Bug 231928 – Show HTTP status code in CORS messages logged to devtools console that can indirectly result from HTTP errors

• Status: RESOLVED
• Resolution: FIXED
• Component: Web Inspector

[sideshowbarker]

### Problem
 When a CORS request is sent to a server and the server responds with a 4xx or 5xx HTTP status code in the response, and the response doesn’t include the Access-Control-Allow-Origin header, the browser will always log a CORS error message to the devtools console — but may not (or even usually doesn’t) also log the HTTP status code to the console.

As a result, many (or most) developers who are in the process of trying to troubleshoot the server-side CORS config (on the server the request was sent to) only see the CORS error — but don’t see the 4xx or 5xx HTTP status code — and so then often (or usually) assume that there’s a mistake in their CORS config, when in fact the actual cause is that a 4xx or 5xx error occurred.

For some evidence and discussion of that problem, see the following:

* https://stackoverflow.com/questions/54795541/503-return-from-server-is-branded-as-cors-violation-by-chrome
* https://davidtruxall.com/misleading-cors-errors/

The effect of that problem is that, every day, it causes hundreds if not thousands of developers to waste hours trying to troubleshoot and identify mistakes in their server CORS config — mistakes they never find, because in fact their existing CORS config is already working as expected and instead the real cause is some 400 or 500 or 502 or whatever.

The main reason those developers see CORS errors logged to the devtools console in those cases is that many (or most) runtimes/server systems by default do not add application-set response headers to 4xx and 5xx responses — including the Access-Control-Allow-Origin response header. (For example, Apache and nginx do not; in order to make them add the Access-Control-Allow-Origin response header to 4xx or 5xx errors, the `always` keyword needs to be added to the header-setting directive.)

### Proposed solution
Make the HTTP status be included in all CORS messages that might get logged when a 4xx or 5xx error occurs. That means the following:

* Failed to load resource: Origin foo is not allowed by Access-Control-Allow-Origin

* Failed to load resource: Preflight response is not successful

The proposal here is that in the case of, for example, a 500 error, those messages would instead be:

* Failed to load resource: Origin foo is not allowed by Access-Control-Allow-Origin. Status code: 500

* Failed to load resource: Preflight response is not successful. Status code: 500

[sideshowbarker]

I should have mentioned in the issue description that I’ve written Gecko and Blink patches for this:

* https://bugzilla.mozilla.org/show_bug.cgi?id=1736026 is the Gecko bug. The patch for that should be landing in the trunk there this week.

* https://bugs.chromium.org/p/chromium/issues/detail?id=1260776 is the Chromium bug. The patch there is going take a while longer to land.

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Created attachment 441669 [details]
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Created attachment 441690 [details]
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Created attachment 441691 [details]
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Created attachment 441699 [details]
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Created attachment 441703 [details]
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Created attachment 441704 [details]
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Created attachment 441710 [details]
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Created attachment 441726 [details]
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[sideshowbarker]

I’m looking at the test failures and I see this:

https://ews-build.s3-us-west-2.amazonaws.com/Windows-EWS/r441710-110694/results.html

…and that’s failing because there’s a difference between the expected output and the actual output:

https://ews-build.s3-us-west-2.amazonaws.com/Windows-EWS/r441710-110694/http/tests/xmlhttprequest/access-control-preflight-not-successful-pretty-diff.html

-CONSOLE MESSAGE: Preflight response is not successful
+CONSOLE MESSAGE: Preflight response is not successful. Status code: 302

That is, specifically, the expected output has no `Status code: 302` while the actual output does.

But the patch does actually include a change to the expected output at http/tests/xmlhttprequest/access-control-preflight-not-successful-expected.txt — see https://bugs.webkit.org/attachment.cgi?id=441726&action=diff#a/LayoutTests/http/tests/xmlhttprequest/access-control-preflight-not-successful-expected.txt_sec1

However, for some reason the expected output that the EWS test is picking up at 
https://ews-build.s3-us-west-2.amazonaws.com/Windows-EWS/r441710-110694/http/tests/xmlhttprequest/access-control-preflight-not-successful-expected.txt doesn’t include the change.

So I don’t understand why the change from the patch isn’t reflected in the version the EWS test is using — and I don’t have an idea how to fix it…

[Alex Christensen]

$ find LayoutTests -name access-control-preflight-not-successful-expected.txt
LayoutTests/platform/win/http/tests/xmlhttprequest/access-control-preflight-not-successful-expected.txt
LayoutTests/platform/mac-wk1/http/tests/xmlhttprequest/access-control-preflight-not-successful-expected.txt
LayoutTests/http/tests/xmlhttprequest/access-control-preflight-not-successful-expected.txt

There are different test expectations files for different platforms.  You'll probably need to update all of them.

