Propagation of inherited mergeinfo during URL->URL/WC copies

On #svn-dev today:

[[[
<danielsh> didn't know it was valid for svn:mergeinfo properties to
have an empty value

<pburba> danielsh: Yes, it allows the possibility for a subtree to
inherit none of its (nearest) parent's mergeinfo.

<danielsh> pburba: ie, prevent inheritance from an ancestor

<pburba> danielsh: yes

<danielsh> combined with a node that has no explicit mergeinfo
<danielsh> pburba: thanks. (I assumed it was not an error, but thanks
for confirming)

<pburba> danielsh: It rarely seen in practice...well at least I've
rarely seen it

<danielsh> pburba: I just saw an instance of it an hour ago.
<danielsh> r820355 in /repos/infra/infrastructure
<danielsh> basically a copy (or replace?) with history in a setting
with no mergeinfo properties at all

<Bert> danielsh: I remember that some very old (1.5 / 1.6 dev) clients
created that property on local copies in some situations

<pburba> Bert: Yeah, I don't recall if there were some cases with
URL->URL/WC copies where it still happened

<danielsh> the commit was done using svn 1.7.3
]]]

So what happened here?

First a little background:

Since 1.5.5WC->WC copies don't record mergeinfo on the destination
unless there is explicit mergeinfo on the source. This was discussed
here http://svn.haxx.se/dev/archive-2008-11/0432.shtml and here
http://svn.haxx.se/dev/archive-2008-11/0297.shtml

However, URL->URL/WC copies still carry the inherited mergeinfo from
the source and make it explicit on the target.

So what happened in r820355?

(Paranoia alert: Since this revision involves a private portion of the
ASF repository, the actual path names and other log info in this
description are fictitious. Probably overkill...)

[[[
>svn log -v -r820355 ^/
------------------------------------------------------------------------
r820355 | bob | 2011-12-06 10:42:38 -0400 (Tue, 06 Dec 2011) | 1 line
Changed paths:
   A /infra/main/boxes/freebsd/harm8/usr/local/etc/postfix (from
/infra/main/boxes/freebsd/harm8/usr/local/etc/postfix:820354)

copy postfix from harm8
]]]

^/infra/main/boxes/freebsd/harm8/usr/local/etc/postfix_at_820354 has no
explicit mergeinfo of its own, but its parent
^/infra/main/boxes/freebsd/harm8/usr_at_820354 has explicit empty
mergeinfo on it:

[[[
>svn pg svn:mergeinfo -vR ^/infra/main/boxes/freebsd/harm8_at_820354
Properties on '^/infra/main/boxes/freebsd/harm8/usr':
  svn:mergeinfo

]]]

So presumably what occurred was a URL->[URL | WC] copy, which
calculated the source's inherited mergeinfo and made it explicit on
the target.

Now while this is expected, in this situation it is hardly ideal.
Certainly cases exist where we *want* the inherited mergeinfo of the
source carried to the destination (e.g. imagine
^/infra/main/boxes/freebsd/harm8 had real merge history representing
merges from some third branch), but since the inherited mergeinfo of
the source in this case is empty, it carries no useful information and
could be safely ignored. Another case comes to mind where this
behavior is unnecessary, when the soruce and destination inherit
mergeinfo from the same parent.

I will file an issue.

-- 
Paul T. Burba
CollabNet, Inc. -- www.collab.net -- Enterprise Cloud Development
Skype: ptburba