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Explain behavior with raised exceptions #228

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Explain behavior with raised exceptions #228

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6 changes: 6 additions & 0 deletions README.md
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Expand Up @@ -113,6 +113,12 @@ A Resource path can be specified as exact string match (`/path/to/file.txt`) or

When specifying an origin, make sure that it does not have a trailing slash.

### Raising from underlying middleware / application

Rack::Cors will not add `Origin` headers if an underlying middleware / application raises an exception. The exception will be permitted to propagate through the call stack, and the client which honors cross-origin policies will refuse to parse the response it receives. The client will report an error similar to `Access to ... has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.`

A way to mitigate that issue is to use a generic error page middleware (or [Rack::ShowExceptions](https://www.rubydoc.info/gems/rack/Rack/ShowExceptions) but it is not safe to use in production). Once the exception gets suppressed `Access-Control-Allow-Origin` and other headers will be set correctly.

### Testing Postman and/or cURL

* Make sure you're passing in an `Origin:` header. That header is required to trigger a CORS response. Here's [a good SO post](https://stackoverflow.com/questions/12173990/how-can-you-debug-a-cors-request-with-curl) about using cURL for testing CORS.
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