Mozilla with 46,492 heads, profiling revealed that list membership
> testing was dominating execution time of applying small changegroups.
> 
> This patch converts the list of old heads to a set. This makes
> membership testing significantly faster. On the aforementioned
> repository with 46,492 heads:
> 
> $ hg unbundle <file with 1 changeset>
> before: 18.535s wall
> after:   1.303s
> 
> Consumers of this variable only check for truthiness (`if oldheads`),
> length (`len(oldheads)`), and (most importantly) item membership
> (`h not in oldheads` - which occurs twice). So, the change to a set
> should be safe and suitable for stable.
> 
> The practical effect of this change is that changegroup application
> and related operations (like `hg push`) no longer exhibit an O(n^2)
> CPU explosion as the number of heads grows.
> 
> diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
> --- a/mercurial/changegroup.py
> +++ b/mercurial/changegroup.py
> @@ -293,7 +293,7 @@ class cg1unpacker(object):
>                 # will not see an inconsistent view
>                 cl = repo.changelog
>                 cl.delayupdate(tr)
> -                oldheads = cl.heads()
> +                oldheads = set(cl.heads())
> 
>                 trp = weakref.proxy(tr)
>                 # pull off the changeset group
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