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RE: Ref-counting for pristine texts

From: Julian Foad <julian.foad_at_wandisco.com>
Date: Thu, 27 Jan 2011 19:44:25 +0000

On Thu, 2011-01-27, Bert Huijben wrote:
>
> > -----Original Message-----
> > From: Julian Foad [mailto:julian.foad_at_wandisco.com]
> > Sent: donderdag 27 januari 2011 18:06
> > To: Bert Huijben
> > Cc: 'Branko Čibej'; dev_at_subversion.apache.org
> > Subject: RE: Ref-counting for pristine texts
> >
> > On Wed, 2011-01-26, Bert Huijben wrote:
> > > > -----Original Message-----
> > > > From: Julian Foad [mailto:julian.foad_at_wandisco.com]
> > > >
> > > > Can anyone help me work out the rules for guaranteeing consistency
> > of
> > > > the pristine text store?
> > >
> > > Previously we used a somewhat reversed definition: as long as there
> > > are work queue items or working-copy locks, you can't assume
> > pristines
> > > are unreferenced.
> >
> > Ah, WC-locks. Yes, that sounds more likely. (I don't recall that we
> > have ever used that rule yet. The pristine cleanup code has only ever
> > checked "WQ is empty", which isn't good enough.)
> >
> > > Note that there can be multiple clients accessing/modifying a working
> > > copy at the same time even though they don't have a sqlite
> > transaction
> > > open. (An update doesn’t keep it's transaction open for the entire
> > > update process).
> >
> > Yes. Nor does the update process keep work items items in the WQ
> > during
> > the whole time - it runs the WQ at various points. But there is a WC
> > lock through the whole update process.
> >
> > So we can write a rule:
> >
> > "You may purge unreferenced pristines only when
> > there are no WC locks in the DB."
> >
> > More precisely, the access control rules for the pristine store in a
> > given WC shall be:
> >
> > * A process may add a new (initially unreferenced) pristine text
> > to the store
> > IFF this process has a WC lock.
> >
> > * A process may add or remove references to any pristine text that
> > is in the store
> > IFF this process has a WC lock.
>
> Maybe this one can be loosened a bit; not sure though... (incrementing
> or decrementing a reference is probably safe when the other rules are
> followed)

My thinking was that if I don't have a lock, I can't safely add a
reference because another process might delete the text just before I
get around to executing that statement. But in fact I don't necessarily
need a WC lock, I need *either* a WC lock *or* a SQL txn. And within
that lock or txn I need to check the pristine text exists in the store
before I add the reference.

To remove a reference, I can't think why I should need a lock or a txn.

> > * A process that has a WC lock may assume that no pristine text,
> > even if unreferenced, will be deleted from the store as long as
> > this process holds any WC lock in this WC.
> >
> > * A process may purge an unreferenced pristine text
> > IFF no other process has a WC lock.
> > ### Must this process have a WC lock?
> >
> > (I use the word "process" loosely.)
> >
> > Does that make sense?
>
> I think it does.

Thanks.

Talking on IRC, we thought maybe involving the WC lock was making the
rules a bit too complex. It would be simpler to just define the rules
with respect to maintaining a valid DB state [1] outside any DB txn. So
something like:

  * To add a new pristine text, you must add it and the NODES row
    that references it within the same txn.

  * To add a reference to an existing text, you must do so within
    the same txn where you checked that it still exists.

  * You may remove a reference to a pristine text; no restrictions.

  * Any process may delete unreferenced pristine texts, as long as
    the check for "unreferenced" and the deletion are in the same txn.

However, to do this, we'd have to upgrade existing code to work like
this. Currently, some important code paths (update and commit) have
been designed to put unreferenced pristines into the store and then
later reference them.

Because of this, I wonder whether it's better to continue down the
existing code's route and ensure unreferenced pristines are never
deleted while the process holds a WC lock. That would appear to be
simpler from the point of view of what changes are required to get it
working.

[1] A "valid DB state" meaning "a state that represents some possible
repository state, but not necessarily the actual repository state".

Also TODO: pristine_install() and pristine_remove() should do the
file-move-into-place or file-delete via the Work Queue.

- Julian
Received on 2011-01-27 20:45:08 CET

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