Refuse an empty fit on /v1/image/ with 400 (closes #139)

A fit in the URL with an empty value (fit=) was treated as missing, so
it was served as cover and verified against a signature made for cover.
It is now a 400 naming fit, the same rule the route applies to an empty
q. It is checked before the existing fit-mode check, which takes an
empty fit as missing; any other value still goes through that check
unchanged. Only a fit missing from the URL is cover.

Model: opus-5-5

README.md

@@ -132,7 +132,8 @@ Where:
   or `85` when the URL has no `q`; a request whose `q` is anything else is
   refused with 400
 - `fit` — the URL's `fit` query parameter (cover, contain, fill, inside,
-  outside), or `cover` when the URL has no `fit`
+  outside), or `cover` when the URL has no `fit`; a request whose `fit` is
+  anything else, an empty `fit=` included, is refused with 400
 
 **Example:** resize `https://cdn.example.com/photos/cat.jpg` to 800x600
 WebP with expiration 1704067200, default quality and fit:

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 refuse an empty `fit` on `/v1/image/` (closes #139): a
+  `fit` in the URL with an empty value (`fit=`) is a 400 naming `fit`,
+  instead of being served as `cover` and verified against a signature
+  made for `cover`; only a `fit` missing from the URL is still `cover`;
+  any other value still goes through the existing fit-mode check;
+  `README.md` says so where it documents `fit`.
 - 2026-09-28 refuse an invalid `q` on `/v1/image/` (closes #134): a `q`
   that is not a whole number from 1 to 100, an empty `q` included, is a
   400 naming `q` and the value, instead of being served at the default

internal/handlers/image.go

@@ -144,8 +144,14 @@ func (s *Handlers) parseImageRequest(
 		return nil, false
 	}
 
-	if fit := query.Get("fit"); fit != "" {
-		req.FitMode = imgcache.FitMode(fit)
+	// Only a fit missing from the URL is cover. A fit in the URL that is not a
+	// fit mode is refused by the fit-mode check below; that check would take an
+	// empty fit as missing, so an empty one is refused here.
+	req.FitMode = imgcache.FitMode(query.Get("fit"))
+	if query.Has("fit") && req.FitMode == "" {
+		s.respondError(w, `invalid fit: not a fit mode, got ""`, http.StatusBadRequest)
+
+		return nil, false
 	}
 
 	// Default fit mode if not set