Add an option to control keep-alive connections

By default http module set keep-alive option in connections to make it
persistent:

$ tarantool examples/middleware.lua
started
entering the event loop
HEAD /

...

$ curl -I  http://127.0.0.1:8080
HTTP/1.1 404 Not found
Content-length: 0
Server: Tarantool http (tarantool v1.10.6-0-g5372cd2fa)
Connection: keep-alive
$

However sometimes we need persistent connection and close it instead. It
is possible with option keepalive_disable. Option accepts a map where key
is a user agent name with assigned non-nil value.

Example:

local httpd = http_server.new('127.0.0.1', 8080, {
    log_requests = true,
    log_errors = true,
    keepalive_disable = {
        ['curl/7.68.0'] = true
    },
})

1. https://nginx.org/en/docs/http/ngx_http_core_module.html#keepalive_disable

Fixes #137

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Added
+- Add option to control keepalive (#137)
+
 ## [2.1.0] - 2020-01-30
 ### Added
 - Return ability to set loggers for a specific route

README.md

@@ -116,6 +116,9 @@ server = require('http.server').new(host, port[, { options } ])
     By default uses `log.info` function for requests logging.
 * `log_errors` - same as the `log_requests` option but is used for error messages logging.
     By default uses `log.error()` function.
+* `keepalive_disable` - disables keep-alive connections with misbehaving
+    clients. Parameter accept a map that contains a user agents with non-nil
+    value. By default map is empty.
 
 ## Creating a router
 

http/server/init.lua

@@ -80,6 +80,17 @@ local function get_error_logger(server_opts, route_opts)
     return log.debug
 end
 
+local function is_no_keepalive(disallowed_map, useragent)
+    if type(disallowed_map) ~= 'table' or useragent == nil then
+        return false
+    end
+    if disallowed_map[useragent] ~= nil then
+        return true
+    end
+
+    return false
+end
+
 ----------
 -- Server
 ----------
@@ -259,6 +270,12 @@ local function process_client(self, s, peer)
             end
         end
 
+        local no_keepalive = is_no_keepalive(self.options.keepalive_disable,
+                                             p.headers['user-agent'])
+        if no_keepalive == true then
+            hdrs.connection = 'close'
+        end
+
         -- generate response {{{
         local response = {
             "HTTP/1.1 ";
@@ -379,6 +396,7 @@ local new = function(host, port, options)
         log_requests        = true,
         log_errors          = true,
         display_errors      = true,
+        keepalive_disable   = {},
     }
 
     local self = {