Yes, Xapian is definitely licensed under GPLv2 or later.

And many distributions following FSDG distribute Xapian. IMO, it's OK to
write a module to empower Emacs with Xapian.

Richard Stallman <rms@gnu.org> writes:

> [[[ To any NSA and FBI agents reading my email: please consider    ]]]
> [[[ whether defending the US Constitution against all enemies,     ]]]
> [[[ foreign or domestic, requires you to follow Snowden's example. ]]]
>
>   > I wrote a dynamic module that exposes a simple interface for
>   > Xapian, the indexing and searching engine. It could be very useful
>   > for indexing thousands of files and search in them. I use this
>   > module for my note-searching package.
>
> This could be a useful thing, but before we decide to install it, we
> need to check some crucial nontechnical issues.
>
> * The moral issues.
> Does Xapian include any nonfree software?
> Does it depend on the presence of any nonfree software?
>
> Does Xapian use some web service?
>
> * The legal issues.
> Assuming Xapian does everything with free software,
> is all that software under GPL-compatible licenses?
> If not, we need to look at how the non-GPL-compatible
> software connects with the GPL-covered body of
> Emacs and its add-ons.  Depending on that, it might be ok.
>
> See https://gnu.org/licenses/license-list.html to check
> whether a license is free, and whether it is GPL-compatible.


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Zihao