fix: return 200 from HEAD on the proxy root index

[neomorphic]

Problem

HEAD / returns 404 NoSuchBucket while GET / returns the 200 index page for the same URL:

$ curl -sSI https://s3.janelia.org/
HTTP/2 404
content-type: application/xml
content-length: 174

$ curl -sS -o /dev/null -w '%{http_code} %{content_type}\n' https://s3.janelia.org/
200 text/html; charset=utf-8

Reproduces on HTTP/1.1, with browser User-Agent/Accept headers, and directly against the backend, so it is the app and not nginx or HTTP/2.

The 404 body is a NoSuchBucket error with an empty bucket name. Byte math confirms it — a bucket name of 30 characters yields 204 bytes, and 204 - 30 = 174:

$ curl -sS https://s3.janelia.org/definitely-not-a-bucket-xyz123   # content-length: 204
<Error><Code>NoSuchBucket</Code>...<BucketName>definitely-not-a-bucket-xyz123</BucketName></Error>

Root cause

target_dispatcher (GET) special-cases an empty target name as "serve the proxy index":

if not target_name or (is_virtual and target_name=='www'):
    # Return target index

head_object has no such case, so the same empty target name falls into the missing-bucket path:

target_name, target_path, _ = get_target(request, path)
if not target_name:
    return get_nosuchbucket_response('')

The two handlers therefore disagree about whether / exists. RFC 9110 §9.3.2 requires HEAD to return the same status and header fields GET would have sent.

Fix

Mirror target_dispatcher's root condition in head_object, including the virtual www host, and reply 200 with the media type Accept would have negotiated — _prefers_html is reused so HEAD and GET cannot drift apart.

Vary: Accept is set for the same reason as #23 / #24: / serves two representations, and nginx's proxy_cache stores HEAD responses too, so the variants must be kept apart. That PR covers the GET fork points; HEAD is outside its scope, and the two do not overlap textually.

The route path also gains its missing leading slash so it matches the GET route. get_target already does removeprefix('/'), so that part is behavior-neutral.

Testing

test_head_root_index asserts the status, the negotiated content type, and Vary for both Accept types, and compares HEAD's status and content type against GET's on the same URL. Verified it fails without the app change (assert 404 == 200) and passes with it.

Full suite: 71 passed.

Out of scope

HEAD /{bucket}/ returns content-type: application/xml unconditionally, even though GET /{bucket}/ serves HTML to browsers. Pre-existing and untested, so left alone here — happy to follow up if you want HEAD to negotiate there too.

🤖 Generated with Claude Code

[krokicki]

Nice catch, thanks!