from_accept returns JPEG unless Accept contains image/webp, and no
thumbnail test sent the header — curl defaults to */*, which does not
match. So the whole suite ran on JPEG and the WebP path was never
exercised over HTTP, despite being what background generation writes and
what the derived tier was built around. The gap was invisible because
the JPEG results were all correct.
Three assertions, one property each.
Content-Type proves negotiation happened: thumbnail_content_type sniffs
the body with `infer` rather than echoing the request, so image/webp
cannot be right by accident — serve JPEG bytes down the WebP path and it
reads image/jpeg and fails.
Differing bytes prove they are genuinely two artifacts rather than one
served twice.
Differing ETags prove the validators are separate. `variant` has carried
the format only since 20261022000000; before that a JPEG request could
match the WebP row and be served the wrong codec, and a shared validator
is exactly how a cache would then hand either to either. A final
conditional request confirms each codec revalidates against its own.
Together these also cover the per-format variant keying that lets one
source hold both codecs — the prerequisite for JPEG clients ever leaving
the sidecar, and therefore for step 10e.
Note on placement: thumb_etag stays in step 4's capture block, beside
thumb_bytes. Every later request omits Accept and so negotiates JPEG, so
the validator must be the JPEG one — captured after the new block it
would describe a different codec than the bytes next to it, and the
copy assertions compare against both.
Edouard Vanbelle
80d5372131