config tests added,

.env file support added with dotenv,
README.md file updated for .evn file support,
database lost connection bug crash fixed,
session server run_session long timeout bug fixed

src/database/database.js

@@ -3,18 +3,21 @@ const { database_url, database_name } = require('../../config')
 const crypto = require("crypto")
 let max_database_connection_timeout = 1000
 
+const client = new MongoClient(database_url, {
+    serverSelectionTimeoutMS: max_database_connection_timeout, 
+    useUnifiedTopology: true
+})
+
 const get_db = async () => {
-    const client = new MongoClient(database_url, {
-        serverSelectionTimeoutMS: max_database_connection_timeout, 
-        useUnifiedTopology: true
-    })
     try {
         await client.connect()
+        await client.db(database_name).command({ ping: 1 })
+        return client.db(database_name)
     } catch (err) {
-        console.log('can not connect to database')
+        console.error('can not connect to database')
+        await client.close()
         return null
     }
-    return client.db(database_name)
 }
 
 const get_titles = async (start, count) => {
@@ -41,14 +44,14 @@ const get_free_session_servers = async (title) => {
     var active_session = db.collection('active_session')
     
     var free_servers = []
-    var servers_length = servers.length
-    for (var i = 0; i < servers_length; ++i) {
-        var sessions = await active_session.find({url:servers[i].url}).toArray()
-        if (servers[i].limit > sessions.length) {
-            free_servers.push(servers[i])
+
+    for (const server of servers) {
+        var sessions = await active_session.find({server_url:server.url}).toArray()
+        if (server.limit > sessions.length) {
+            free_servers.push(server)
         }
     }
-    return free_servers
+    return (free_servers.length) ? free_servers : null
 }
 
 const add_active_session = async (server_url, session_id, websocket_url) => {