Take the signature, the expiry and the URL scheme off the image request (closes #314)

The image request now holds only its source and its transform, exactly what
the variant key covers. The /v1/image/ handler checks the allowlist and
verifies the signature itself with Signer.Verify(req, sig, expires). Both
image handlers keep the URL's expiry as a local value for Cache-Control. The
image service holds no signer, allowlist or AllowHTTP, and ValidateRequest is
gone. The fetcher takes the source and builds its URL with Source.URL from its
own AllowHTTP, so no URL string is built and parsed again. chooseAutoFormat
returns the format it chooses instead of writing into the request. The signed
URL format and every route's answers are unchanged.

Model: opus-5-5

internal/handlers/image_signature_internal_test.go

@@ -38,44 +38,40 @@ func getImage(t *testing.T, fix *testFixtures, target string) int {
 }
 
 // generateSignedURL returns the /v1/image/ URL of req, signed with key and
-// expiring after ttl. It signs a copy of req, so req itself is not changed.
-// An unset quality or fit in the copy is set to 85 or cover, the values the
-// route uses when a URL has no q or fit. The URL carries q and fit next to sig
-// and exp, so the route verifies it for the quality and fit it was signed
-// with.
+// expiring after ttl. An unset quality or fit of req is first set to 85 or
+// cover, the values the route uses when a URL has no q or fit. The URL carries
+// q and fit next to sig and exp, so the route verifies it for the quality and
+// fit it was signed with.
 func generateSignedURL(
 	key string, req *imagerequest.Request, ttl time.Duration,
 ) string {
-	signed := *req
-
-	if signed.Transform.Quality == 0 {
-		signed.Transform.Quality = 85
+	if req.Transform.Quality == 0 {
+		req.Transform.Quality = 85
 	}
 
-	if signed.Transform.FitMode == "" {
-		signed.Transform.FitMode = imagerequest.FitCover
+	if req.Transform.FitMode == "" {
+		req.Transform.FitMode = imagerequest.FitCover
 	}
 
 	expires := time.Now().Add(ttl)
-	sig := signature.New(key).Sign(&signed, expires)
+	sig := signature.New(key).Sign(req, expires)
 
 	// A source query follows the source path after a "?". The image route
 	// reads the path decoded once, so the whole path is written in Go's own
 	// percent-encoding of a URL path, which writes "?" as %3F and "%" as %25,
 	// and leaves "/", ";", ",", "=" and "&" as they are.
-	sourcePathAndQuery := signed.Source.Path
-	if signed.Source.Query != "" {
-		sourcePathAndQuery += "?" + signed.Source.Query
+	sourcePathAndQuery := req.Source.Path
+	if req.Source.Query != "" {
+		sourcePathAndQuery += "?" + req.Source.Query
 	}
 
 	unescapedPath := fmt.Sprintf("/v1/image/%s%s/%dx%d.%s",
-		signed.Source.Host, sourcePathAndQuery,
-		signed.Transform.Size.Width, signed.Transform.Size.Height,
-		signed.Transform.Format)
+		req.Source.Host, sourcePathAndQuery,
+		req.Transform.Size.Width, req.Transform.Size.Height, req.Transform.Format)
 
 	return fmt.Sprintf("%s?sig=%s&exp=%d&q=%d&fit=%s",
 		(&url.URL{Path: unescapedPath}).EscapedPath(), sig, expires.Unix(),
-		signed.Transform.Quality, signed.Transform.FitMode)
+		req.Transform.Quality, req.Transform.FitMode)
 }
 
 // TestHandleImage_SignatureCoversQualityAndFit signs a URL for quality 85

internal/imagerequest/urlparser.go

@@ -126,8 +126,9 @@ func parseImageComponents(remainder string) (*Request, error) {
 		return nil, ErrMissingHost
 	}
 
-	// Later steps put the host unparsed into the allowlist check, the cache key
-	// and the upstream URL, so only the forms hostRegex describes are accepted
+	// Only the forms hostRegex describes are accepted. A "?", "#" or "@" in
+	// the host would make Go's URL parser, which the allowlist check and the
+	// fetcher use, read a different host from the one allowlisted and cached
 	hostMatch := hostRegex.FindStringSubmatch(host)
 	if hostMatch == nil {
 		return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
@@ -146,8 +147,9 @@ func parseImageComponents(remainder string) (*Request, error) {
 	// host
 	host = strings.ToLower(host)
 
-	// A "#" starts a URL's fragment, so neither the source path nor the source
-	// query may hold one
+	// Go's URL parser, which the fetcher uses, reads a "#" as the start of a
+	// fragment and drops the rest, so each text after it would be another
+	// spelling, cached under its own keys, of one upstream request
 	if strings.Contains(path, "#") {
 		return nil, fmt.Errorf(
 			`%w: the source path or query holds a "#"`, ErrInvalidPath)