/* ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Google Safe Browsing.
 *
 * The Initial Developer of the Original Code is Google Inc.
 * Portions created by the Initial Developer are Copyright (C) 2006
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Tony Chang <tc@google.com> (original author)
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the "GPL"), or
 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

// This implements logic for stopping requests if the server starts to return
// too many errors.  If we get MAX_ERRORS errors in ERROR_PERIOD minutes, we
// back off for TIMEOUT_INCREMENT minutes.  If we get another error
// immediately after we restart, we double the timeout and add
// TIMEOUT_INCREMENT minutes, etc.
// 
// This is similar to the logic used by the search suggestion service.

// HTTP responses that count as an error.  We also include any 5xx response
// as an error.
const HTTP_FOUND                 = 302;
const HTTP_SEE_OTHER             = 303;
const HTTP_TEMPORARY_REDIRECT    = 307;

/**
 * @param maxErrors Number the number of errors needed to trigger backoff
 * @param errorPeriod Number time (ms) in which maxErros have to occur to
 *     trigger the backoff behavior
 * @param timeoutIncrement Number time (ms) the starting timeout period
 *     we double this time for consecutive errors
 * @param maxTimeout Number time (ms) maximum timeout period
 */
function RequestBackoff(maxErrors, errorPeriod, timeoutIncrement, maxTimeout) {
  this.MAX_ERRORS_ = maxErrors;
  this.ERROR_PERIOD_ = errorPeriod;
  this.TIMEOUT_INCREMENT_ = timeoutIncrement;
  this.MAX_TIMEOUT_ = maxTimeout;

  // Queue of ints keeping the time of errors.
  this.errorTimes_ = [];
  this.errorTimeout_ = 0;
  this.nextRequestTime_ = 0;
  this.backoffTriggered_ = false;
}

/**
 * Reset the object for reuse.
 */
RequestBackoff.prototype.reset = function() {
  this.errorTimes_ = [];
  this.errorTimeout_ = 0;
  this.nextRequestTime_ = 0;
  this.backoffTriggered_ = false;
}

/**
 * Check to see if we can make a request.
 */
RequestBackoff.prototype.canMakeRequest = function() {
  return Date.now() > this.nextRequestTime_;
}

/**
 * Notify this object of the last server response.  If it's an error, we
 * check to see if we should trigger a backoff.
 * @return Boolean true if this counts as an error.
 */
RequestBackoff.prototype.noteServerResponse = function(status) {
  var isError = this.isErrorStatus_(status);
  if (isError) {
    var now = Date.now();
    this.errorTimes_.push(now);

    // We only care about keeping track of MAX_ERRORS
    if (this.errorTimes_.length > this.MAX_ERRORS_)
      this.errorTimes_.shift();

    // See if we hit the backoff case
    // This either means we hit MAX_ERRORS in ERROR_PERIOD
    // *or* we were already in a backoff state, in which case we
    // increase our timeout.
    if ((this.errorTimes_.length == this.MAX_ERRORS_ &&
         now - this.errorTimes_[0] < this.ERROR_PERIOD_)
        || this.backoffTriggered_) {
      this.errorTimeout_ = (this.errorTimeout_ * 2)  + this.TIMEOUT_INCREMENT_;
      this.errorTimeout_ = Math.min(this.errorTimeout_, this.MAX_TIMEOUT_);
      this.nextRequestTime_ = now + this.errorTimeout_;
      this.backoffTriggered_ = true;
    }
  } else {
    // Reset error timeout, allow requests to go through, and switch out
    // of backoff state.
    this.errorTimeout_ = 0;
    this.nextRequestTime_ = 0;
    this.backoffTriggered_ = false;
  }
  return isError;
}

/**
 * We consider 302, 303, 307, and 5xx http responses to be errors.
 * @param status Number http status
 * @return Boolean true if we consider this http status an error
 */
RequestBackoff.prototype.isErrorStatus_ = function(status) {
  return ((500 <= status && status <= 599) ||
          HTTP_FOUND == status ||
          HTTP_SEE_OTHER == status ||
          HTTP_TEMPORARY_REDIRECT == status);
}

#ifdef 0
// Some unittests (e.g., paste into JS shell)
var jslib = Cc["@mozilla.org/url-classifier/jslib;1"].
            getService().wrappedJSObject;
var _Datenow = jslib.Date.now;
function setNow(time) {
  jslib.Date.now = function() {
    return time;
  }
}

// 2 errors, 5ms time period, 5ms backoff interval, 20ms max delay
var rb = new jslib.RequestBackoff(2, 5, 5, 20);
setNow(1);
rb.noteServerResponse(200)
if (!rb.canMakeRequest()) throw "expected ok";

setNow(2);
rb.noteServerResponse(500);
if (!rb.canMakeRequest()) throw "expected ok";

setNow(3);
rb.noteServerResponse(200)
if (!rb.canMakeRequest()) throw "expected ok";

// Trigger backoff
setNow(4);
rb.noteServerResponse(502)
if (rb.canMakeRequest()) throw "expected failed";
if (rb.nextRequestTime_ != 9) throw "wrong next request time";

// Trigger backoff again
setNow(10);
if (!rb.canMakeRequest()) throw "expected ok";
rb.noteServerResponse(503)
if (rb.canMakeRequest()) throw "expected failed";
if (rb.nextRequestTime_ != 25) throw "wrong next request time";

// Trigger backoff a third time and hit max timeout
setNow(30);
if (!rb.canMakeRequest()) throw "expected ok";
rb.noteServerResponse(302)
if (rb.canMakeRequest()) throw "expected failed";
if (rb.nextRequestTime_ != 50) throw "wrong next request time";

// Request goes through
setNow(100);
if (!rb.canMakeRequest()) throw "expected ok";
rb.noteServerResponse(200)
if (!rb.canMakeRequest()) throw "expected ok";
if (rb.nextRequestTime_ != 0) throw "wrong next request time";

// Another error (shouldn't trigger backoff)
setNow(101);
rb.noteServerResponse(500);
if (!rb.canMakeRequest()) throw "expected ok";

// Another error, but not in ERROR_PERIOD, so it should be ok
setNow(107);
rb.noteServerResponse(500);
if (!rb.canMakeRequest()) throw "expected ok";

jslib.Date.now = _Datenow;
#endif