glue.js

var wrap = require('shimmer').wrap;

/*
 *
 * CONSTANTS
 *
 */
var HAS_CREATE_AL = 1 << 0;
var HAS_BEFORE_AL = 1 << 1;
var HAS_AFTER_AL = 1 << 2;
var HAS_ERROR_AL = 1 << 3;

/**
 * There is one list of currently active listeners that is mutated in place by
 * addAsyncListener and removeAsyncListener. This complicates error-handling,
 * for reasons that are discussed below.
 */
var listeners = [];

/**
 * There can be multiple listeners with the same properties, so disambiguate
 * them by assigning them an ID at creation time.
 */
var uid = 0;

/**
 * Ensure that errors coming from within listeners are handed off to domains,
 * process._fatalException, or uncaughtException without being treated like
 * user errors.
 */
var inAsyncTick = false;

/**
 * Because asynchronous contexts can be nested, and errors can come from anywhere
 * in the stack, a little extra work is required to keep track of where in the
 * nesting we are. Because JS arrays are frequently mutated in place
 */
var listenerStack = [];

/**
 * The error handler on a listener can capture errors thrown during synchronous
 * execution immediately after the listener is added. To capture both
 * synchronous and asynchronous errors, the error handler just uses the
 * "global" list of active listeners, and the rest of the code ensures that the
 * listener list is correct by using a stack of listener lists during
 * asynchronous execution.
 */
var asyncCatcher;

/**
 * The guts of the system -- called each time an asynchronous event happens
 * while one or more listeners are active.
 */
var asyncWrap;

/**
 * Simple helper function that's probably faster than using Array
 * filter methods and can be inlined.
 */
function union(dest, added) {
  var destLength = dest.length;
  var addedLength = added.length;
  var returned = [];

  if (destLength === 0 && addedLength === 0) return returned;

  for (var j  = 0; j < destLength; j++) returned[j] = dest[j];

  if (addedLength === 0) return returned;

  for (var i = 0; i < addedLength; i++) {
    var missing = true;
    for (j = 0; j < destLength; j++) {
      if (dest[j].uid === added[i].uid) {
        missing = false;
        break;
      }
    }
    if (missing) returned.push(added[i]);
  }

  return returned;
}

/*
 * For performance, split error-handlers and asyncCatcher up into two separate
 * code paths.
 */

// 0.9+
if (process._fatalException) {
  /**
   * Error handlers on listeners can throw, the catcher needs to be able to
   * discriminate between exceptions thrown by user code, and exceptions coming
   * from within the catcher itself. Use a global to keep track of which state
   * the catcher is currently in.
   */
  var inErrorTick = false;

  /**
   * Throwing always happens synchronously. If the current array of values for
   * the current list of asyncListeners is put in a module-scoped variable right
   * before a call that can throw, it will always be correct when the error
   * handlers are run.
   */
  var errorValues;

  asyncCatcher = function asyncCatcher(er) {
    var length = listeners.length;
    if (inErrorTick || length === 0) return false;

    var handled = false;

    /*
     * error handlers
     */
    inErrorTick = true;
    for (var i = 0; i < length; ++i) {
      var listener = listeners[i];
      if ((listener.flags & HAS_ERROR_AL) === 0) continue;

      var value = errorValues && errorValues[listener.uid];
      handled = listener.error(value, er) || handled;
    }
    inErrorTick = false;

    /* Test whether there are any listener arrays on the stack. In the case of
     * synchronous throws when the listener is active, there may have been
     * none pushed yet.
     */
    if (listenerStack.length > 0) listeners = listenerStack.pop();
    errorValues = undefined;

    return handled && !inAsyncTick;
  };

  asyncWrap = function asyncWrap(original, list, length) {
    var values = [];

    /*
     * listeners
     */
    inAsyncTick = true;
    for (var i = 0; i < length; ++i) {
      var listener = list[i];
      values[listener.uid] = listener.data;

      if ((listener.flags & HAS_CREATE_AL) === 0) continue;

      var value = listener.create(listener.data);
      if (value !== undefined) values[listener.uid] = value;
    }
    inAsyncTick = false;

    /* One of the main differences between this polyfill and the core
     * asyncListener support is that core avoids creating closures by putting a
     * lot of the state managemnt on the C++ side of Node (and of course also it
     * bakes support for async listeners into the Node C++ API through the
     * AsyncWrap class, which means that it doesn't monkeypatch basically every
     * async method like this does).
     */
    return function () {
      // put the current values where the catcher can see them
      errorValues = values;

