[enh] Add timeout limit per request (#1640)

The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.

In addition, the query can start with <[number] to set the timeout limit.

For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds

For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.

In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).

If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.

Related to #1077
Updated version of PR #1413 from @isj-privacore

searx/search.py

@@ -45,6 +45,16 @@ if sys.version_info[0] == 3:
 logger = logger.getChild('search')
 
 number_of_searches = 0
+max_request_timeout = settings.get('outgoing', {}).get('max_request_timeout' or None)
+if max_request_timeout is None:
+    logger.info('max_request_timeout={0}'.format(max_request_timeout))
+else:
+    if isinstance(max_request_timeout, float):
+        logger.info('max_request_timeout={0} second(s)'.format(max_request_timeout))
+    else:
+        logger.critical('outgoing.max_request_timeout if defined has to be float')
+        from sys import exit
+        exit(1)
 
 
 def send_http_request(engine, request_params):
@@ -265,6 +275,15 @@ def get_search_query_from_webapp(preferences, form):
     # query_engines
     query_engines = raw_text_query.engines
 
+    # timeout_limit
+    query_timeout = raw_text_query.timeout_limit
+    if query_timeout is None and 'timeout_limit' in form:
+        raw_time_limit = form.get('timeout_limit')
+        try:
+            query_timeout = float(raw_time_limit)
+        except ValueError:
+            raise SearxParameterException('timeout_limit', raw_time_limit)
+
     # query_categories
     query_categories = []
 
@@ -338,7 +357,8 @@ def get_search_query_from_webapp(preferences, form):
     query_engines = deduplicate_query_engines(query_engines)
 
     return (SearchQuery(query, query_engines, query_categories,
-                        query_lang, query_safesearch, query_pageno, query_time_range),
+                        query_lang, query_safesearch, query_pageno,
+                        query_time_range, query_timeout),
             raw_text_query)
 
 
@@ -351,6 +371,7 @@ class Search(object):
         super(Search, self).__init__()
         self.search_query = search_query
         self.result_container = ResultContainer()
+        self.actual_timeout = None
 
     # do search-request
     def search(self):
@@ -380,7 +401,7 @@ class Search(object):
         search_query = self.search_query
 
         # max of all selected engine timeout
-        timeout_limit = 0
+        default_timeout = 0
 
         # start search-reqest for all selected engines
         for selected_engine in search_query.engines:
@@ -420,12 +441,32 @@ class Search(object):
             # append request to list
             requests.append((selected_engine['name'], search_query.query, request_params))
 
-            # update timeout_limit
-            timeout_limit = max(timeout_limit, engine.timeout)
+            # update default_timeout
+            default_timeout = max(default_timeout, engine.timeout)
 
+        # adjust timeout
+        self.actual_timeout = default_timeout
+        query_timeout = self.search_query.timeout_limit
+
+        if max_request_timeout is None and query_timeout is None:
+            # No max, no user query: default_timeout
+            pass
+        elif max_request_timeout is None and query_timeout is not None:
+            # No max, but user query: From user query except if above default
+            self.actual_timeout = min(default_timeout, query_timeout)
+        elif max_request_timeout is not None and query_timeout is None:
+            # Max, no user query: Default except if above max
+            self.actual_timeout = min(default_timeout, max_request_timeout)
+        elif max_request_timeout is not None and query_timeout is not None:
+            # Max & user query: From user query except if above max
+            self.actual_timeout = min(query_timeout, max_request_timeout)
+
+        logger.debug("actual_timeout={0} (default_timeout={1}, ?timeout_limit={2}, max_request_timeout={3})"
+                     .format(self.actual_timeout, default_timeout, query_timeout, max_request_timeout))
+
+        # send all search-request
         if requests:
-            # send all search-request
-            search_multiple_requests(requests, self.result_container, start_time, timeout_limit)
+            search_multiple_requests(requests, self.result_container, start_time, self.actual_timeout)
             start_new_thread(gc.collect, tuple())
 
         # return results, suggestions, answers and infoboxes