When the `search` plugin is enabled, with the options:

```
# search plugin
# path to the omega cgi program
omega_cgi: /usr/lib/cgi-bin/omega/omega
# use google site search rather than internal xapian index?
google_search: 0
```

Then we see the following error when editing a page from the web browser UI or running `ikiwiki --rebuild`:

```
Error: Exception: Db block overwritten - are there multiple writers?
```

The issue goes away when we disable the 'search' plugin.

We're running ikiwiki version 3.20180228-1.

> Every time?
> I used to see an error (this one, I think) intermittently on OS X, on incremental rebuilds only.
> Haven't seen it in a while, though.
> Maybe something got fixed upstream.
> What OS, and what version of Xapian and Omega?
> (1.4.11 for me.)
> Maybe worth blowing away `.ikiwiki/xapian/`, as described in [[tips/inside_dot_ikiwiki]]?
> 
> --[[schmonz]]
> 
>> We've removed the `.ikiwiki/xapian` directory before performing a rebuild, including actually rebooting the host OS. The search appears to work for a while, but once the wiki has been live for a while (and subject to edits on the web interface), the issue reappears.
>> 
>> * OS: Ubuntu 18.04.2 LTS (GNU/Linux 4.15.0-47-generic x86_64)
>> * xapian-tools/bionic-updates,bionic-security,now 1.4.5-1ubuntu0.1 amd64
>> * xapian-omega/bionic,now 1.4.5-1 amd64
>> 
>> I've upgraded our installations of both Xapian components and will let you know if that fixes it.
>> -- jsewell
>>
>>> Updating appeared to fix it. Sorry for not trying that sooner. Thanks for the help.
>>> -- jsewell
