component AP gateway: convert XMPP mentions to AP:

When a XEP-0372 mention is received, the linked pubsub item is looked after in cache, and
if found, it is send to mentioned entity with `mention` tag added.

However, this doesn't work in some cases (see incoming doc for details). To work around
that, `@user@server.tld` type mention are also scanned in body, and mentions are added
when found (this can be disabled with `auto_mentions` setting).

Mention are only scanned in "public" messages, i.e. for pubsub items, and not direct
messages.

rel 369

sat/plugins/plugin_comp_ap_gateway/regex.py

@@ -16,7 +16,11 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-"""Regular Expression to parse "Signature" header"""
+"""Various Regular Expression for AP gateway"""
+
+import re
+
+## "Signature" header parsing
 
 # those expression have been generated with abnf-to-regex
 # (https://github.com/aas-core-works/abnf-to-regexp)
@@ -38,8 +42,6 @@
 # obs-text = %x80-FF
 # ---
 
-import re
-
 ows = '[ \t]*'
 bws = f'{ows}'
 obs_text = '[\\x80-\\xff]'
@@ -52,3 +54,11 @@ RE_SIG_PARAM = re.compile(
     f'(?P<key>{token}{bws})={bws}'
     f'((?P<uq_value>{token})|(?P<quoted_value>{quoted_string}))'
 )
+
+
+## Account/Mention
+
+# FIXME: naive regex, should be approved following webfinger, but popular implementations
+#   such as Mastodon use a very restricted subset
+RE_ACCOUNT = re.compile(r"[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-]+")
+RE_MENTION = re.compile(rf"(?<!\w)@{RE_ACCOUNT.pattern}\b")