async._eachLimit halts one of N>1 concurrent tasks on error; keeps processing

[aronatkins]

All async._eachLimit callers are subject to this issue, which includes:

• async.eachLimit
• async.parallelLimit
• async.mapLimit

The effect is that when run with n=1, processing halts after the first error. When run with n>1 and a single error occurs, all other work is processed (though with less concurrency).

Here is an example of this behavior using mapLimit:

'use strict';

var async = require('async');

var parts = [ 1,2,3,4,5,6,7,8,9,10 ];
async.mapLimit(parts, 2, function(part, callback) {
  console.log('part',part);
  if (part === 4) {
    callback(new Error('four is fantastic'));
  } else {
    setTimeout(function() {
      console.log('finished',part);
      callback(null);
    }, 1000);
  }
}, function(err, result) {
     console.log('in finalizer');
     console.log(err,result);
});

When run with n=2, you will see that we call the finalizer and then continue processing the remaining items.

When run with n=1, work stops after an error occurs.

The _eachLimit inner replenish function should stop iteration if any concurrent task encounters an error. Something like:

            var erred = false;

            (function replenish () {
                if (completed >= arr.length) {
                    return callback();
                }

                while (running < limit && started < arr.length && !erred) {
                    started += 1;
                    running += 1;
                    iterator(arr[started - 1], function (err) {
                        if (err) {
                            callback(err);
                            callback = function () {};
                            //erred = true;
                        }
                        else {
                            completed += 1;
                            running -= 1;
                            if (completed >= arr.length) {
                                callback();
                            }
                            else {
                                replenish();
                            }
                        }
                    });
                }
            })();