svnsync backwards-compatibility with older servers (Was: Re: svn commit: r999421)

Hi,

svnsync has always had locking, to prevent two svnsync processes from
writing to the same mirror at the same time. (If that happens, then the
mirror repo gets corrupted. You have to either delete the mirror repo
and resync, dump/load it, or restore from backup. Either way, there's
significant downtime).

Unfortunately, this locking has always been broken (there's a race
condition). It's impossible to fix it if the mirror server is running
<=1.6 - anyone using 1.6 needs to use an external locking program like
flock(1)*. If the mirror server is running 1.7, which has the "atomic
revprops" feature, and svnsync is also version 1.7, then the locking
will work properly.

So... what do we do if a 1.7 svnsync connects to a <=1.6 mirror server?
Some options are:

1) It works like 1.6 - i.e. it does buggy locking that works most of the
time, then one day it corrupts the mirror repo.

2) It runs without even trying to do locking. This is likely to be more
obvious to the sysadmin, so they might notice it in testing.

3) It errors out with an informative message. If the user has control
of the mirror server, they can upgrade it to 1.7. Alternatively, if the
user doesn't need locking (e.g. they have set up external locking), they
can pass --disable-locking to svnsync.

I really don't like option 1. I think Subversion should be reliable,
and random corruption is not good. I'd like to get rid the old, buggy
locking code.

I'm not keen on option 2 either. Silently disabling a
required-for-correctness feature seems like a really bad idea. And the
locking has always been a documented feature (even if it's never
worked).

So I guess that leaves option 3. That's not fully backwards-compatible
- you can't just drop in a 1.7 svnsync to replace a 1.6 svnsync, unless
you update the server at the same time. But I feel that it's the least
bad option.

Thoughts?

Kind regards,

Jon

* http://linux.die.net/man/1/flock

-----Original Message-----
[snip commit that does option 1]

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Received on 2010-09-21 18:08:28 CEST