[PATCH] transaction: move changelog finalizer to be before bookmark finalizer

Durham Goode durham at fb.com
Sun Mar 20 13:17:45 EDT 2016



On 3/19/16 2:43 PM, FUJIWARA Katsunori wrote:
> At Sat, 19 Mar 2016 10:04:23 -0700,
> Durham Goode wrote:
>> On 3/18/16 6:07 PM, Pierre-Yves David wrote:
>>> On 03/16/2016 11:31 PM, Durham Goode wrote:
>>>> # HG changeset patch
>>>> # User Durham Goode <durham at fb.com>
>>>> # Date 1458196059 25200
>>>> #      Wed Mar 16 23:27:39 2016 -0700
>>>> # Node ID cdf4a94fdc46d27196f5411fc7a4008690834fba
>>>> # Parent  ed75909c4c670a7d9db4a2bef9817a0d5f0b4d9c
>>>> transaction: move changelog finalizer to be before bookmark finalizer
>>>>
>>>> Previously, transaction close would run the file generators before
>>>> running the
>>>> finalizers (see the list below for what is in each). Since file
>>>> generators
>>>> contain the bookmarks and the dirstate, this meant we made the
>>>> dirstate and
>>>> bookmarks visible to external readers before we actually wrote the
>>>> commits into
>>>> the changelog, which could result in missing bookmarks and missing
>>>> working copy
>>>> parents (especially on servers with high commit throughput, since
>>>> pulls might
>>>> fail to see certain bookmarks in this situation).
>>>>
>>>> By moving the changelog writing to be before the bookmark/dirstate
>>>> writing, we
>>>> ensure the commits are present before they are referenced.
>>>>
>>>> For reference, file generators currently consist of: bookmarks,
>>>> dirstate, and
>>>> phases. Finalizers currently consist of: changelog, revbranchcache,
>>>> and fncache.
>>>> All of the former reference the latter, so therefore the latter
>>>> should be
>>>> written first.
>>>>
>>>> Technically there's still plenty of race conditions (can the order of
>>>> finalizers
>>>> affect how external readers see the repo?), but this is a step
>>>> forward at least.
>>> Can we get a more complete analyse on the race condition we have here
>>> and which ones we are trading for which others one?
>>>
>>> For example, this changes could result in phase root being visible
>>> after the changesets, making public being seen public while they are not.
>>>
>>> I would like to have a clear idea of where we are walking there and
>>> what tradeoff we are performing
>> K, given that file generators are "bookmarks, dirstate, and phases" and
>> finalizers are "revlogs, revbranchcache, fncache", the old race
>> conditions are:
>>
>> - bookmarks written before revlogs - causes bookmarks to appear missing
>> during read
>> - dirstate written before revlogs - causes missing working copy parent
>> - phases written before revlogs - should be benign, since any phase on a
>> commit that is missing only affects commits that are also missing
>> - bookmarks/dirstate/phases written before revbranch cache - no issues
>> - bookmarks/dirstate/phases written before fncache - no issues
>>
>> and the new race conditions are:
>>
>> - revlogs written before bookmarks - may see commits where the bookmark
>> hasn't moved on top of them yet; odd but not an invalid state
>> - revlogs written before dirstate - no issues
>> - revlogs written before phases - some commits may show up with the
>> wrong phase (potentially public)
>> - revbranch cache written before bookmarks/dirstate/phases - no issues
>> - fncache written before bookmarks/dirstate/phases - no issues
>>
>> Race conditions that existed before (within the two categories):
>> - fncache before revlog - fncache could point to file that doesn't exist
>> - revlog before fncache - fncache could be missing a file
>>
>>
>> So in summary, my change removes the invalid bookmarks and invalid
>> dirstate case, and replaces it with an out-of-date phase case.  The out
>> of date phase case could potentially be bad, since if we send public
>> commits to the client, it may be hard to make them draft later. On the
>> flip side, most central-server setups are publishing servers, so this
>> wouldn't actually be an issue for them. This could be fixed by moving
>> the phase writes to be before the revlogs (or by redoing the entire
>> storage system to be more widely transactional).
> Is there any reason why all file generators should be executed at once ?
>
> If not, how about categorizing file generators into two groups
> ("prior" to finalizing and other) by changes like below ?
>
> ====================
> diff --git a/mercurial/phases.py b/mercurial/phases.py
> --- a/mercurial/phases.py
> +++ b/mercurial/phases.py
> @@ -268,7 +268,7 @@ class phasecache(object):
>           self.invalidate()
>           self.dirty = True
>
> -        tr.addfilegenerator('phase', ('phaseroots',), self._write)
> +        tr.addfilegenerator('phase', ('phaseroots',), self._write, prior=True)
>           tr.hookargs['phases_moved'] = '1'
>
>       def advanceboundary(self, repo, tr, targetphase, nodes):
>
I talked with Pierre-Yves and Fujiwara in real life.  I'm going to send 
a patch similar to this one, except instead of changing the 
addfilegenerator api, we'll add a list to transaction.py of generators 
that should be run after the finalizers.  That way if we ever fix the 
transaction logic to not require ordering, we can just delete the list 
instead of having to update all the callers.


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