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Just a quickie for future reference. Svnsync is a tool that comes with Subversion that allows to create read-only mirrors of Subversion repositories.

Creating a mirror is quite easy and usually involves the following steps:

• First we create a standard subversion repository.

   mkdir mirror
   svnadmin create mirror

• Now we have to create a pre-revprop-change hook that only allows the svnsync write access to it. Remember that the mirror is read-only. Put the following contents in mirror/hooks/pre-revprop-change.

   #!/bin/sh
  
   if [ "$3" = "svnsync" ]; then exit 0; fi
   echo "Only svnsync user may edit revision properties through svnsync" >&2
   exit 1

• To activate the hook we need to give it executable permissions.

   chmod +x mirror/hooks/pre-revprop-change

• Configure the synchronization

   svnsync init --username svnsync \
       file:///path/to/mirror \
       https://url.to/repository

• Finally perform the synchronization! You can put this command in the cron system for example to ensure the mirror is kept up-to-date.

   svnsync sync file:///path/to/mirror

There is a problem though with this setup if we want to use a post-commit hook. svnsync works by first committing a synced revision with its own user and then, in a second step, modifies the revision properties to change the author and date to those of the original repository. This means that post-commit hooks which need to obtain the revision author or date cannot be property run, since the reported author and date will be wrong.

The solution is to use a different hook, post-revprop-change, which is triggered each time a revision property is changed. We can have something as the following script to emulate a normal post-commit hook.

  #!/bin/sh

  REPOS="$1"
  REV="$2"
  USER="$3"
  PROPNAME="$4"
  ACTION="$5"

  # Only do actual work when the "svn:date" property is modified, which seems
  # to be the last revision property modified by the svnsync process.
  if [ "$ACTION" = "M" -a "$PROPNAME" = "svn:date" ]; then
    AUTHOR=`svnlook author $REPOS -r $REV`
    DATE=`svnlook date $REPOS -r $REV`
    LOG=`svnlook log $REPOS -r $REV`

    echo "$REPOS@$REV: $LOG by $AUTHOR at $DATE"
  fi