mu-find: insist on --linksdir for links

Fixes #2790.

man/mu-find.1.org

@@ -127,15 +127,16 @@ Output results in the specified format.
 - The default is *plain*, i.e normal output with one line per message.
 - *links* outputs the results as a maildir with symbolic links to the found
   messages. This enables easy integration with mail-clients (see below for more
-  information).
+  information). This requires *--linksdir*.
 - *xml* formats the search results as XML.
 - *sexp* formats the search results as an s-expression as used in Lisp programming
   environments.
 
 ** --linksdir _dir_ and -c, --clearlinks
-When using *--format=links*, output the results as a maildir with symbolic links to
-the found messages. This enables easy integration with mail-clients (see below
-for more information). *mu* will create the maildir if it does not exist yet.
+When using *--format=links*, output the results as a maildir with symbolic links
+to the found messages. This enables easy integration with mail-clients (see
+below for more information). *mu* will create the maildir if it does not exist
+yet.
 
 If you specify *--clearlinks*, existing symlinks will be cleared from the target
 directories; this allows for re-use of the same maildir. However, this option

mu/mu-cmd-find.cc

@@ -164,6 +164,10 @@ get_query(const Store& store, const Options& opts)
 static Result<void>
 prepare_links(const Options& opts)
 {
+	// XXX: can this be done as part of the option-parsing?
+	if (opts.find.linksdir.empty())
+		return Err(Error::Code::InvalidArgument, "--linksdir is required");
+
 	/* note, mu_maildir_mkdir simply ignores whatever part of the
 	 * mail dir already exists */
 	if (auto&& res = maildir_mkdir(opts.find.linksdir, 0700, true); !res)