[mirror-admin] Only run rsync if something has actually changed

[Adrian Reber]

I am using dl.fedoraproject.org::fedora-buffet0/ to mirror Fedora and
a rsync run which downloads no new files still needs around 2500 seconds
to check if there have been any changes.

To avoid running rsync without actually downloading anything I have
written a script which queries Fedora's message bus to see if there
have been changes or not.

The script is available at:

https://fedorahosted.org/fedora-infrastructure/attachment/ticket/4539/last-sync

and details about its usage are right now only documented in this ticket:

https://fedorahosted.org/fedora-infrastructure/ticket/4539

As I am recording for each mirror the starttime, endtime and status I
can avoid running rsync if nothing has actually changed. I have added
the following lines to my mirror script:

# check if mirror needs to be updated
CURDATE=`date +%s`
LASTRUN=$( # some sql command to query the last successful run )
DELTA=`echo ${CURDATE}-${LASTRUN} | bc`

/path/to/last-sync -d ${DELTA} -q

if [ "$?" -ne "0" ]; then
        # no changes on the master mirror
        # abort
        exit 0
fi

As this might be helpful for other mirrors I am posting it here.

		Adrian
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