Lower-case the image URL's host and require a signature for a port (closes #246)

With cdn.example.com allowlisted, CDN.Example.com and
cdn.example.com:8443 passed the allowlist too, and each spelling had its
own cache entries and its own connections under
upstream_connections_per_host. The image URL parser now lower-cases the
host and refuses one holding "@" with ErrInvalidHost (400).
ValidateRequest counts a host followed by ":", with or without a port,
as not allowlisted, so it needs a signature. That check is not in
allowlist.IsAllowed, which also matches referer_blocklist. A URL is now
signed over the lower-cased host; README.md says so.

Model: opus-5-5

internal/imgcache/service.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"net"
 	"net/url"
 	"runtime/debug"
 	"time"
@@ -218,7 +219,14 @@ func (s *Service) ValidateRequest(req *ImageRequest) error {
 		return fmt.Errorf("invalid source URL: %w", err)
 	}
 
-	if s.allowlist.IsAllowed(parsedURL) {
+	// An allowlist entry names no port, so a host followed by ":", with or
+	// without a port, needs a signature. This is checked here and not in
+	// allowlist.IsAllowed, which also matches referer_blocklist, where a
+	// Referer must be blocked whatever its port.
+	_, _, splitErr := net.SplitHostPort(parsedURL.Host)
+	hostHasPort := splitErr == nil
+
+	if !hostHasPort && s.allowlist.IsAllowed(parsedURL) {
 		return nil
 	}
 

internal/imgcache/urlparser.go

@@ -144,6 +144,16 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 		return nil, ErrMissingHost
 	}
 
+	// A URL reads what comes before "@" as a user name and password, and the
+	// rest as the host
+	if strings.Contains(host, "@") {
+		return nil, fmt.Errorf("%w: %q holds @", ErrInvalidHost, host)
+	}
+
+	// Host names do not depend on letter case, so the allowlist, the upstream
+	// connection limits and the cache see one spelling of each host
+	host = strings.ToLower(host)
+
 	// Extract query string if present in path
 	if qIndex := strings.Index(path, "?"); qIndex != -1 {
 		query = path[qIndex+1:]