fix(master): drop registry response bodies from poll errors (#9)

internal/master/registry/registry.go

@@ -4,11 +4,16 @@ package registry
 
 import (
 	"context"
+	"errors"
 	"fmt"
+	"net/http"
+	"regexp"
+	"strings"
 
 	"github.com/google/go-containerregistry/pkg/authn"
 	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
+	"github.com/google/go-containerregistry/pkg/v1/remote/transport"
 
 	"github.com/shcizo/package-updater/internal/imageref"
 )
@@ -49,7 +54,33 @@ func (c *Client) Digest(ctx context.Context, ref string) (string, error) {
 		remote.WithAuthFromKeychain(c.keychain),
 		remote.WithUserAgent(c.userAgent))
 	if err != nil {
-		return "", fmt.Errorf("head %s: %w", r, err)
+		var terr *transport.Error
+		if errors.As(err, &terr) {
+			return "", fmt.Errorf("lookup %s: %s", r, describe(terr))
+		}
+		return "", fmt.Errorf("lookup %s: %w", r, err)
 	}
 	return desc.Digest.String(), nil
 }
+
+// errorCodePattern matches the registry error codes of the distribution
+// spec ("MANIFEST_UNKNOWN"). Anything else is dropped from error text.
+var errorCodePattern = regexp.MustCompile(`^[A-Z_]{1,40}$`)
+
+// describe returns the HTTP status and the structured error codes of a
+// registry error, without the messages, details or raw body that ggcr copies
+// into its error text. The registry host comes from an image name, so
+// whatever it answers must not end up in the UI.
+func describe(e *transport.Error) string {
+	msg := fmt.Sprintf("registry answered %d %s", e.StatusCode, http.StatusText(e.StatusCode))
+	var codes []string
+	for _, d := range e.Errors {
+		if errorCodePattern.MatchString(string(d.Code)) {
+			codes = append(codes, string(d.Code))
+		}
+	}
+	if len(codes) > 0 {
+		msg += " (" + strings.Join(codes, ", ") + ")"
+	}
+	return msg
+}