Intermittent failure with TypeError: Network error occurred

[Manishearth]

This happened with /html/dom/idlharness.https.html?exclude=(Document|Window|HTML.*) but I strongly suspect it's not about that specific test.

  ▶ Unexpected subtest result in /html/dom/idlharness.https.html?exclude=(Document|Window|HTML.*):
  │ FAIL [expected PASS] idl_test setup
  └   → promise_test: Unhandled rejection with value: object "TypeError: Network error occurred"

[Manishearth]

The relevant error comes from fetch:

servo/components/script/fetch.rs

Line 222 in 58c61d3

promise.reject_error(Error::Type("Network error occurred".to_string()));

(should that be an Error::Network?)

[pshaughn]

Most of the times I've seen this error in WPT tests, it's been a response header that Servo interpreted as CORS-disallowed or otherwise unacceptable. And unfortunately it IS supposed to be a TypeError! https://fetch.spec.whatwg.org/ specifies TypeError in many situations that are not really type-related.

[jdm]

I have observed this problem in 7474b30, but it appears to have become much more frequent between 3996326 and d657a6c.