The tests in this directory exercise the user agent's implementation of Network Error Logging and Reporting.
Each test case generates a unique reportID that is used to distinguish the NEL reports generated by that test case.
The support/report.py file is a Python file handler that can be used as a Reporting collector. Its default operation is to save any reports that it receives into the stash. If you pass in the optional op URL parameter, with a value of retrieve_report, it will instead return a list of all of the reports received for a particular reportID.
NEL reports are only generated if the user agent has received a NEL policy for the origin of the request. The current request counts; if its response contains a policy, that policy is used for the current request and all future requests, modulo the policy's max_age field.
Most of the test cases will therefore make a request or two to install NEL policies, and then make another request that should or should not be covered by those policies. It will then assert that NEL reports were or were not created, as required by the spec.
The support directory contains several images, each of which defines a particular “kind” of NEL policy (e.g., include_subdomains set vs unset, no policy at all, etc.). The support/nel.sub.js file contains helper JavaScript methods for requesting those images, so that the test cases themselves are more descriptive.
NEL policies apply to all future requests to the origin. We therefore serve all of the test case‘s “infrastructure” (the test case itself, support/report.py and support/nel.sub.js) on a different origin than the requests that exercise the NEL implementation. That ensures that we don’t have to wade through NEL reports about the infrastructure when verifying the NEL reports about the requests that we care about.
You must configure your browser's Reporting implementation to upload reports for a request immediately. The test cases do not currently have any timeouts; they assume that as soon as the Fetch API promise resolves, any NEL reports for the request have already been uploaded.
Because NEL policies are stored in a global cache in the user agent, we need to run the tests in this directory serially instead of in parallel. We implement a simple spin-lock in support/lock.py to ensure that only one test is allowed to perform any NEL-related requests at a time.
Reporting uploads are subject to CORS preflights. We want to test normal operation (when preflight requests succeed) as well as failures of the CORS preflight logic in the user agent. To support this, our test collector is configured to always reject the CORS preflight for a single domain (www2), and to always grant the CORS preflight for all other test subdomains.