Fix redundant keep-alive requests when session_lifetime is greater than ~20000 (#5273)

CHANGELOG

@@ -6,6 +6,7 @@ CHANGELOG Roundcube Webmail
 - Fix so minified publickey.js (with cache-buster) is used when available (#5254)
 - Fix (replace) application/x-tar file extension test as it might not exist in nginx config (#5253)
 - Fix PHP warning when password_hosts is set, but is not an array (#5260)
+- Fix redundant keep-alive requests when session_lifetime is greater than ~20000 (#5273)
 
 RELEASE 1.2.0
 -------------

program/js/app.js

@@ -8534,7 +8534,10 @@ function rcube_webmail()
     if (this._keepalive)
       clearInterval(this._keepalive);
 
-    this._keepalive = setInterval(function(){ ref.keep_alive(); }, this.env.session_lifetime * 0.5 * 1000);
+    // use Math to prevent from an integer overflow (#5273)
+    // maximum interval is 15 minutes, minimum is 30 seconds
+    var interval = Math.min(1800, this.env.session_lifetime) * 0.5 * 1000;
+    this._keepalive = setInterval(function() { ref.keep_alive(); }, interval < 30000 ? 30000 : interval);
   };
 
   // starts interval for refresh signal