Synchronous XHR request through zonejs results in: More tasks executed then were scheduled

[thelgevold]

I am logging a bug report:

There seems to be a bug in the bookkeeping of XMLHttpRequest in zone.js.

If I trigger a synchronous XMLHttpRequest when zonejs is loaded on the page I get the following error:
Error: More tasks executed then were scheduled.

The error does not occur if the request is asynchronous.

Here is a simple plunker with a repro: https://plnkr.co/edit/7Cn3JknfDIIHMIdqcuo0?p=preview

I have not looked into this extensively, but the error occurs here:
next is set to -1 which triggers the error condition.

ZoneDelegate.prototype._updateTaskCount = function (type, count) {
                var counts = this._taskCounts;
                var prev = counts[type];
                var next = counts[type] = prev + count;
                if (next < 0) {
                    throw new Error('More tasks executed then were scheduled.');
                }
                if (prev == 0 || next == 0) {
                    var isEmpty = {
                        microTask: counts.microTask > 0,
                        macroTask: counts.macroTask > 0,
                        eventTask: counts.eventTask > 0,
                        change: type
                    };
                    try {
                        this.hasTask(this.zone, isEmpty);
                    }
                    finally {
                        if (this._parentDelegate) {
                            this._parentDelegate._updateTaskCount(type, count);
                        }
                    }
                }
            };

[thelgevold]

I agree that sync xhr requests are discouraged, but I think a warning is more appropriate than an error in this scenario.

[garethdn]

Having the same issue specifically when trying to use a Marketo lead-tracking plugin. Granted, the requests the plugin perform should really be async but throwing an error and essentially breaking change detection seems a bit much.

[garethdn]

#287 (comment)

[dfahlander]

The awesome karma-source-map-support needs to do synchronous xhr requests to make the property Error.prototype.stack behave nice, because a getter is always synchronous. Zonejs should neither warn or fail about that. Synchronous request is a public and acceptable use of XMLHttpRequest and zonejs should respect that. It should really be transparent. This is a bug and I suppose it could be easily fixed for those who are already familiar with the zonejs source code.