fix: add support for farmOS/OAuth2 Authorization Code grant/workflow

src/wuttafarm/web/templates/auth/login.mako

@@ -0,0 +1,33 @@
+## -*- coding: utf-8; -*-
+<%inherit file="wuttaweb:templates/auth/login.mako" />
+<%namespace name="base_meta" file="/base_meta.mako" />
+
+<%def name="page_content()">
+  <div class="wutta-page-content">
+    <div class="wutta-logo">${base_meta.full_logo(image_url or None)}</div>
+    <div style="display: flex; gap: 1rem; align-items: center;">
+
+        <div class="card">
+          <div class="card-content">
+            ${form.render_vue_tag()}
+          </div>
+        </div>
+
+        <div class="has-text-italic">
+          - OR -
+        </div>
+
+        <div class="card">
+          <div class="card-content">
+            <wutta-button type="is-primary"
+                          tag="a"
+                          href="${url('farmos_oauth_login')}"
+                          icon-left="user"
+                          label="Login via farmOS / OAuth2"
+                          once />
+          </div>
+        </div>
+
+    </div>
+  </div>
+</%def>

src/wuttafarm/web/views/auth.py

@@ -23,7 +23,12 @@
 Auth views
 """
 
+from oauthlib.oauth2 import AccessDeniedError
+from requests_oauthlib import OAuth2Session
+
 from wuttaweb.views import auth as base
+from wuttaweb.auth import login_user
+from wuttaweb.db import Session
 
 
 class AuthView(base.AuthView):
@@ -47,6 +52,87 @@ class AuthView(base.AuthView):
 
         return None
 
+    def get_farmos_oauth2_session(self):
+        return OAuth2Session(
+            client_id="farm",
+            scope="farm_manager",
+            redirect_uri=self.request.route_url("farmos_oauth_callback"),
+        )
+
+    def farmos_oauth_login(self):
+        """
+        View to initiate OAuth2 workflow.
+        """
+        oauth = self.get_farmos_oauth2_session()
+        auth_url, state = oauth.authorization_url(
+            self.app.get_farmos_url("/oauth/authorize")
+        )
+        return self.redirect(auth_url)
+
+    def farmos_oauth_callback(self):
+        """
+        View for OAuth2 workflow, provided as redirect URL when
+        authorizing.
+        """
+        session = Session()
+        auth = self.app.get_auth_handler()
+        oauth = self.get_farmos_oauth2_session()
+
+        try:
+            # get oauth token from farmOS
+            token = oauth.fetch_token(
+                self.app.get_farmos_url("/oauth/token"),
+                authorization_response=self.request.current_route_url(),
+                include_client_id=True,
+            )
+
+        except AccessDeniedError:
+            self.request.session.flash("Access to farmOS was denied.", "error")
+            return self.redirect(self.request.route_url("login"))
+
+        # save token in user session
+        self.request.session["farmos.oauth2.token"] = token
+
+        # get (or create) native app user
+        farmos_client = self.app.get_farmos_client(token=token)
+        info = farmos_client.info()
+        farmos_user = farmos_client.resource.get_id(
+            "user", "user", info["meta"]["links"]["me"]["meta"]["id"]
+        )
+        user = auth.get_or_make_farmos_user(
+            session, farmos_user["data"]["attributes"]["name"]
+        )
+        if not user:
+            self.request.session.flash(
+                "farmOS authentication was successful, but user is "
+                "not allowed login to {self.app.get_node_title()}.",
+                "error",
+            )
+            return self.redirect(self.request.route_url("login"))
+
+        # delare user is logged in
+        headers = login_user(self.request, user)
+        referrer = self.request.get_referrer()
+        return self.redirect(referrer, headers=headers)
+
+    @classmethod
+    def defaults(cls, config):
+        cls._auth_defaults(config)
+        cls._wuttafarm_defaults(config)
+
+    @classmethod
+    def _wuttafarm_defaults(cls, config):
+
+        # farmos oauth login
+        config.add_route("farmos_oauth_login", "/farmos/oauth/login")
+        config.add_view(cls, attr="farmos_oauth_login", route_name="farmos_oauth_login")
+
+        # farmos oauth callback
+        config.add_route("farmos_oauth_callback", "/farmos/oauth/callback")
+        config.add_view(
+            cls, attr="farmos_oauth_callback", route_name="farmos_oauth_callback"
+        )
+
 
 def defaults(config, **kwargs):
     local = globals()