Read the source host's port from the parsed URL (part of #246)

net.SplitHostPort fails on an IPv6 address written without brackets,
so with 2001:db8::1 allowlisted, 2001:db8::1:8443 and 2001:db8::1:
counted as having no port and were served without a signature, though
the fetcher connects to port 8443. ValidateRequest now takes a host to
have a port when the parsed URL's Port() is set or the host ends in
":", as Go's URL parser and so the fetcher read it.

Model: opus-5-5

internal/imgcache/service.go

@@ -7,9 +7,9 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
-	"net"
 	"net/url"
 	"runtime/debug"
+	"strings"
 	"time"
 
 	"github.com/dustin/go-humanize"
@@ -220,11 +220,13 @@ func (s *Service) ValidateRequest(req *ImageRequest) error {
 	}
 
 	// 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
+	// without a port, needs a signature. The port is read from the parsed
+	// URL, as the fetcher reads it: in an IPv6 address written without
+	// brackets, such as 2001:db8::1:8443, it is what follows the last ":".
+	// This is checked here and not in allowlist.IsAllowed, which also matches
+	// referer_blocklist, where a Referer must be blocked whatever its port.
+	hostHasPort := parsedURL.Port() != "" ||
+		strings.HasSuffix(parsedURL.Host, ":")
 
 	if !hostHasPort && s.allowlist.IsAllowed(parsedURL) {
 		return nil