Last week I browsed the SourceForge site status and found an announcement about enabling revprops. This is good news, fpsvnsync does it job, but it needs both trees checked out and it is not as efficient as svnsync itself, because it starts a svn process several times for each changed file.
Last Monday I made a svndump of the Lazarus repository to get all those names and dates (which fpsvnsync cannot / could not set correctly) right. For your information, is a 700 MB file after bzip2-ing. I imported it on SourceForge using the svn migration page. Then I had a surprise when started svnsync init:
svnsync init https://lazarus.svn.sourceforge.net/svnroot/lazarus \
svnsync: Cannot initialize a repository with content in it
It only works on empty repositories. Three hours of import wasted.
So I imported an empty repository and called svn init on that repository. Using svnsync sync would be an option to make the mirror up to date, but importing more than 19000 revision this way didn't seem to be a wise choice. So I imported the 700 MB dump again, but without clearing the existing one. But still the svnsync utility didn't want to work. I had to set the svn:sync-last-merged-rev property manually, svnsync keeps this revprop for its administrative puproses at revision 0:
svn propset svn:sync-last-merged-rev --revprop -r0 19091 \
I updated my crontab to call svnsync every three minutes and the Lazarus svn mirror at SourceForge has been updated nicely after.