blog.crox.net

While trying to restore two files with BackupPC I got the following error message:

Remote[1]: Internal hashtable error: illegal key supplied!
Remote[1]: rsync error: errors with program diagnostics (code 13) at hashtable.c(65) [Receiver=3.0.9]
Read EOF: 
Tried again: got 0 bytes
Done: 2 files, 760646 bytes
restore failed: Unable to read 4 bytes

Other restores worked flawlessly. All hosts involved were running the same version of Debian Linux, with the same version of rsync. The same two files could be restored as a zip file from the BackupPC web front-end.

After spending some time on the issue with a colleague we eventually found the explanation, albeit not the solution: one of the files was a hard link. It looks like we hit a bug similar to what is described here: http://sourceforge.net/p/backuppc/mailman/message/20762995/

> This results in one subtle bug that can't be easily fixed: if you
> switch the Xfer method from tar to rsync, old backups that have
> hardlinks stored with tar won't be correctly restored with rsync.
> The workaround is generate a tar file and extract it, or switch
> the Xfer method back to tar before you do the restore.

We did not switch Xfer method (we've always used rsync over ssh). Possibly the issue was caused by attempting to restore two hard links to the same file at once - more testing would be required to confirm this.