      /* More than one listener can end up inside these closures, so save the
       * current listeners on a stack.
       */
      listenerStack.push(listeners);

      /* Activate both the listeners that were active when the closure was
       * created and the listeners that were previously active.
       */
      listeners = union(list, listeners);

      /*
       * before handlers
       */
      inAsyncTick = true;
      for (var i = 0; i < length; ++i) {
        if ((list[i].flags & HAS_BEFORE_AL) > 0) {
          list[i].before(this, values[list[i].uid]);
        }
      }
      inAsyncTick = false;

      // save the return value to pass to the after callbacks
      var returned = original.apply(this, arguments);

      /*
       * after handlers (not run if original throws)
       */
      inAsyncTick = true;
      for (i = 0; i < length; ++i) {
        if ((list[i].flags & HAS_AFTER_AL) > 0) {
          list[i].after(this, values[list[i].uid]);
        }
      }
      inAsyncTick = false;

      // back to the previous listener list on the stack
      listeners = listenerStack.pop();
      errorValues = undefined;

      return returned;
    };
  };

  wrap(process, '_fatalException', function (_fatalException) {
    return function _asyncFatalException(er) {
      return asyncCatcher(er) || _fatalException(er);
    };
  });
}
// 0.8 and below
else {
  /**
   * If an error handler in asyncWrap throws, the process must die. Under 0.8
   * and earlier the only way to put a bullet through the head of the process
   * is to rethrow from inside the exception handler, so rethrow and set
   * errorThrew to tell the uncaughtHandler what to do.
   */
  var errorThrew = false;

  /**
   * Under Node 0.8, this handler *only* handles synchronously thrown errors.
   * This simplifies it, which almost but not quite makes up for the hit taken
   * by putting everything in a try-catch.
   */
  asyncCatcher = function uncaughtCatcher(er) {
    // going down hard
    if (errorThrew) throw er;

    var handled = false;

    /*
     * error handlers
     */
    var length = listeners.length;
    for (var i = 0; i < length; ++i) {
      var listener = listeners[i];
      if ((listener.flags & HAS_ERROR_AL) === 0) continue;
      handled = listener.error(null, er) || handled;
    }

    /* Rethrow if one of the before / after handlers fire, which will bring the
     * process down immediately.
     */
    if (!handled && inAsyncTick) throw er;
  };

  asyncWrap = function asyncWrap(original, list, length) {
    var values = [];

    /*
     * listeners
     */
    inAsyncTick = true;
    for (var i = 0; i < length; ++i) {
      var listener = list[i];
      values[listener.uid] = listener.data;

      if ((listener.flags & HAS_CREATE_AL) === 0) continue;

      var value = listener.create(listener.data);
      if (value !== undefined) values[listener.uid] = value;
    }
    inAsyncTick = false;

    /* One of the main differences between this polyfill and the core
     * asyncListener support is that core avoids creating closures by putting a
     * lot of the state managemnt on the C++ side of Node (and of course also it
     * bakes support for async listeners into the Node C++ API through the
     * AsyncWrap class, which means that it doesn't monkeypatch basically every
     * async method like this does).
     */
    return function () {
      /*jshint maxdepth:4*/

      // after() handlers don't run if threw
      var threw = false;

      // ...unless the error is handled
      var handled = false;

      /* More than one listener can end up inside these closures, so save the
       * current listeners on a stack.
       */
      listenerStack.push(listeners);

      /* Activate both the listeners that were active when the closure was
       * created and the listeners that were previously active.
       */
      listeners = union(list, listeners);

      /*
       * before handlers
       */
      inAsyncTick = true;
      for (var i = 0; i < length; ++i) {
        if ((list[i].flags & HAS_BEFORE_AL) > 0) {
          list[i].before(this, values[list[i].uid]);
        }
      }
      inAsyncTick = false;

