Make one image URL per source on the generator page for host case and an empty path (closes #293)

The generator page passed the source host and path into the encrypted
URL as entered, so a host spelled in another letter case, or a source
with no path against the same source with /, made separate cache keys
and fetched one upstream image twice. The page now lower-cases the
source host, as /v1/image/ does, and writes an empty source path as /,
before it builds the encrypted URL. The checks the page gained for
#288 still hold.

Model: opus-5-5

README.md

@@ -473,9 +473,11 @@ nor change what it asks for.
    `jxl` for a form with no format.
 
 The token holds the source's host, path and query and the size, format, quality,
-fit and expiry, encrypted with a key derived from `signing_key`. The source
-URL's scheme is not kept: the image is fetched like any other (see Routes), and
-the blocked networks still apply.
+fit and expiry, encrypted with a key derived from `signing_key`. The host is
+kept in lower case and an empty path as `/`, as for `/v1/image/`, so
+`https://CDN.Example.com` and `https://cdn.example.com/` are one source image,
+fetched and cached once. The source URL's scheme is not kept: the image is
+fetched like any other (see Routes), and the blocked networks still apply.
 
 How long the URL lasts is chosen on the page, from 1 minute to 1 year, or never.
 The expiry is fixed in the token when the URL is made and cannot be changed or

TODO.md

@@ -30,6 +30,11 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-16 the generator page lower-cases the source host and writes an empty
+  source path as `/` before it makes the encrypted URL, as `/v1/image/` does
+  (closes #293). A host written in upper and in lower case, or a source URL with
+  no path and with the path `/`, made two cache keys and two upstream requests
+  for one image.
 - 2026-10-16 the source URL a request fetches is built with `net/url` from the
   source host, path and query, not joined as text (closes #288). The source
   path, decoded, is fetched percent-encoded again, so `100%25.png` fetches

internal/handlers/auth.go

@@ -127,7 +127,7 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 		sourceURL := r.FormValue("url")
 
 		// Validate source URL
-		parsed, err := url.Parse(sourceURL)
+		parsed, err := parseSourceURL(sourceURL)
 		if err != nil || parsed.Host == "" {
 			s.renderGeneratorWithForm(w, r, "Invalid source URL", r.Form,
 				http.StatusBadRequest)
@@ -198,6 +198,25 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 	}
 }
 
+// parseSourceURL parses the source URL entered on the generator page with its
+// host lower-cased and an empty path written as "/", as /v1/image/ reads a
+// source URL, so that each spelling of one source URL makes one cache key and
+// one upstream request.
+func parseSourceURL(sourceURL string) (*url.URL, error) {
+	parsed, err := url.Parse(sourceURL)
+	if err != nil {
+		return nil, err
+	}
+
+	parsed.Host = strings.ToLower(parsed.Host)
+
+	if parsed.Path == "" {
+		parsed.Path = "/"
+	}
+
+	return parsed, nil
+}
+
 // fetchedAsWritten reports whether fetchURL, the URL pixa fetches for an
 // image URL made from the source URL source, is source as it was written,
 // apart from the scheme, which the token does not keep. It is not when source

internal/server/source_url_internal_test.go

@@ -308,6 +308,70 @@ func TestGeneratedURLFetchesTheSourceURLAsEntered(t *testing.T) {
 	}
 }
 
+// TestGeneratorMakesOneImageURLForTwoSpellingsOfASourceURL enters two
+// spellings of one source URL on the generator page, requests the encrypted
+// URL made for each, and checks that the upstream receives one request and
+// that the second encrypted URL is served from the cache. One pair writes the
+// host in upper and in lower case, which /v1/image/ lower-cases. The host is
+// an IPv6 address, whose hex digits have letter case, because the fetcher
+// would look a host name up in DNS. It has a port, as the fetcher refuses an
+// IPv6 address written without one. The other pair is a source URL with no
+// path and with the path "/", both of which fetch "/".
+func TestGeneratorMakesOneImageURLForTwoSpellingsOfASourceURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		// sourceURLs are the two spellings, after "https://"
+		sourceURLs []string
+		// wantRequest is the path the upstream must receive
+		wantRequest string
+	}{
+		{"host in upper and in lower case",
+			[]string{"[2001:DB8::A]:8443/a.png", "[2001:db8::a]:8443/a.png"},
+			"/a.png"},
+		{"no path and the path written as a slash",
+			[]string{upstreamHost, upstreamHost + "/"}, "/"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			upstream := startRecordingUpstream(t)
+			s, _, _ := startImageProxy(t, upstream.Server)
+
+			var imageURLs []string
+
+			for _, sourceURL := range tt.sourceURLs {
+				rec := postGenerate(t, s, "https://"+sourceURL)
+
+				match := generatedURLPattern.FindStringSubmatch(rec.Body.String())
+				if rec.Code != http.StatusOK || match == nil {
+					t.Fatalf("POST /generate: %d with no encrypted URL; body %s",
+						rec.Code, rec.Body.String())
+				}
+
+				imageURLs = append(imageURLs, match[1])
+			}
+
+			getImage(t, s, imageURLs[0])
+
+			second := getImage(t, s, imageURLs[1])
+			if got := second.Header().Get("X-Pixa-Cache"); got != string(imgcache.CacheHit) {
+				t.Errorf("second X-Pixa-Cache = %q, want HIT", got)
+			}
+
+			got := upstream.requestTargets()
+			t.Logf("upstream received %q", got)
+
+			if !slices.Equal(got, []string{tt.wantRequest}) {
+				t.Errorf("upstream received %q, want [%q]", got, tt.wantRequest)
+			}
+		})
+	}
+}
+
 // TestGeneratorRefusesSourceURLItCannotFetchAsWritten submits source URLs the
 // generator page must make no encrypted URL for, and checks that it answers
 // 400 and shows why. pixa keeps the source path decoded and fetches it