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Created attachment 442005 [details]
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Created attachment 442009 [details]
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Created attachment 442022 [details]
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Created attachment 442076 [details]
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Created attachment 442085 [details]
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Created attachment 442097 [details]
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Created attachment 442114 [details]
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Created attachment 442125 [details]
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[sideshowbarker]

(In reply to Alex Christensen from comment #11)
>
> There are different test expectations files for different platforms.  You'll
> probably need to update all of them.

Aha, thanks — I’ve now updated all the tests, and EWS is all green. So I think this is ready for review from whoever might have time.

Incidentally, the equivalent Firefox change has now landed and will ship in Firefox 95.

- https://hg.mozilla.org/mozilla-central/rev/2b65a0fbcf8b
- https://bugzilla.mozilla.org/show_bug.cgi?id=1736026

[EWS]

ChangeLog entry in Source/WebKit/ChangeLog contains OOPS!.

[Radar WebKit Bug Importer]

<rdar://problem/84640965>

[Chris Dumez]

Comment on attachment 442125 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=442125&action=review

> Source/WebKit/ChangeLog:28
> +        Reviewed by NOBODY (OOPS!).

double NOBODY line.

[sideshowbarker]

Created attachment 443028 [details]
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[EWS]

Committed r285145 (243782@main): <https://commits.webkit.org/243782@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 443028 [details].

[Eric Hutchison]

Patch broke a few tests on iOS EWS: 

http/tests/app-privacy-report/user-attribution-preflight-async.html
http/tests/app-privacy-report/user-attribution-preflight-sync.html
http/tests/app-privacy-report/app-attribution-preflight-async.html
http/tests/app-privacy-report/app-attribution-preflight-sync.html
http/tests/privateClickMeasurement/attribution-conversion-through-fetch-keepalive.html

Re-opening for a patch.

[Kate Cheney]

Fixed test expectations in http://trac.webkit.org/changeset/285166/webkit.

[sideshowbarker]

(In reply to Eric Hutchison from comment #26)
> Patch broke a few tests on iOS EWS: 
> 
> http/tests/app-privacy-report/user-attribution-preflight-async.html
> http/tests/app-privacy-report/user-attribution-preflight-sync.html
> http/tests/app-privacy-report/app-attribution-preflight-async.html
> http/tests/app-privacy-report/app-attribution-preflight-sync.html
> http/tests/privateClickMeasurement/attribution-conversion-through-fetch-
> keepalive.html

(In reply to Kate Cheney from comment #27)
> Fixed test expectations in http://trac.webkit.org/changeset/285166/webkit.

Thanks much — and sorry for having not been more thorough myself — to have identified those tests as needing changes.

[Kate Cheney]

(In reply to Michael[tm] Smith from comment #28)
> (In reply to Eric Hutchison from comment #26)
> > Patch broke a few tests on iOS EWS: 
> > 
> > http/tests/app-privacy-report/user-attribution-preflight-async.html
> > http/tests/app-privacy-report/user-attribution-preflight-sync.html
> > http/tests/app-privacy-report/app-attribution-preflight-async.html
> > http/tests/app-privacy-report/app-attribution-preflight-sync.html
> > http/tests/privateClickMeasurement/attribution-conversion-through-fetch-
> > keepalive.html
> 
> (In reply to Kate Cheney from comment #27)
> > Fixed test expectations in http://trac.webkit.org/changeset/285166/webkit.
> 
> Thanks much — and sorry for having not been more thorough myself — to have
> identified those tests as needing changes.

No worries, those tests don't run on open source EWS bots for various reasons so it would have been very tricky to catch them the first time around!

[Eric Hutchison]

http/tests/privateClickMeasurement/attribution-conversion-through-fetch-keepalive.html continues to fail on iOS15 on EWS and Opensource.

[Eric Hutchison]

Creating new bug report for failing test: http/tests/privateClickMeasurement/attribution-conversion-through-fetch-keepalive.html

[Eric Hutchison]

Rebaselined http/tests/privateClickMeasurement/attribution-conversion-through-fetch-keepalive.html at https://trac.webkit.org/changeset/285218/webkit

[Eric Hutchison]

(In reply to Eric Hutchison from comment #31)
> Creating new bug report for failing test:
> http/tests/privateClickMeasurement/attribution-conversion-through-fetch-
> keepalive.html

Disregard, re-baselined instead.