      // save the return value to pass to the after callbacks
      var returned;
      try {
        returned = original.apply(this, arguments);
      }
      catch (er) {
        threw = true;
        for (var i = 0; i < length; ++i) {
          if ((listeners[i].flags & HAS_ERROR_AL) == 0) continue;
          try {
            handled = listeners[i].error(values[list[i].uid], er) || handled;
          }
          catch (x) {
            errorThrew = true;
            throw x;
          }
        }

        if (!handled) {
          // having an uncaughtException handler here alters crash semantics
          process.removeListener('uncaughtException', asyncCatcher);
          process._originalNextTick(function () {
            process.addListener('uncaughtException', asyncCatcher);
          });

          throw er;
        }
      }
      finally {
        /*
         * after handlers (not run if original throws)
         */
        if (!threw || handled) {
          inAsyncTick = true;
          for (i = 0; i < length; ++i) {
            if ((list[i].flags & HAS_AFTER_AL) > 0) {
              list[i].after(this, values[list[i].uid]);
            }
          }
          inAsyncTick = false;
        }

        // back to the previous listener list on the stack
        listeners = listenerStack.pop();
      }


      return returned;
    };
  };

  // will be the first to fire if async-listener is the first module loaded
  process.addListener('uncaughtException', asyncCatcher);
}

// for performance in the case where there are no handlers, just the listener
function simpleWrap(original, list, length) {
  inAsyncTick = true;
  for (var i = 0; i < length; ++i) {
    var listener = list[i];
    if (listener.create) listener.create(listener.data);
  }
  inAsyncTick = false;

  // still need to make sure nested async calls are made in the context
  // of the listeners active at their creation
  return function () {
    listenerStack.push(listeners);
    listeners = union(list, listeners);

    var returned = original.apply(this, arguments);

    listeners = listenerStack.pop();

    return returned;
  };
}

/**
 * Called each time an asynchronous function that's been monkeypatched in
 * index.js is called. If there are no listeners, return the function
 * unwrapped.  If there are any asyncListeners and any of them have callbacks,
 * pass them off to asyncWrap for later use, otherwise just call the listener.
 */
function wrapCallback(original) {
  var length = listeners.length;

  // no context to capture, so avoid closure creation
  if (length === 0) return original;

  // capture the active listeners as of when the wrapped function was called
  var list = listeners.slice();

  for (var i = 0; i < length; ++i) {
    if (list[i].flags > 0) return asyncWrap(original, list, length);
  }

  return simpleWrap(original, list, length);
}

function AsyncListener(callbacks, data) {
  if (typeof callbacks.create === 'function') {
    this.create = callbacks.create;
    this.flags |= HAS_CREATE_AL;
  }

  if (typeof callbacks.before === 'function') {
    this.before = callbacks.before;
    this.flags |= HAS_BEFORE_AL;
  }

  if (typeof callbacks.after === 'function') {
    this.after = callbacks.after;
    this.flags |= HAS_AFTER_AL;
  }

  if (typeof callbacks.error === 'function') {
    this.error = callbacks.error;
    this.flags |= HAS_ERROR_AL;
  }

  this.uid = ++uid;
  this.data = data === undefined ? null : data;
}
AsyncListener.prototype.create = undefined;
AsyncListener.prototype.before = undefined;
AsyncListener.prototype.after  = undefined;
AsyncListener.prototype.error  = undefined;
AsyncListener.prototype.data   = undefined;
AsyncListener.prototype.uid    = 0;
AsyncListener.prototype.flags  = 0;

function createAsyncListener(callbacks, data) {
  if (typeof callbacks !== 'object' || !callbacks) {
    throw new TypeError('callbacks argument must be an object');
  }

  if (callbacks instanceof AsyncListener) {
    return callbacks;
  }
  else {
    return new AsyncListener(callbacks, data);
  }
}

function addAsyncListener(callbacks, data) {
  var listener;
  if (!(callbacks instanceof AsyncListener)) {
    listener = createAsyncListener(callbacks, data);
  }
  else {
    listener = callbacks;
  }

  // Make sure the listener isn't already in the list.
  var registered = false;
  for (var i = 0; i < listeners.length; i++) {
    if (listener === listeners[i]) {
      registered = true;
      break;
    }
  }

  if (!registered) listeners.push(listener);

  return listener;
}

function removeAsyncListener(listener) {
  for (var i = 0; i < listeners.length; i++) {
    if (listener === listeners[i]) {
      listeners.splice(i, 1);
      break;
    }
  }
}

process.createAsyncListener = createAsyncListener;
process.addAsyncListener    = addAsyncListener;
process.removeAsyncListener = removeAsyncListener;

module.exports = wrapCallback;