Add RFC XXX - URLManifestItem.url shouldn't contain non-URL-unit characters

Author: gsnedders

rfcs/valid-item-urls.md

@@ -0,0 +1,40 @@
+# RFC XXX - URLManifestItem.url shouldn't contain non-URL-unit characters
+
+## Summary
+
+Currently we end up storing URLs such as: `/xhr/xmlhttprequest-timeout-worker-aborted.html?aborted immediately after send()`. If you pass this to the [URL parser](https://url.spec.whatwg.org/#concept-url-parser) with a base URL of `http://web-platform.test`, this results in a number of [invalid-URL-unit](https://url.spec.whatwg.org/#invalid-url-unit) [validation errors](https://url.spec.whatwg.org/#validation-error).
+
+We should instead store the output of [URL serialize](https://url.spec.whatwg.org/#concept-url-serializer) ∘ [URL parse](https://url.spec.whatwg.org/#concept-url-parser) of the URL.
+
+## Details
+
+There are various systems at various vendors which expect test identifiers to not contain spaces, and when in principle the test identifier is a URL one might expect it to not contain any spaces.
+
+This is primarily a concern with variants, which vastly more often include spaces, such as the example above.
+
+We do have a few files in WPT which contain spaces:
+
+```
+gsnedders@gsnedders-marsha web-platform-tests % find . -name '* *'
+./tools/wave/test/WAVE Local.postman_environment.json
+./tools/wave/test/WAVE Server REST API Tests.postman_collection.json
+./tools/wave/tests/WAVE Local.postman_environment.json
+./tools/wave/tests/WAVE Server REST API Tests.postman_collection.json
+./css/CSS2/syntax/support/'green block.png
+./html/semantics/document-metadata/the-meta-element/pragma-directives/attr-meta-http-equiv-refresh/support/url foo
+./html/canvas/offscreen/path-objects/2d.path.roundrect.1.radius.dompoint.single argument.worker.js
+./html/canvas/offscreen/path-objects/2d.path.roundrect.1.radius.dompoint.single argument.html
+./html/canvas/element/path-objects/2d.path.roundrect.1.radius.dompoint.single argument.html
+```
+
+Only the last three of these are test files, which generate three test items.
+
+One question here is whether we should disallow test file paths that allow spaces, or whether we should just escape these in the URL.
+
+## Risks
+
+Any test system that keys off the test URLs will have to deal with URLs changing.
+
+This includes the wptmanifest (i.e., wptrunner's expectation manifest), WebKit (and I believe Blink)'s TestExpectations files and results expectations files.
+
+This may also affect items in wpt-metadata.