Refuse a q outside 1-100 on /v1/image/ with 400 (closes #134)

The route ignored a q that was not a number or was outside 1-100 and
used 85, so q=banana or q=500 was served as if q were absent and
verified against a signature made for 85.

It now reads q with the check the URL generator uses for its quality
field (parseFormInt with minQuality and maxQuality, default
encurl.DefaultQuality) and answers anything else with a 400 naming q
and the value. That check takes an empty value as missing, so an empty
q in the URL is refused before it. Only a q missing from the URL is
85. README.md states the range.

Model: opus-5-5

README.md

@@ -125,8 +125,9 @@ Where:
 - `height` — requested height in pixels, `0` for original
 - `format` — output format (jpeg, png, webp, avif, gif, orig)
 - `expiration` — Unix timestamp when signature expires
-- `quality` — the URL's `q` query parameter (1-100), or `85` when the URL
-  has no `q`
+- `quality` — the URL's `q` query parameter, a whole number from 1 to 100,
+  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`
 

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 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
+  85; the route reads `q` with the generator's quality check
+  (`parseFormInt` with `minQuality` and `maxQuality`); only a `q` missing
+  from the URL is still 85; `README.md` states the range.
 - 2026-09-28 unknown `PIXA_` environment variables abort startup (closes
   #133): a variable whose name starts with `PIXA_` but is neither a
   setting's variable nor `PIXA_CONFIG_PATH` aborts startup naming it, as

internal/handlers/auth.go

@@ -18,13 +18,14 @@ import (
 	"sneak.berlin/go/pixa/internal/templates"
 )
 
-// errInvalidFormField reports a generator form field whose value is
-// non-numeric or out of range. The offending field name is wrapped in so the
-// response can name it.
+// errInvalidFormField reports a generator form field, or the q parameter of
+// /v1/image/, whose value is non-numeric or out of range. The offending field
+// name is wrapped in so the response can name it.
 var errInvalidFormField = errors.New("invalid")
 
-// Bounds for the generator's quality and ttl fields. maxTTL is in seconds:
-// the expiry calculation time.Duration(ttl) * time.Second overflows above it.
+// Bounds for the generator's quality and ttl fields; the quality bounds also
+// apply to the q parameter of /v1/image/. maxTTL is in seconds: the expiry
+// calculation time.Duration(ttl) * time.Second overflows above it.
 const (
 	minQuality = 1
 	maxQuality = 100
@@ -248,9 +249,9 @@ func parseFormDimension(form url.Values, field string) (int, error) {
 	return value, nil
 }
 
-// parseFormInt reads an optional integer form field, returning def when the
-// field is empty and an error naming the field when the value is non-numeric
-// or outside minValue to maxValue.
+// parseFormInt reads an optional integer form field or URL query parameter,
+// returning def when the field is empty and an error naming the field when the
+// value is non-numeric or outside minValue to maxValue.
 func parseFormInt(
 	form url.Values, field string, def, minValue, maxValue int,
 ) (int, error) {

internal/handlers/image.go

@@ -2,12 +2,14 @@ package handlers
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"strconv"
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
@@ -100,23 +102,31 @@ func (s *Handlers) parseImageRequest(
 		}
 	}
 
-	// Parse optional quality and fit params
-	if qStr := query.Get("q"); qStr != "" {
-		q, parseErr := strconv.Atoi(qStr)
-		if parseErr == nil && q > 0 && q <= 100 {
-			req.Quality = q
-		}
+	// Parse optional quality and fit params. Only a q missing from the URL is
+	// 85. A q in the URL that is not a whole number from 1 to 100, an empty
+	// one included, is refused, checked as the generator checks its quality
+	// field; that check alone would take an empty q as missing.
+	qStr := query.Get("q")
+	if query.Has("q") && qStr == "" {
+		s.respondError(w, `invalid q: not a number, got ""`,
+			http.StatusBadRequest)
+
+		return nil, false
+	}
+
+	req.Quality, err = parseFormInt(query, "q",
+		encurl.DefaultQuality, minQuality, maxQuality)
+	if err != nil {
+		s.respondError(w, fmt.Sprintf("%v, got %q", err, qStr),
+			http.StatusBadRequest)
+
+		return nil, false
 	}
 
 	if fit := query.Get("fit"); fit != "" {
 		req.FitMode = imgcache.FitMode(fit)
 	}
 
-	// Default quality if not set
-	if req.Quality == 0 {
-		req.Quality = 85
-	}
-
 	// Default fit mode if not set
 	if req.FitMode == "" {
 		req.FitMode = imgcache.FitCover