[Webkit-unassigned] [Bug 187386] New: Sometimes "x instanceof Worker" inappropriately returns false

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=187386

            Bug ID: 187386
           Summary: Sometimes "x instanceof Worker" inappropriately
                    returns false
           Product: WebKit
           Version: Safari 11
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jdscheff at gmail.com

This code should clearly print "true":

    blob = new Blob(["self.onmessage = function() {};"], { type: "text/javascript" });
    worker = new Worker(window.URL.createObjectURL(blob));
    console.log(worker instanceof Worker);

I tested that in Safari 11.1 and it does indeed print "true" when I run it. However for some small subset of Safari 11.x users, it will print "false".

>From my client-side error tracker, this bug has occurred in Safari versions 11.0.2, 11.0.3, 11.1.0, 11.1.1. But only to a small fraction of Safari users. Out of a total of ~5000 Safari 11 users in the past month, only 14 of them experienced this error. I even got one of these users to run the above code in the JS console and he confirmed that even that simple code does indeed print "false" for him.

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