[PATCH stable] revisionbranchcache: fall back to slow path if starting readonly (issue4531)

[Mads Kiilerich]

On 02/10/2015 08:04 PM, Augie Fackler wrote:
> On Fri, Feb 06, 2015 at 02:53:31AM +0100, Mads Kiilerich wrote:
>> # HG changeset patch
>> # User Mads Kiilerich <madski at unity3d.com>
>> # Date 1423187530 -3600
>> #      Fri Feb 06 02:52:10 2015 +0100
>> # Branch stable
>> # Node ID 83796637ce12f8d0e913f6db062eae5b835dbea9
>> # Parent  942a5a34b2d0611ab284380fbe45b9bb1897af98
>> revisionbranchcache: fall back to slow path if starting readonly (issue4531)
>>
>> Transitioning to Mercurial versions with revision branch cache could be slow as
>> long as all operations were readonly (revset queries) and the cache would be
>> populated but not written back.
>>
>> Instead, fall back to using the consistently slow path when readonly and the
>> cache doesn't exist yet. That avoids the overhead of populating the cache
>> without writing it back.
>>
>> If not readonly, it will still populate all missing entries initially. That
>> avoids repeated writing of the cache file with small updates, and it also makes
>> sure a fully populated cache available for the readonly operations.
>>
>> diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
>> --- a/mercurial/branchmap.py
>> +++ b/mercurial/branchmap.py
>> @@ -330,7 +330,7 @@ class revbranchcache(object):
>>       and will grow with it but be 1/8th of its size.
>>       """
>>
>> -    def __init__(self, repo):
>> +    def __init__(self, repo, readonly=True):
>>           assert repo.filtername is None
>>           self._names = [] # branch names in local encoding with static index
>>           self._rbcrevs = array('c') # structs of type _rbcrecfmt
>> @@ -342,6 +342,10 @@ class revbranchcache(object):
>>           except (IOError, OSError), inst:
>>               repo.ui.debug("couldn't read revision branch cache names: %s\n" %
>>                             inst)
>> +            if readonly:
>> +                # don't try to use cache - fall back to the slow path
>> +                self.branchinfo = self._branchinfo
> Do we have any way we can use the cache if it's available (already
> loaded), but skip loading/fixing it up if we're on the readonly path
> and it's not already loaded?

The revision branch cache is currently only shortlived. It is not stored 
anywhere and will thus never already have been loaded. So no, there is 
currently no win there.

The same question could be asked for 'fully populated' instead of 
'loaded'. That is however also not feasible; all the revision entries 
are invalidated separately and there is no grand total that can be checked.

So: the best we can do is to make sure we don't spend time populating 
the cache when using it readonly.

/Mads