[fix] url_for(..., _external=True) in templates

The `url_for` function in the template context is not the one from Flask, it is
the one from `webapp`.  The `webapp.url_for_theme` is different from its
namesake of Flask and has it quirks, when called with argument `_external=True`.

The `webapp.url_for_theme` can't handle absolute URLs since it pokes a leading
'/', here is the snippet of the old code::

    url = url_for(endpoint, **values)
    if settings['server']['base_url']:
        if url.startswith('/'):
            url = url[1:]
        url = urljoin(settings['server']['base_url'], url)

Next drawback of (Flask's) `_external=True` is, that it will not return the HTTP
scheme when searx (the Flask app) listens on http and is proxied by a https
server.

To get the right scheme `HTTP_X_SCHEME` is needed by Flask (werkzeug).  Since
this is not provided in every environment (e.g. behind Apache mod_wsgi or the
HTTP header is not fully set for some other reasons) it is recommended to
get *script_name*, *server* and *scheme* from the configured `base_url`.  If
`base_url` is specified, then these values from are given preference over any
Flask's generics.

BTW this patch normalize to use `url_for` in the `opensearch.xml` and drop the
need of `host` and `urljoin` in template's context.

Signed-off-by: Markus Heiser <markus@darmarit.de>

searx/templates/__common__/opensearch.xml

@@ -3,7 +3,7 @@
   <ShortName>{{ instance_name }}</ShortName>
   <Description>a privacy-respecting, hackable metasearch engine</Description>
   <InputEncoding>UTF-8</InputEncoding>
-  <Image>{{ urljoin(host, url_for('static', filename='img/favicon.png')) }}</Image>
+  <Image>{{ url_for('static', filename='img/favicon.png', _external=True) }}</Image>
   <LongName>searx metasearch</LongName>
   {% if opensearch_method == 'get' %}
     <Url rel="results" type="text/html" method="get" template="{{ url_for('search', _external=True) }}?q={searchTerms}"/>
@@ -13,7 +13,7 @@
     </Url>
   {% endif %}
   {% if autocomplete %}
-    <Url rel="suggestions" type="application/x-suggestions+json" template="{{ host }}autocompleter?q={searchTerms}"/>
+    <Url rel="suggestions" type="application/x-suggestions+json" template="{{ url_for('autocompleter', _external=True) }}?q={searchTerms}"/>
   {% endif %}
 
   <Url type="application/opensearchdescription+xml"

searx/webapp.py

@@ -40,7 +40,7 @@ from datetime import datetime, timedelta
 from time import time
 from html import escape
 from io import StringIO
-from urllib.parse import urlencode, urljoin, urlparse
+from urllib.parse import urlencode, urlparse
 
 from pygments import highlight
 from pygments.lexers import get_lexer_by_name
@@ -269,14 +269,7 @@ def extract_domain(url):
 
 
 def get_base_url():
-    if settings['server']['base_url']:
-        hostname = settings['server']['base_url']
-    else:
-        scheme = 'http'
-        if request.is_secure:
-            scheme = 'https'
-        hostname = url_for('index', _external=True, _scheme=scheme)
-    return hostname
+    return url_for('index', _external=True)
 
 
 def get_current_theme_name(override=None):
@@ -309,10 +302,6 @@ def url_for_theme(endpoint, override_theme=None, **values):
         if filename_with_theme in static_files:
             values['filename'] = filename_with_theme
     url = url_for(endpoint, **values)
-    if settings['server']['base_url']:
-        if url.startswith('/'):
-            url = url[1:]
-        url = urljoin(settings['server']['base_url'], url)
     return url
 
 
@@ -812,7 +801,7 @@ def preferences():
 
     # save preferences
     if request.method == 'POST':
-        resp = make_response(redirect(urljoin(settings['server']['base_url'], url_for('index'))))
+        resp = make_response(url_for('index', _external=True))
         try:
             request.preferences.parse_form(request.form)
         except ValidationException:
@@ -1002,11 +991,11 @@ def opensearch():
     if request.headers.get('User-Agent', '').lower().find('webkit') >= 0:
         method = 'get'
 
-    ret = render('opensearch.xml',
-                 opensearch_method=method,
-                 host=get_base_url(),
-                 urljoin=urljoin,
-                 override_theme='__common__')
+    ret = render(
+        'opensearch.xml',
+        opensearch_method=method,
+        override_theme='__common__'
+    )
 
     resp = Response(response=ret,
                     status=200,
@@ -1027,7 +1016,7 @@ def favicon():
 
 @app.route('/clear_cookies')
 def clear_cookies():
-    resp = make_response(redirect(urljoin(settings['server']['base_url'], url_for('index'))))
+    resp = make_response(redirect(url_for('index', _external=True)))
     for cookie_name in request.cookies:
         resp.delete_cookie(cookie_name)
     return resp
@@ -1128,19 +1117,38 @@ class ReverseProxyPathFix:
     '''
 
     def __init__(self, app):
+
         self.app = app
+        self.script_name = None
+        self.scheme = None
+        self.server = None
+
+        if settings['server']['base_url']:
+
+            # If base_url is specified, then these values from are given
+            # preference over any Flask's generics.
+
+            base_url = urlparse(settings['server']['base_url'])
+            self.script_name = base_url.path
+            self.scheme = base_url.scheme
+            self.server = base_url.netloc
 
     def __call__(self, environ, start_response):
-        script_name = environ.get('HTTP_X_SCRIPT_NAME', '')
+
+        script_name = self.script_name or environ.get('HTTP_X_SCRIPT_NAME', '')
         if script_name:
             environ['SCRIPT_NAME'] = script_name
             path_info = environ['PATH_INFO']
             if path_info.startswith(script_name):
                 environ['PATH_INFO'] = path_info[len(script_name):]
 
-        scheme = environ.get('HTTP_X_SCHEME', '')
+        scheme = self.scheme or environ.get('HTTP_X_SCHEME', '')
         if scheme:
             environ['wsgi.url_scheme'] = scheme
+
+        server = self.server or environ.get('HTTP_X_FORWARDED_HOST', '')
+        if server:
+            environ['HTTP_HOST'] = server
         return self.app(environ, start_response)